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Abstract class and Interface in Java

  1. Introduction
  2. Abstract Class
  3. Abstract class, the examples
  4. Overview of the interface
  5. Structure of an Interface
  6. Class implements Interface

1. Introduction

In this document, I will instruct you in Interface and Abstract Class, also analyze the similarities and differences between them.

2. Abstract Class

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Abstract class. for example:
// This is an abstract class.
// It must be declared as abstract because it has an abstract method.
public abstract class ClassA  { 
	// This is an abstract method
	// It has no body.
	// Its access modifier is public
	public abstract void doSomething(); 
	// The access modifier of this method is protected.
	protected abstract String doNothing();
	// This method does not declare access modifier
        // It has the default access modifier.
	abstract void todo() ;
} 
// This is an abstract class.
// It is declared as abstract, 
// though it does not have any abstract methods.
public abstract class ClassB   {

}
Characteristics of an abstract class is:
  1. It is declared abstract.
  2. It can declare 0, 1 or more of inside abstract methods.
  3. Unable to initialize one object directly from an abstract class.

3. Abstract class, the examples

See illustration:
AbstractJob.java
package org.o7planning.tutorial.abs;

// An abstract class (Simulating a job). 
// There are two methods declared abstract.
public abstract class AbstractJob { 
	public AbstractJob() {

	} 
	// This is an abstract method.
	// return Job Name.
	public abstract String getJobName(); 
	// This is an abstract method.
	public abstract void doJob(); 
}
JavaCoding.java
package org.o7planning.tutorial.abs;

// Class implements all abstract methods of the parent class.
public class JavaCoding extends AbstractJob { 
	public JavaCoding() {
	} 
	// Implements abstract method of parent class.
	@Override
	public void doJob() {
		System.out.println("Coding Java...");
	} 
	// Implements abstract method of parent.
	// Method with body.
	// Return name of Job. 
	@Override
	public String getJobName() {
		return "Coding Java";
	} 
}
ManualJob.java
package org.o7planning.tutorial.abs;

// AbstractJob class has two abstract methods.
// This class implements only one abstract method of parent class. 
// Therefore it must be declared as abstract.
public abstract class ManualJob extends AbstractJob { 
	public ManualJob() {

	} 
	// Implements abstract method of parent class.
	@Override
	public String getJobName() {
		return "Manual Job";
	} 
}
BuildHouse.java
package org.o7planning.tutorial.abs;

// This class inherited from ManualJob
// BuildHouse is not declared as abstract.
// so it must implements all remaining abstract methods. 
public class BuildHouse extends ManualJob { 
	public BuildHouse() {

	} 
	// Implements abstract methods of the parent class. 
	@Override
	public void doJob() {
		System.out.println("Build a House");
	} 
}
Demo
JobDemo.java
package org.o7planning.tutorial.abs;

public class JobDemo { 
	public static void main(String[] args) { 
		// Create AbstractJob object
		// from Constructor of JavaCoding.
		AbstractJob job1 = new JavaCoding();

		// Call doJob() method.
		job1.doJob();

		// getJobName is the abstract method in AbstractJob class.
		// But it was implemented in a certain subclass (JavaCoding).
		// So can call it.
		String jobName = job1.getJobName(); 
		System.out.println("Job Name 1= " + jobName);

		// Create AbstractJob object
		// from constructor of BuildHouse. 
		AbstractJob job2 = new BuildHouse(); 
		job2.doJob(); 
		String jobName2 = job2.getJobName(); 
		System.out.println("Job Name 2= " + jobName2); 
	}
}
Running the example:
Coding Java...
Job Name 1= Coding Java
Build a House
Job Name 2= Manual Job

4. Overview of the interface

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We know that a class can extends from only one parent class.
// B is subclass of class A, or in other words B is extended from A.
// Java only allows a class to extends from only one another class.
public class B extends A  {
     // ....
} 
// In the absence of specifying class B extends from a particular class.
// By default, understand that B extends from the Object class.
public class B  {

} 
// This declaration, and the above is equivalent.
public class B extends Object  {

}
But a class can extend from multiple Interfaces.
// A class can extends from only one parent class.
// But can implements (extends) from multiple Interfaces.
public class Cat extends Animal implements CanEat, CanDrink { 
     // ....
}
Các đặc điểm của interface
  • Interface always has modifier: public interface, whether you declare clearly or not.
  • If there are fields, they are: public static final, whether you declare clearly or not.
  • Its methods are abstract method, ie without function body, and contains modifier is: public abstract, whether you declare or not.
  • Interface have no Constructor.

5. Structure of an Interface

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NoAccessModifierInterface.java
package org.o7planning.tutorial.itf;

// This is an interface not specified access modifier.
// Its access modifier is default.
// Only classes in the same package can implements this interface. 
interface NoAccessModifierInterface {

}
CanMove.java
package org.o7planning.tutorial.itf;

// This interface defines a standard, 
// about things capable of moving.
public interface CanMove { 
	// The methods in Interface are always abstract and public.
	public abstract void run();

	// Even if you do not specify 'public abstract', java always understands.
	void back();

	// Velocity.
	public int getVelocity(); 
}
CanDrink.java
package org.o7planning.tutorial.itf;

// This interface defines a standard, 
// about things capable of drinking.
public interface CanDrink { 
	// Fields in Interface are always 'public static final'.
	// Whether you declare it or not.
	public static final String PEPSI = "PEPSI";
	final String NUMBER_ONE = "NUMBER ONE";
	String SEVENUP = "SEVEN UP";

	public void drink(); 
}
CanEat.java
package org.o7planning.tutorial.itf;

// This interface defines a standard, 
// about things capable of eating.
public interface CanEat { 
	public void eat();
}

6. Class implements Interface

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Animal.java
package org.o7planning.tutorial.cls;

import org.o7planning.tutorial.itf.CanMove;

// This class extends from Object.
// And implements CanMove interface.
// CanMove has 3 abstract methods.
// This class implements only one abstract method of CanMove.
// Therefore it must be declared as abstract.
// The remaining abstract methods to be implemented in the subclasses. 
public abstract class Animal implements CanMove { 
	// Implements run() method of CanMove.
	@Override
	public void run() {
		System.out.println("Animal run...");
	} 
}
Cat.java
package org.o7planning.tutorial.cls;

import org.o7planning.tutorial.itf.CanDrink;
import org.o7planning.tutorial.itf.CanEat;

// This class extends Animal and implements CanEat, CanDrink interfaces.
// This class is not declared as abstract.
// So it must implements all abstract methods of the interfaces. 
public class Cat extends Animal implements CanEat, CanDrink { 
	private String name;

	public Cat(String name) {
		this.name = name;
	} 
	public String getName() {
		return this.name;
	} 
	// Implements method of CanMove
	@Override
	public void back() {
		System.out.println(name + " cat back ...");
	} 
	// Implements method of CanMove.
	@Override
	public int getVelocity() {
		return 110;
	} 
	// Implements method of CanEat.
	@Override
	public void eat() {
		System.out.println(name + " cat eat ...");
	} 
	// Implements method of CanDrink.
	@Override
	public void drink() {
		System.out.println(name + " cat drink ...");
	} 
}
Mouse.java
package org.o7planning.tutorial.cls;

import org.o7planning.tutorial.itf.CanDrink;
import org.o7planning.tutorial.itf.CanEat;

public class Mouse extends Animal  implements CanEat, CanDrink {
  @Override
  public void back() {
      System.out.println("Mouse back ...");
  }
  @Override
  public int getVelocity() {
      return 85;
  }
  @Override
  public void drink() {
      System.out.println("Mouse drink ...");
  }
  @Override
  public void eat() {
      System.out.println("Mouse eat ...");
  }
}
AnimalDemo.java
package org.o7planning.tutorial.cls;

import org.o7planning.tutorial.itf.CanEat;

public class AnimalDemo { 
	public static void main(String[] args) { 
		// Inherit static field from CanDrink interface.
		System.out.println("Drink " + Cat.SEVENUP); 
		// Create CanEat object
		// via constructor of Cat. 
		CanEat canEat1 = new Cat("Tom");

		// An object declared as CanEat.
		// But in fact is Mouse.
		CanEat canEat2 = new Mouse();

		// Polymorphism shown here.
		// Java know the actual types of objects.
		// ==> Tom cat eat ...
		canEat1.eat();
		// ==> Mouse eat ...
		canEat2.eat(); 
		boolean isCat = canEat1 instanceof Cat; 
		System.out.println("catEat1 is Cat? " + isCat);

		// Cast
		if (canEat2 instanceof Mouse) {
			Mouse mouse = (Mouse) canEat2; 
			// Call drink method (Inherited from CanDrink).
			mouse.drink();
		} 
	}
}
Running the example:
Drink SEVEN UP
Tom cat eat ...
Mouse eat ...
catEat1 is Cat? true
Mouse drink ...
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