The data structures provided by the Java utility package are very powerful and perform a wide range of functions. These data structures consist of the following interface and classes:
Enumeration
BitSet
Vector
Stack
Dictionary
Hashtable
Properties
All these classes are now legacy and Java-2 has introduced a new framework called Collections Framework, which is discussed in next tutorial:
- The Enumeration:
The Enumeration interface isn't itself a data structure, but it is very important within the context of other data structures. The Enumeration interface defines a means to retrieve successive elements from a data structure.
For example, Enumeration defines a method called nextElement that is used to get the next element in a data structure that contains multiple elements.
To have more detail about this interface, check The Enumeration.
- The BitSet
The BitSet class implements a group of bits or flags that can be set and cleared individually.
This class is very useful in cases where you need to keep up with a set of Boolean values; you just assign a bit to each value and set or clear it as appropriate.
To have more detail about this class, check The BitSet.
- The Vector
The Vector class is similar to a traditional Java array, except that it can grow as necessary to accommodate new elements.
Like an array, elements of a Vector object can be accessed via an index into the vector.
The nice thing about using the Vector class is that you don't have to worry about setting it to a specific size upon creation; it shrinks and grows automatically when necessary.
To have more detail about this class, check The Vector.
- The Stack
The Stack class implements a last-in-first-out (LIFO) stack of elements.
You can think of a stack literally as a vertical stack of objects; when you add a new element, it gets stacked on top of the others.
When you pull an element off the stack, it comes off the top. In other words, the last element you added to the stack is the first one to come back off.
To have more detail about this class, check The Stack.
- The Dictionary
The Dictionary class is an abstract class that defines a data structure for mapping keys to values.
This is useful in cases where you want to be able to access data via a particular key rather than an integer index.
Since the Dictionary class is abstract, it provides only the framework for a key-mapped data structure rather than a specific implementation.
To have more detail about this class, check The Dictionary.
- The Hashtable
The Hashtable class provides a means of organizing data based on some user-defined key structure.
For example, in an address list hash table you could store and sort data based on a key such as ZIP code rather than on a person's name.
The specific meaning of keys in regard to hash tables is totally dependent on the usage of the hash table and the data it contains.
To have more detail about this class, check The Hashtable.
- The Properties
Properties is a subclass of Hashtable. It is used to maintain lists of values in which the key is a String and the value is also a String.
The Properties class is used by many other Java classes. For example, it is the type of object returned by System.getProperties( ) when obtaining environmental values.
To have more detail about this class, check The Properties.
Sunday, June 1, 2014
[Java core 25] Data structures
[Java core 24] packages
Packages are used in Java in order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations and annotations easier, etc.
A Package can be defined as a grouping of related types(classes, interfaces, enumerations and annotations ) providing access protection and name space management.
Some of the existing packages in Java are::
java.lang - bundles the fundamental classes
java.io - classes for input , output functions are bundled in this package
Programmers can define their own packages to bundle group of classes/interfaces, etc. It is a good practice to group related classes implemented by you so that a programmer can easily determine that the classes, interfaces, enumerations, annotations are related.
Since the package creates a new namespace there won't be any name conflicts with names in other packages. Using packages, it is easier to provide access control and it is also easier to locate the related classes.
Creating a package:
When creating a package, you should choose a name for the package and put a package statement with that name at the top of every source file that contains the classes, interfaces, enumerations, and annotation types that you want to include in the package.
The package statement should be the first line in the source file. There can be only one package statement in each source file, and it applies to all types in the file.
If a package statement is not used then the class, interfaces, enumerations, and annotation types will be put into an unnamed package.
Example:
Let us look at an example that creates a package called animals. It is common practice to use lowercased names of packages to avoid any conflicts with the names of classes, interfaces.
Put an interface in the package animals:
------------------------------------------------
/* File name : Animal.java */
package animals;
interface Animal {
public void eat();
public void travel();
}
Now, put an implementation in the same package animals:
package animals;
/* File name : MammalInt.java */
public class MammalInt implements Animal{
public void eat(){
System.out.println("Mammal eats");
}
public void travel(){
System.out.println("Mammal travels");
}
public int noOfLegs(){
return 0;
}
public static void main(String args[]){
MammalInt m = new MammalInt();
m.eat();
m.travel();
}
}
-----------------------------------------
Now, you compile these two files and put them in a sub-directory called animals and try to run as follows:
$ mkdir animals
$ cp Animal.class MammalInt.class animals
$ java animals/MammalInt
Mammal eats
Mammal travels
The import Keyword:
If a class wants to use another class in the same package, the package name does not need to be used. Classes in the same package find each other without any special syntax.
Example:
Here, a class named Boss is added to the payroll package that already contains Employee. The Boss can then refer to the Employee class without using the payroll prefix, as demonstrated by the following Boss class.
-------------------------------
package payroll;
public class Boss
{
public void payEmployee(Employee e)
{
e.mailCheck();
}
}
--------------------------------
What happens if Boss is not in the payroll package? The Boss class must then use one of the following techniques for referring to a class in a different package.
The fully qualified name of the class can be used. For example:
payroll.Employee
The package can be imported using the import keyword and the wild card (*). For example:
import payroll.*;
The class itself can be imported using the import keyword. For example:
import payroll.Employee;
Note: A class file can contain any number of import statements. The import statements must appear after the package statement and before the class declaration.
The Directory Structure of Packages:
Two major results occur when a class is placed in a package:
The name of the package becomes a part of the name of the class, as we just discussed in the previous section.
The name of the package must match the directory structure where the corresponding bytecode resides.
Here is simple way of managing your files in Java:
Put the source code for a class, interface, enumeration, or annotation type in a text file whose name is the simple name of the type and whose extension is .java. For example:
// File Name : Car.java
package vehicle;
public class Car {
// Class implementation.
}
Now, put the source file in a directory whose name reflects the name of the package to which the class belongs:
....\vehicle\Car.java
Now, the qualified class name and pathname would be as below:
Class name -> vehicle.Car
Path name -> vehicle\Car.java (in windows)
In general, a company uses its reversed Internet domain name for its package names. Example: A company's Internet domain name is apple.com, then all its package names would start with com.apple. Each component of the package name corresponds to a subdirectory.
Example: The company had a com.apple.computers package that contained a Dell.java source file, it would be contained in a series of subdirectories like this:
....\com\apple\computers\Dell.java
At the time of compilation, the compiler creates a different output file for each class, interface and enumeration defined in it. The base name of the output file is the name of the type, and its extension is .class
For example:
// File Name: Dell.java
package com.apple.computers;
public class Dell{
}
class Ups{
}
Now, compile this file as follows using -d option:
$javac -d . Dell.java
This would put compiled files as follows:
.\com\apple\computers\Dell.class
.\com\apple\computers\Ups.class
You can import all the classes or interfaces defined in \com\apple\computers\ as follows:
import com.apple.computers.*;
Like the .java source files, the compiled .class files should be in a series of directories that reflect the package name. However, the path to the .class files does not have to be the same as the path to the .java source files. You can arrange your source and class directories separately, as:
<path-one>\sources\com\apple\computers\Dell.java
<path-two>\classes\com\apple\computers\Dell.class
By doing this, it is possible to give the classes directory to other programmers without revealing your sources. You also need to manage source and class files in this manner so that the compiler and the Java Virtual Machine (JVM) can find all the types your program uses.
The full path to the classes directory, <path-two>\classes, is called the class path, and is set with the CLASSPATH system variable. Both the compiler and the JVM construct the path to your .class files by adding the package name to the class path.
Say <path-two>\classes is the class path, and the package name is com.apple.computers, then the compiler and JVM will look for .class files in <path-two>\classes\com\apple\compters.
A class path may include several paths. Multiple paths should be separated by a semicolon (Windows) or colon (Unix). By default, the compiler and the JVM search the current directory and the JAR file containing the Java platform classes so that these directories are automatically in the class path.
Set CLASSPATH System Variable:
To display the current CLASSPATH variable, use the following commands in Windows and UNIX (Bourne shell):
In Windows -> C:\> set CLASSPATH
In UNIX -> % echo $CLASSPATH
To delete the current contents of the CLASSPATH variable, use :
In Windows -> C:\> set CLASSPATH=
In UNIX -> % unset CLASSPATH; export CLASSPATH
To set the CLASSPATH variable:
In Windows -> set CLASSPATH=C:\users\jack\java\classes
In UNIX -> % CLASSPATH=/home/jack/java/classes; export CLASSPATH
[java core 23] Interfaces
An interface is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.
An interface is not a class. Writing an interface is similar to writing a class, but they are two different concepts. A class describes the attributes and behaviors of an object. An interface contains behaviors that a class implements.
Unless the class that implements the interface is abstract, all the methods of the interface need to be defined in the class.
An interface is similar to a class in the following ways:
An interface can contain any number of methods.
An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.
The bytecode of an interface appears in a .class file.
Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.
However, an interface is different from a class in several ways, including:
You cannot instantiate an interface.
An interface does not contain any constructors.
All of the methods in an interface are abstract.
An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.
An interface is not extended by a class; it is implemented by a class.
An interface can extend multiple interfaces.
Declaring Interfaces:
The interface keyword is used to declare an interface. Here is a simple example to declare an interface:
Example:
Let us look at an example that depicts encapsulation:
---------------------------------------------------
/* File name : NameOfInterface.java */
import java.lang.*;
//Any number of import statements
public interface NameOfInterface
{
//Any number of final, static fields
//Any number of abstract method declarations\
}
------------------------------------------------
Interfaces have the following properties:
An interface is implicitly abstract. You do not need to use the abstract keyword when declaring an interface.
Each method in an interface is also implicitly abstract, so the abstract keyword is not needed.
Methods in an interface are implicitly public.
Example:
-----------------------------------------
/* File name : Animal.java */
interface Animal {
public void eat();
public void travel();
}
---------------------------------------
Implementing Interfaces:
When a class implements an interface, you can think of the class as signing a contract, agreeing to perform the specific behaviors of the interface. If a class does not perform all the behaviors of the interface, the class must declare itself as abstract.
A class uses the implements keyword to implement an interface. The implements keyword appears in the class declaration following the extends portion of the declaration.
----------------------------------------------
/* File name : MammalInt.java */
public class MammalInt implements Animal{
public void eat(){
System.out.println("Mammal eats");
}
public void travel(){
System.out.println("Mammal travels");
}
public int noOfLegs(){
return 0;
}
public static void main(String args[]){
MammalInt m = new MammalInt();
m.eat();
m.travel();
}
}
----------------------------------------------
This would produce the following result:
Mammal eats
Mammal travels
When overriding methods defined in interfaces there are several rules to be followed:
Checked exceptions should not be declared on implementation methods other than the ones declared by the interface method or subclasses of those declared by the interface method.
The signature of the interface method and the same return type or subtype should be maintained when overriding the methods.
An implementation class itself can be abstract and if so interface methods need not be implemented.
When implementation interfaces there are several rules:
A class can implement more than one interface at a time.
A class can extend only one class, but implement many interfaces.
An interface can extend another interface, similarly to the way that a class can extend another class.
Extending Interfaces:
An interface can extend another interface, similarly to the way that a class can extend another class. The extends keyword is used to extend an interface, and the child interface inherits the methods of the parent interface.
The following Sports interface is extended by Hockey and Football interfaces.
------------------------------------
//Filename: Sports.java
public interface Sports
{
public void setHomeTeam(String name);
public void setVisitingTeam(String name);
}
//Filename: Football.java
public interface Football extends Sports
{
public void homeTeamScored(int points);
public void visitingTeamScored(int points);
public void endOfQuarter(int quarter);
}
//Filename: Hockey.java
public interface Hockey extends Sports
{
public void homeGoalScored();
public void visitingGoalScored();
public void endOfPeriod(int period);
public void overtimePeriod(int ot);
}
---------------------------------------------
The Hockey interface has four methods, but it inherits two from Sports; thus, a class that implements Hockey needs to implement all six methods. Similarly, a class that implements Football needs to define the three methods from Football and the two methods from Sports.
Extending Multiple Interfaces:
A Java class can only extend one parent class. Multiple inheritance is not allowed. Interfaces are not classes, however, and an interface can extend more than one parent interface.
The extends keyword is used once, and the parent interfaces are declared in a comma-separated list.
For example, if the Hockey interface extended both Sports and Event, it would be declared as:
public interface Hockey extends Sports, Event
Tagging Interfaces:
The most common use of extending interfaces occurs when the parent interface does not contain any methods. For example, the MouseListener interface in the java.awt.event package extended java.util.EventListener, which is defined as:
package java.util;
public interface EventListener
{}
An interface with no methods in it is referred to as a tagging interface. There are two basic design purposes of tagging interfaces:
Creates a common parent: As with the EventListener interface, which is extended by dozens of other interfaces in the Java API, you can use a tagging interface to create a common parent among a group of interfaces. For example, when an interface extends EventListener, the JVM knows that this particular interface is going to be used in an event delegation scenario.
Adds a data type to a class: This situation is where the term tagging comes from. A class that implements a tagging interface does not need to define any methods (since the interface does not have any), but the class becomes an interface type through polymorphism.
[Java core 22] Encapsulation
Encapsulation is one of the four fundamental OOP concepts. The other three are inheritance, polymorphism, and abstraction.
Encapsulation is the technique of making the fields in a class private and providing access to the fields via public methods. If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. For this reason, encapsulation is also referred to as data hiding.
Encapsulation can be described as a protective barrier that prevents the code and data being randomly accessed by other code defined outside the class. Access to the data and code is tightly controlled by an interface.
The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this feature Encapsulation gives maintainability, flexibility and extensibility to our code.
Example:
Let us look at an example that depicts encapsulation:
--------------------------------------------
/* File name : EncapTest.java */
public class EncapTest{
private String name;
private String idNum;
private int age;
public int getAge(){
return age;
}
public String getName(){
return name;
}
public String getIdNum(){
return idNum;
}
public void setAge( int newAge){
age = newAge;
}
public void setName(String newName){
name = newName;
}
public void setIdNum( String newId){
idNum = newId;
}
}
-----------------------------------------------
The public methods are the access points to this class' fields from the outside java world. Normally, these methods are referred as getters and setters. Therefore any class that wants to access the variables should access them through these getters and setters.
The variables of the EncapTest class can be access as below::
----------------------------------------------
/* File name : RunEncap.java */
public class RunEncap{
public static void main(String args[]){
EncapTest encap = new EncapTest();
encap.setName("James");
encap.setAge(20);
encap.setIdNum("12343ms");
System.out.print("Name : " + encap.getName()+
" Age : "+ encap.getAge());
}
}
--------------------------------------------
This would produce the following result:
Name : James Age : 20
Benefits of Encapsulation:
The fields of a class can be made read-only or write-only.
A class can have total control over what is stored in its fields.
The users of a class do not know how the class stores its data. A class can change the data type of a field and users of the class do not need to change any of their code.
[Java core 21] Abstraction
Abstraction refers to the ability to make a class abstract in OOP. An abstract class is one that cannot be instantiated. All other functionality of the class still exists, and its fields, methods, and constructors are all accessed in the same manner. You just cannot create an instance of the abstract class.
If a class is abstract and cannot be instantiated, the class does not have much use unless it is subclass. This is typically how abstract classes come about during the design phase. A parent class contains the common functionality of a collection of child classes, but the parent class itself is too abstract to be used on its own.
Abstract Class:
Use the abstract keyword to declare a class abstract. The keyword appears in the class declaration somewhere before the class keyword.
--------------------------------------
/* File name : Employee.java */
public abstract class Employee
{
private String name;
private String address;
private int number;
public Employee(String name, String address, int number)
{
System.out.println("Constructing an Employee");
this.name = name;
this.address = address;
this.number = number;
}
public double computePay()
{
System.out.println("Inside Employee computePay");
return 0.0;
}
public void mailCheck()
{
System.out.println("Mailing a check to " + this.name
+ " " + this.address);
}
public String toString()
{
return name + " " + address + " " + number;
}
public String getName()
{
return name;
}
public String getAddress()
{
return address;
}
public void setAddress(String newAddress)
{
address = newAddress;
}
public int getNumber()
{
return number;
}
}
-------------------------------------------
Notice that nothing is different in this Employee class. The class is now abstract, but it still has three fields, seven methods, and one constructor.
Now if you would try as follows:
-------------------------------------------
/* File name : AbstractDemo.java */
public class AbstractDemo
{
public static void main(String [] args)
{
/* Following is not allowed and would raise error */
Employee e = new Employee("George W.", "Houston, TX", 43);
System.out.println("\n Call mailCheck using Employee reference--");
e.mailCheck();
}
}
-----------------------------------------------
When you would compile above class then you would get the following error:
Employee.java:46: Employee is abstract; cannot be instantiated
Employee e = new Employee("George W.", "Houston, TX", 43);
^
1 error
Extending Abstract Class:
We can extend Employee class in normal way as follows:
---------------------------------------
/* File name : Salary.java */
public class Salary extends Employee
{
private double salary; //Annual salary
public Salary(String name, String address, int number, double
salary)
{
super(name, address, number);
setSalary(salary);
}
public void mailCheck()
{
System.out.println("Within mailCheck of Salary class ");
System.out.println("Mailing check to " + getName()
+ " with salary " + salary);
}
public double getSalary()
{
return salary;
}
public void setSalary(double newSalary)
{
if(newSalary >= 0.0)
{
salary = newSalary;
}
}
public double computePay()
{
System.out.println("Computing salary pay for " + getName());
return salary/52;
}
}
------------------------------------------
Here, we cannot instantiate a new Employee, but if we instantiate a new Salary object, the Salary object will inherit the three fields and seven methods from Employee.
------------------------------------------
/* File name : AbstractDemo.java */
public class AbstractDemo
{
public static void main(String [] args)
{
Salary s = new Salary("Mohd Mohtashim", "Ambehta, UP", 3, 3600.00);
Employee e = new Salary("John Adams", "Boston, MA", 2, 2400.00);
System.out.println("Call mailCheck using Salary reference --");
s.mailCheck();
System.out.println("\n Call mailCheck using Employee reference--");
e.mailCheck();
}
}
---------------------------------------------
This would produce the following result:
Constructing an Employee
Constructing an Employee
Call mailCheck using Salary reference --
Within mailCheck of Salary class
Mailing check to Mohd Mohtashim with salary 3600.0
Call mailCheck using Employee reference--
Within mailCheck of Salary class
Mailing check to John Adams with salary 2400.
Abstract Methods:
If you want a class to contain a particular method but you want the actual implementation of that method to be determined by child classes, you can declare the method in the parent class as abstract.
The abstract keyword is also used to declare a method as abstract. An abstract method consists of a method signature, but no method body.
Abstract method would have no definition, and its signature is followed by a semicolon, not curly braces as follows:
------------------------------------------
public abstract class Employee
{
private String name;
private String address;
private int number;
public abstract double computePay();
//Remainder of class definition
}
------------------------------------------
Declaring a method as abstract has two results:
The class must also be declared abstract. If a class contains an abstract method, the class must be abstract as well.
Any child class must either override the abstract method or declare itself abstract.
A child class that inherits an abstract method must override it. If they do not, they must be abstract and any of their children must override it.
Eventually, a descendant class has to implement the abstract method; otherwise, you would have a hierarchy of abstract classes that cannot be instantiated.
If Salary is extending Employee class, then it is required to implement computePay() method as follows:
------------------------------------------------
/* File name : Salary.java */
public class Salary extends Employee
{
private double salary; // Annual salary
public double computePay()
{
System.out.println("Computing salary pay for " + getName());
return salary/52;
}
//Remainder of class definition
}
---------------------------------------------------
[Java core 20] Polymorphism
Polymorphism is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.
Any Java object that can pass more than one IS-A test is considered to be polymorphic. In Java, all Java objects are polymorphic since any object will pass the IS-A test for their own type and for the class Object.
It is important to know that the only possible way to access an object is through a reference variable. A reference variable can be of only one type. Once declared, the type of a reference variable cannot be changed.
The reference variable can be reassigned to other objects provided that it is not declared final. The type of the reference variable would determine the methods that it can invoke on the object.
A reference variable can refer to any object of its declared type or any subtype of its declared type. A reference variable can be declared as a class or interface type.
Example:
Let us look at an example.
public interface Vegetarian{...}
public class Animal{...}
public class Deer extends Animal implements Vegetarian{...}
Now, the Deer class is considered to be polymorphic since this has multiple inheritance. Following are true for the above example:
A Deer IS-A Animal
A Deer IS-A Vegetarian
A Deer IS-A Deer
A Deer IS-A Object
When we apply the reference variable facts to a Deer object reference, the following declarations are legal:
Deer d = new Deer();
Animal a = d;
Vegetarian v = d;
Object o = d;
All the reference variables d,a,v,o refer to the same Deer object in the heap.
Virtual Methods:
In this section, I will show you how the behavior of overridden methods in Java allows you to take advantage of polymorphism when designing your classes.
We already have discussed method overriding, where a child class can override a method in its parent. An overridden method is essentially hidden in the parent class, and is not invoked unless the child class uses the super keyword within the overriding method.
------------------------------------------
/* File name : Employee.java */
public class Employee
{
private String name;
private String address;
private int number;
public Employee(String name, String address, int number)
{
System.out.println("Constructing an Employee");
this.name = name;
this.address = address;
this.number = number;
}
public void mailCheck()
{
System.out.println("Mailing a check to " + this.name
+ " " + this.address);
}
public String toString()
{
return name + " " + address + " " + number;
}
public String getName()
{
return name;
}
public String getAddress()
{
return address;
}
public void setAddress(String newAddress)
{
address = newAddress;
}
public int getNumber()
{
return number;
}
}
-----------------------------------------
Now suppose we extend Employee class as follows:
----------------------------------------
/* File name : Salary.java */
public class Salary extends Employee
{
private double salary; //Annual salary
public Salary(String name, String address, int number, double
salary)
{
super(name, address, number);
setSalary(salary);
}
public void mailCheck()
{
System.out.println("Within mailCheck of Salary class ");
System.out.println("Mailing check to " + getName()
+ " with salary " + salary);
}
public double getSalary()
{
return salary;
}
public void setSalary(double newSalary)
{
if(newSalary >= 0.0)
{
salary = newSalary;
}
}
public double computePay()
{
System.out.println("Computing salary pay for " + getName());
return salary/52;
}
}
-----------------------------------------
Now, you study the following program carefully and try to determine its output:
------------------------------------------
/* File name : VirtualDemo.java */
public class VirtualDemo
{
public static void main(String [] args)
{
Salary s = new Salary("Mohd Mohtashim", "Ambehta, UP", 3, 3600.00);
Employee e = new Salary("John Adams", "Boston, MA", 2, 2400.00);
System.out.println("Call mailCheck using Salary reference --");
s.mailCheck();
System.out.println("\n Call mailCheck using Employee reference--");
e.mailCheck();
}
}
-----------------------------------------
This would produce the following result:
Constructing an Employee
Constructing an Employee
Call mailCheck using Salary reference --
Within mailCheck of Salary class
Mailing check to Mohd Mohtashim with salary 3600.0
Call mailCheck using Employee reference--
Within mailCheck of Salary class
Mailing check to John Adams with salary 2400.0
Here, we instantiate two Salary objects . one using a Salary reference s, and the other using an Employee reference e.
While invoking s.mailCheck() the compiler sees mailCheck() in the Salary class at compile time, and the JVM invokes mailCheck() in the Salary class at run time.
Invoking mailCheck() on e is quite different because e is an Employee reference. When the compiler sees e.mailCheck(), the compiler sees the mailCheck() method in the Employee class.
Here, at compile time, the compiler used mailCheck() in Employee to validate this statement. At run time, however, the JVM invokes mailCheck() in the Salary class.
This behavior is referred to as virtual method invocation, and the methods are referred to as virtual methods. All methods in Java behave in this manner, whereby an overridden method is invoked at run time, no matter what data type the reference is that was used in the source code at compile time.
[Java core 19] Overriding
In the previous chapter, we talked about super classes and sub classes. If a class inherits a method from its super class, then there is a chance to override the method provided that it is not marked final.
The benefit of overriding is: ability to define a behavior that's specific to the subclass type which means a subclass can implement a parent class method based on its requirement.
In object-oriented terms, overriding means to override the functionality of an existing method.
Example:
Let us look at an example.
--------------------------------
class Animal{
public void move(){
System.out.println("Animals can move");
}
}
class Dog extends Animal{
public void move(){
System.out.println("Dogs can walk and run");
}
}
public class TestDog{
public static void main(String args[]){
Animal a = new Animal(); // Animal reference and object
Animal b = new Dog(); // Animal reference but Dog object
a.move();// runs the method in Animal class
b.move();//Runs the method in Dog class
}
}
-------------------------------
This would produce the following result:
Animals can move
Dogs can walk and run
In the above example, you can see that the even though b is a type of Animal it runs the move method in the Dog class. The reason for this is: In compile time, the check is made on the reference type. However, in the runtime, JVM figures out the object type and would run the method that belongs to that particular object.
Therefore, in the above example, the program will compile properly since Animal class has the method move. Then, at the runtime, it runs the method specific for that object.
Consider the following example :
--------------------------------------
class Animal{
public void move(){
System.out.println("Animals can move");
}
}
class Dog extends Animal{
public void move(){
System.out.println("Dogs can walk and run");
}
public void bark(){
System.out.println("Dogs can bark");
}
}
public class TestDog{
public static void main(String args[]){
Animal a = new Animal(); // Animal reference and object
Animal b = new Dog(); // Animal reference but Dog object
a.move();// runs the method in Animal class
b.move();//Runs the method in Dog class
b.bark();
}
}
---------------------------------------
This would produce the following result:
TestDog.java:30: cannot find symbol
symbol : method bark()
location: class Animal
b.bark();
^
This program will throw a compile time error since b's reference type Animal doesn't have a method by the name of bark.
Rules for method overriding:
The argument list should be exactly the same as that of the overridden method.
The return type should be the same or a subtype of the return type declared in the original overridden method in the superclass.
The access level cannot be more restrictive than the overridden method's access level. For example: if the superclass method is declared public then the overridding method in the sub class cannot be either private or protected.
Instance methods can be overridden only if they are inherited by the subclass.
A method declared final cannot be overridden.
A method declared static cannot be overridden but can be re-declared.
If a method cannot be inherited, then it cannot be overridden.
A subclass within the same package as the instance's superclass can override any superclass method that is not declared private or final.
A subclass in a different package can only override the non-final methods declared public or protected.
An overriding method can throw any uncheck exceptions, regardless of whether the overridden method throws exceptions or not. However the overriding method should not throw checked exceptions that are new or broader than the ones declared by the overridden method. The overriding method can throw narrower or fewer exceptions than the overridden method.
Constructors cannot be overridden.
Using the super keyword:
When invoking a superclass version of an overridden method the super keyword is used.
-------------------------------------------
class Animal{
public void move(){
System.out.println("Animals can move");
}
}
class Dog extends Animal{
public void move(){
super.move(); // invokes the super class method
System.out.println("Dogs can walk and run");
}
}
public class TestDog{
public static void main(String args[]){
Animal b = new Dog(); // Animal reference but Dog object
b.move(); //Runs the method in Dog class
}
}
-------------------------------------
This would produce the following result:
Animals can move
Dogs can walk and run
[Java core 18] Inheritance
Inheritance can be defined as the process where one object acquires the properties of another. With the use of inheritance the information is made manageable in a hierarchical order.
When we talk about inheritance, the most commonly used keyword would be extends and implements. These words would determine whether one object IS-A type of another. By using these keywords we can make one object acquire the properties of another object.
1 - IS-A Relationship:
IS-A is a way of saying : This object is a type of that object. Let us see how the extends keyword is used to achieve inheritance.
public class Animal{
...
}
public class Mammal extends Animal{
...
}
public class Reptile extends Animal{
...
}
public class Dog extends Mammal{
...
}
Now, based on the above example, In Object Oriented terms, the following are true:
- Animal is the superclass of Mammal class.
- Animal is the superclass of Reptile class.
- Mammal and Reptile are subclasses of Animal class.
- Dog is the subclass of both Mammal and Animal classes.
Now, if we consider the IS-A relationship, we can say:
- Mammal IS-A Animal
- Reptile IS-A Animal
- Dog IS-A Mammal
- Hence : Dog IS-A Animal as well
With use of the extends keyword the subclasses will be able to inherit all the properties of the superclass except for the private properties of the superclass.
We can assure that Mammal is actually an Animal with the use of the instance operator.
Example:
-----------------------------------
public class Dog extends Mammal{
public static void main(String args[]){
Animal a = new Animal();
Mammal m = new Mammal();
Dog d = new Dog();
System.out.println(m instanceof Animal);
System.out.println(d instanceof Mammal);
System.out.println(d instanceof Animal);
}
}
--------------------------------------
This would produce the following result:
true
true
true
Since we have a good understanding of the extends keyword let us look into how the implements keyword is used to get the IS-A relationship.
The implements keyword is used by classes by inherit from interfaces. Interfaces can never be extended by the classes.
Example:
-----------------------------------------
public interface Animal {}
public class Mammal implements Animal{
...
}
public class Dog extends Mammal{
...
}
-------------------------------------------
2 - The instanceof Keyword:
Let us use the instanceof operator to check determine whether Mammal is actually an Animal, and dog is actually an Animal
-----------------------------------------
interface Animal{ ... }
class Mammal implements Animal{ ... }
public class Dog extends Mammal{
public static void main(String args[]){
Mammal m = new Mammal();
Dog d = new Dog();
System.out.println(m instanceof Animal);
System.out.println(d instanceof Mammal);
System.out.println(d instanceof Animal);
}
}
------------------------------------------
This would produce the following result:
true
true
true
3- HAS-A relationship:
These relationships are mainly based on the usage. This determines whether a certain class HAS-A certain thing. This relationship helps to reduce duplication of code as well as bugs.
Lets us look into an example:
-----------------------------------------------
public class Vehicle{ ... }
public class Speed{ ... }
public class Van extends Vehicle{
private Speed sp;
}
-----------------------------------------------
This shows that class Van HAS-A Speed. By having a separate class for Speed, we do not have to put the entire code that belongs to speed inside the Van class., which makes it possible to reuse the Speed class in multiple applications.
In Object-Oriented feature, the users do not need to bother about which object is doing the real work. To achieve this, the Van class hides the implementation details from the users of the Van class. So basically what happens is the users would ask the Van class to do a certain action and the Van class will either do the work by itself or ask another class to perform the action.
A very important fact to remember is that Java only supports only single inheritance. This means that a class cannot extend more than one class. Therefore following is illegal:
public class extends Animal, Mammal{ ... }
However, a class can implement one or more interfaces. This has made Java get rid of the impossibility of multiple inheritance.
[Java core 17] Exceptions handling
An exception is a problem that arises during the execution of a program. An exception can occur for many different reasons, including the following:
- A user has entered invalid data.
- A file that needs to be opened cannot be found.
- A network connection has been lost in the middle of communications or the JVM has run out of memory.
- Some of these exceptions are caused by user error, others by programmer error, and others by physical resources that have failed in some manner.
1 - To understand how exception handling works in Java, you need to understand the three categories of exceptions:
Checked exceptions: A checked exception is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation.
Runtime exceptions: A runtime exception is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compilation.
Errors: These are not exceptions at all, but problems that arise beyond the control of the user or the programmer. Errors are typically ignored in your code because you can rarely do anything about an error. For example, if a stack overflow occurs, an error will arise. They are also ignored at the time of compilation.
2 - Exception Hierarchy:
All exception classes are subtypes of the java.lang.Exception class. The exception class is a subclass of the Throwable class. Other than the exception class there is another subclass called Error which is derived from the Throwable class.
Errors are not normally trapped form the Java programs. These conditions normally happen in case of severe failures, which are not handled by the java programs. Errors are generated to indicate errors generated by the runtime environment. Example : JVM is out of Memory. Normally programs cannot recover from errors.
The Exception class has two main subclasses: IOException class and RuntimeException Class.
3 - Java Exceptions
Here is a list of most common checked and unchecked Java's Built-in Exceptions.
Exceptions Methods:
Following is the list of important medthods available in the Throwable class.
1) public String getMessage() Returns a detailed message about the exception that has occurred. This message is initialized in the Throwable constructor.
2) public Throwable getCause() Returns the cause of the exception as represented by a Throwable object.
3) public String toString() Returns the name of the class concatenated with the result of getMessage()
4) public void printStackTrace() Prints the result of toString() along with the stack trace to System.err, the error output stream.
5) public StackTraceElement [] getStackTrace() Returns an array containing each element on the stack trace. The element at index 0 represents the top of the call stack, and the last element in the array represents the method at the bottom of the call stack.
6) public Throwable fillInStackTrace() Fills the stack trace of this Throwable object with the current stack trace, adding to any previous information in the stack trace.
4- Catching Exceptions:
A method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following:
try
{
//Protected code
}catch(ExceptionName e1)
{
//Catch block
}
A catch statement involves declaring the type of exception you are trying to catch. If an exception occurs in protected code, the catch block (or blocks) that follows the try is checked. If the type of exception that occurred is listed in a catch block, the exception is passed to the catch block much as an argument is passed into a method parameter.
Example:
The following is an array is declared with 2 elements. Then the code tries to access the 3rd element of the array which throws an exception.
--------------------------------------
// File Name : ExcepTest.java
import java.io.*;
public class ExcepTest{
public static void main(String args[]){
try{
int a[] = new int[2];
System.out.println("Access element three :" + a[3]);
}catch(ArrayIndexOutOfBoundsException e){
System.out.println("Exception thrown :" + e);
}
System.out.println("Out of the block");
}
}
--------------------------------------------
This would produce the following result:
Exception thrown :java.lang.ArrayIndexOutOfBoundsException: 3
Out of the block
Multiple catch Blocks:
A try block can be followed by multiple catch blocks. The syntax for multiple catch blocks looks like the following:
try
{
//Protected code
}catch(ExceptionType1 e1)
{
//Catch block
}catch(ExceptionType2 e2)
{
//Catch block
}catch(ExceptionType3 e3)
{
//Catch block
}
The previous statements demonstrate three catch blocks, but you can have any number of them after a single try. If an exception occurs in the protected code, the exception is thrown to the first catch block in the list. If the data type of the exception thrown matches ExceptionType1, it gets caught there. If not, the exception passes down to the second catch statement. This continues until the exception either is caught or falls through all catches, in which case the current method stops execution and the exception is thrown down to the previous method on the call stack.
Example:
Here is code segment showing how to use multiple try/catch statements.
--------------------------------------
try
{
file = new FileInputStream(fileName);
x = (byte) file.read();
}catch(IOException i)
{
i.printStackTrace();
return -1;
}catch(FileNotFoundException f) //Not valid!
{
f.printStackTrace();
return -1;
}
---------------------------------------
5- The throws/throw Keywords:
If a method does not handle a checked exception, the method must declare it using the throws keyword. The throws keyword appears at the end of a method's signature.
You can throw an exception, either a newly instantiated one or an exception that you just caught, by using the throw keyword. Try to understand the different in throws and throw keywords.
The following method declares that it throws a RemoteException:
-----------------------------------------
import java.io.*;
public class className
{
public void deposit(double amount) throws RemoteException
{
// Method implementation
throw new RemoteException();
}
//Remainder of class definition
}
-----------------------------------------
A method can declare that it throws more than one exception, in which case the exceptions are declared in a list separated by commas. For example, the following method declares that it throws a RemoteException and an InsufficientFundsException:
----------------------------------------
import java.io.*;
public class className
{
public void withdraw(double amount) throws RemoteException,
InsufficientFundsException
{
// Method implementation
}
//Remainder of class definition
}
------------------------------------
6 - The finally Keyword
The finally keyword is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred.
Using a finally block allows you to run any cleanup-type statements that you want to execute, no matter what happens in the protected code.
A finally block appears at the end of the catch blocks and has the following syntax:
try
{
//Protected code
}catch(ExceptionType1 e1)
{
//Catch block
}catch(ExceptionType2 e2)
{
//Catch block
}catch(ExceptionType3 e3)
{
//Catch block
}finally
{
//The finally block always executes.
}
Example:
---------------------------------------------------
public class ExcepTest{
public static void main(String args[]){
int a[] = new int[2];
try{
System.out.println("Access element three :" + a[3]);
}catch(ArrayIndexOutOfBoundsException e){
System.out.println("Exception thrown :" + e);
}
finally{
a[0] = 6;
System.out.println("First element value: " +a[0]);
System.out.println("The finally statement is executed");
}
}
}
------------------------------------------------------
This would produce the following result:
Exception thrown :java.lang.ArrayIndexOutOfBoundsException: 3
First element value: 6
The finally statement is executed
7- Note the following:
A catch clause cannot exist without a try statement.
It is not compulsory to have finally clauses when ever a try/catch block is present.
The try block cannot be present without either catch clause or finally clause.
Any code cannot be present in between the try, catch, finally blocks.
8- Declaring you own Exception:
You can create your own exceptions in Java. Keep the following points in mind when writing your own exception classes:
All exceptions must be a child of Throwable.
If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class.
If you want to write a runtime exception, you need to extend the RuntimeException class.
We can define our own Exception class as below:
class MyException extends Exception{
}
You just need to extend the Exception class to create your own Exception class. These are considered to be checked exceptions. The following InsufficientFundsException class is a user-defined exception that extends the Exception class, making it a checked exception. An exception class is like any other class, containing useful fields and methods.
Example:
------------------------------------------------------------
// File Name InsufficientFundsException.java
import java.io.*;
public class InsufficientFundsException extends Exception
{
private double amount;
public InsufficientFundsException(double amount)
{
this.amount = amount;
}
public double getAmount()
{
return amount;
}
}
--------------------------------------------------------------
To demonstrate using our user-defined exception, the following CheckingAccount class contains a withdraw() method that throws an InsufficientFundsException.
--------------------------------------------------------------
// File Name CheckingAccount.java
import java.io.*;
public class CheckingAccount
{
private double balance;
private int number;
public CheckingAccount(int number)
{
this.number = number;
}
public void deposit(double amount)
{
balance += amount;
}
public void withdraw(double amount) throws
InsufficientFundsException
{
if(amount <= balance)
{
balance -= amount;
}
else
{
double needs = amount - balance;
throw new InsufficientFundsException(needs);
}
}
public double getBalance()
{
return balance;
}
public int getNumber()
{
return number;
}
}
--------------------------------------------------------------------
The following BankDemo program demonstrates invoking the deposit() and withdraw() methods of CheckingAccount.
-------------------------------------------------------------------
// File Name BankDemo.java
public class BankDemo
{
public static void main(String [] args)
{
CheckingAccount c = new CheckingAccount(101);
System.out.println("Depositing $500...");
c.deposit(500.00);
try
{
System.out.println("\nWithdrawing $100...");
c.withdraw(100.00);
System.out.println("\nWithdrawing $600...");
c.withdraw(600.00);
}catch(InsufficientFundsException e)
{
System.out.println("Sorry, but you are short $"
+ e.getAmount());
e.printStackTrace();
}
}
}
----------------------------------------------------------------------
Compile all the above three files and run BankDemo, this would produce the following result:
Depositing $500...
Withdrawing $100...
ithdrawing $600...
Sorry, but you are short $200.0
InsufficientFundsException
at CheckingAccount.withdraw(CheckingAccount.java:25)
at BankDemo.main(BankDemo.java:13)
9- Common Exceptions:
In Java, it is possible to define two catergories of Exceptions and Errors.
JVM Exceptions: - These are exceptions/errors that are exclusively or logically thrown by the JVM. Examples : NullPointerException, ArrayIndexOutOfBoundsException, ClassCastException,
Programmatic exceptions: - These exceptions are thrown explicitly by the application or the API programmers Examples: IllegalArgumentException, IllegalStateException.
[Java core 16] Streams files and IO
The java.io package contains nearly every class you might ever need to perform input and output (I/O) in Java. All these streams represent an input source and an output destination. The stream in the java.io package supports many data such as primitives, Object, localized characters, etc.
A stream can be defined as a sequence of data. The InputStream is used to read data from a source and the OutputStream is used for writing data to a destination.
Java provides strong but flexible support for I/O related to Files and networks but this tutorial covers very basic functionality related to streams and I/O. We would see most commonly used example one by one:
1 - Byte Streams
Java byte streams are used to perform input and output of 8-bit bytes. Though there are many classes related to byte streams but the most frequently used classes are , FileInputStream and FileOutputStream. Following is an example which makes use of these two classes to copy an input file into an output file:
Example:
------------------------------------------
import java.io.*;
public class CopyFile {
public static void main(String args[]) throws IOException
{
FileInputStream in = null;
FileOutputStream out = null;
try {
in = new FileInputStream("input.txt");
out = new FileOutputStream("output.txt");
int c;
while ((c = in.read()) != -1) {
out.write(c);
}
}finally {
if (in != null) {
in.close();
}
if (out != null) {
out.close();
}
}
}
}
-------------------------------------
Now let's have a file input.txt with the following content:
Java Programming - http://java-programming-tdu.blogspot.com/
- This is test for copy file.
As a next step, compile above program and execute it, which will result in creating output.txt file with the same content as we have in input.txt. So let's put above code in CopyFile.java file.
2- Character Streams
Java Byte streams are used to perform input and output of 8-bit bytes, where as Java Character streams are used to perform input and output for 16-bit unicode. Though there are many classes related to character streams but the most frequently used classes are , FileReader and FileWriter.. Though internally FileReader uses FileInputStream and FileWriter uses FileOutputStream but here major difference is that FileReader reads two bytes at a time and FileWriter writes two bytes at a time.
We can re-write above example which makes use of these two classes to copy an input file (having unicode characters) into an output file:
Example
-----------------------------
import java.io.*;
public class CopyFile {
public static void main(String args[]) throws IOException
{
FileReader in = null;
FileWriter out = null;
try {
in = new FileReader("input.txt");
out = new FileWriter("output.txt");
int c;
while ((c = in.read()) != -1) {
out.write(c);
}
}finally {
if (in != null) {
in.close();
}
if (out != null) {
out.close();
}
}
}
}
--------------------------------
Now let's have a file input.txt with the following content:
Java Programming - http://java-programming-tdu.blogspot.com/
- This is test for copy file.
As a next step, compile above program and execute it, which will result in creating output.txt file with the same content as we have in input.txt. So let's put above code in CopyFile.java file.
3 - Standard Streams
All the programming languages provide support for standard I/O where user's program can take input from a keyboard and then produce output on the computer screen. If you are aware if C or C++ programming languages, then you must be aware of three standard devices STDIN, STDOUT and STDERR. Similar way Java provides following three standard streams
Standard Input: This is used to feed the data to user's program and usually a keyboard is used as standard input stream and represented as System.in.
Standard Output: This is used to output the data produced by the user's program and usually a computer screen is used to standard output stream and represented as System.out.
Standard Error: This is used to output the error data produced by the user's program and usually a computer screen is used to standard error stream and represented as System.err.
Following is a simple program which creates InputStreamReader to read standard input stream until the user types a "q":
Example------------------------------
import java.io.*;
public class ReadConsole {
public static void main(String args[]) throws IOException
{
InputStreamReader cin = null;
try {
cin = new InputStreamReader(System.in);
System.out.println("Enter characters, 'q' to quit.");
char c;
do {
c = (char) cin.read();
System.out.print(c);
} while(c != 'q');
}finally {
if (cin != null) {
cin.close();
}
}
}
}
--------------------------------
Let's keep above code in ReadConsole.java file and try to compile and execute it as below. This program continues reading and outputting same character until we press 'q':
Enter characters, 'q' to quit.
1
1
e
e
q
q
4 - Reading and Writing Files
As described earlier, A stream can be defined as a sequence of data. The InputStream is used to read data from a source and the OutputStream is used for writing data to a destination.
Here is a hierarchy of classes to deal with Input and Output streams.
+ Java I/O Streams
The two important streams are FileInputStream and FileOutputStream, which would be discussed in this tutorial:
- FileInputStream:
This stream is used for reading data from the files. Objects can be created using the keyword new and there are several types of constructors available.
Following constructor takes a file name as a string to create an input stream object to read the file.:
InputStream f = new FileInputStream("C:/java/hello");
Following constructor takes a file object to create an input stream object to read the file. First we create a file object using File() method as follows:
File f = new File("C:/java/hello");
InputStream f = new FileInputStream(f);
Once you have InputStream object in hand, then there is a list of helper methods which can be used to read to stream or to do other operations on the stream.
1) public void close() throws IOException{} This method closes the file output stream. Releases any system resources associated with the file. Throws an IOException.
2) protected void finalize()throws IOException {} This method cleans up the connection to the file. Ensures that the close method of this file output stream is called when there are no more references to this stream. Throws an IOException.
3) public int read(int r)throws IOException{} This method reads the specified byte of data from the InputStream. Returns an int. Returns the next byte of data and -1 will be returned if it's end of file.
4) public int read(byte[] r) throws IOException{} This method reads r.length bytes from the input stream into an array. Returns the total number of bytes read. If end of file -1 will be returned.
5 ) public int available() throws IOException{} Gives the number of bytes that can be read from this file input stream. Returns an int.
There are other important input streams available, for more detail you can refer to the following links:
ByteArrayInputStream
DataInputStream
- FileOutputStream:
FileOutputStream is used to create a file and write data into it. The stream would create a file, if it doesn't already exist, before opening it for output.
Here are two constructors which can be used to create a FileOutputStream object.
Following constructor takes a file name as a string to create an input stream object to write the file:
OutputStream f = new FileOutputStream("C:/java/hello")
Following constructor takes a file object to create an output stream object to write the file. First, we create a file object using File() method as follows:
Example:
File f = new File("C:/java/hello");
OutputStream f = new FileOutputStream(f);
Once you have OutputStream object in hand, then there is a list of helper methods, which can be used to write to stream or to do other operations on the stream.
1) public void close() throws IOException{} This method closes the file output stream. Releases any system resources associated with the file. Throws an IOException.
2) protected void finalize()throws IOException {} This method cleans up the connection to the file. Ensures that the close method of this file output stream is called when there are no more references to this stream. Throws an IOException.
3) public void write(int w)throws IOException{} This methods writes the specified byte to the output stream.
4 ) public void write(byte[] w) Writes w.length bytes from the mentioned byte array to the OutputStream.
There are other important output streams available, for more detail you can refer to the following links:
ByteArrayOutputStream
DataOutputStream
Example:
Following is the example to demonstrate InputStream and OutputStream:
----------------------------------------
import java.io.*;
public class fileStreamTest{
public static void main(String args[]){
try{
byte bWrite [] = {11,21,3,40,5};
OutputStream os = new FileOutputStream("test.txt");
for(int x=0; x < bWrite.length ; x++){
os.write( bWrite[x] ); // writes the bytes
}
os.close();
InputStream is = new FileInputStream("test.txt");
int size = is.available();
for(int i=0; i< size; i++){
System.out.print((char)is.read() + " ");
}
is.close();
}catch(IOException e){
System.out.print("Exception");
}
}
}
--------------------------------------
The above code would create file test.txt and would write given numbers in binary format. Same would be output on the stdout screen.
- File Navigation and I/O:
There are several other classes that we would be going through to get to know the basics of File Navigation and I/O.
File Class
FileReader Class
FileWriter Class
- Directories in Java:
A directory is a File which can contains a list of other files and directories. You use File object to create directories, to list down files available in a directory. For complete detail check a list of all the methods which you can call on File object and what are related to directories.
- Creating Directories:
There are two useful File utility methods, which can be used to create directories:
The mkdir( ) method creates a directory, returning true on success and false on failure. Failure indicates that the path specified in the File object already exists, or that the directory cannot be created because the entire path does not exist yet.
The mkdirs() method creates both a directory and all the parents of the directory.
Following example creates "/tmp/user/java/bin" directory:
-------------------------------------------------
import java.io.File;
public class CreateDir {
public static void main(String args[]) {
String dirname = "/tmp/user/java/bin";
File d = new File(dirname);
// Create directory now.
d.mkdirs();
}
}
-----------------------------------
Compile and execute above code to create "/tmp/user/java/bin".
Note: Java automatically takes care of path separators on UNIX and Windows as per conventions. If you use a forward slash (/) on a Windows version of Java, the path will still resolve correctly.
Listing Directories:
You can use list( ) method provided by File object to list down all the files and directories available in a directory as follows:
Example------------------------------
import java.io.File;
public class ReadDir {
public static void main(String[] args) {
File file = null;
String[] paths;
try{
// create new file object
file = new File("/tmp");
// array of files and directory
paths = file.list();
// for each name in the path array
for(String path:paths)
{
// prints filename and directory name
System.out.println(path);
}
}catch(Exception e){
// if any error occurs
e.printStackTrace();
}
}
}
---------------------------------------
This would produce following result based on the directories and files available in your /tmp directory:
test1.txt
test2.txt
ReadDir.java
ReadDir.class