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Showing posts with label Java Object Oriented. Show all posts
Showing posts with label Java Object Oriented. Show all posts

Saturday, June 6, 2015

1. Java - Inheritance

5:54 AM Unknown No comments
Java Tutorial
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.

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{
}

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

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.

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2. Java - Overriding

5:53 AM Unknown No comments
Java Tutorial

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


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3. Java - Polymorphism

5:49 AM Unknown No comments
Java Tutorial 


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.


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4. Java - Abstraction

5:48 AM Unknown No comments
Java Tutorial

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
}


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5. Java - Encapsulation

5:45 AM Unknown No comments
Java Tutorial

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.
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6. Java - Interfaces

5:44 AM Unknown No comments
Java Tutorial
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.

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7. Java - Packages

5:41 AM Unknown No comments
Java Tutorial
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: 
\sources\com\apple\computers\Dell.java

\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, \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 \classes is the class path, and the package name is com.apple.computers, then the compiler and JVM will look for .class files in \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

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