Explain the meaning of abstract keyword in relation to classes and methods?

Answer 1

The abstract keyword is used to denote abstract classes and abstract methods in Java.

An abstract method is a method which is defined, but not implemented:

public abstract void doStuff();

An abstract class is a class which contains one or more abstract methods*, and which cannot be instantiated directly. All subclasses of an abstract class must implement the abstract methods.

* Note that abstract classes can include no abstract methods, but this rather defeats the purpose of using an abstract class.

The quintessential example of an abstract class is the Shape class.

// Definition of our abstract class

public abstract class Shape {

// Notice how we can actually declare and implement variables and methods.

// This is what differentiates between an abstract class and an interface.

// The location of this shape

private double x,y;

// Change our location - all subclasses will have this by default

public void moveTo(final double newX, final double newY) {

x = newX;

y = newY;

}

// Definitions of our abstract classes.

// All classes which extend from Shape must implement these.

public abstract double getArea();

public abstract double getPerimiter();

}

// Definition of our concrete example class

public class Rectangle extends Shape {

// Beyond the x,y location of Shape, Rectangle must have width and height

private double width, height;

// Implement abstract methods

public double getArea() {

return width * height;

}

public double getPerimiter() {

return width + width + height + height;

}

}

Answer 2

Abstract Methods

An abstract method is a method that's been declared as abstract but not implemented. In other words, the method contains no code. You mark a method abstract when you want to force subclasses to provide the implementation.

Ex: public abstract void showSample();

Notice that the abstract method ends with a semicolon instead of curly braces. It is illegal to have even a single abstract method in a class that is not explicitly declared abstract! Look at the following illegal class:

public class IllegalAbstractClass{

public abstract void test();

}

The preceding class will produce the following error if you try to compile it:

IllegalClass.java:1: class IllegalAbstractClass must be declared

abstract.

It does not define void test() from class IllegalAbstractClass.

public class IllegalAbstractClass {

1 error

You can, however, have an abstract class with no abstract methods. The following example will compile fine:

public abstract class LegalAbstractClass{

void test() {

// you can write lots of code here

}

}

In the preceding example, test() is not abstract. Three different clues tell you it's not an abstract method:

• The method is not marked abstract.

• The method declaration includes curly braces, as opposed to ending in a semicolon. In other words, the method has a method body.

• The method contains actual implementation code.

Any class that extends an abstract class must implement all abstract methods of the superclass, unless the subclass is also abstract. The rule is this:

The first concrete subclass of an abstract class must implement all abstract methods of the superclass.

Concrete just means nonabstract, so if you have an abstract class extending another abstract class, the abstract subclass doesn't need to provide implementations for the inherited abstract methods. Sooner or later, though, somebody's going to make a nonabstract subclass (in other words, a class that can be instantiated), and that subclass will have to implement all the abstract methods from up the inheritance tree.