abstract methods

Learn abstract methods.

abstract methods

An abstract method is a method declaration without a body. It starts with the abstract keyword; the subclass must write its body.

Example: abstract void sound();. Here sound has a signature; no body. It is declared inside an abstract class. A subclass overrides this method and writes its body. Otherwise the subclass itself must be abstract. This mechanism lets the base class say what to do; it leaves how to the subclass. Different subclasses can implement the same method differently. This is the basis of polymorphism.

The override requirement

Abstract methods of an abstract class must be overridden in the subclass. Otherwise the subclass itself must be abstract. This guarantees the contract is fulfilled.

Example: if you define abstract class Animal { abstract void sound(); } and then write class Dog extends Animal, you must write void sound() with its body. If you do not, the compiler gives an error like 'Dog is not abstract and does not override abstract method sound'. The @Override annotation strengthens this check. The subclass may also override concrete methods of the abstract class. But abstract methods must absolutely be written.

Exercise Add an abstract sound method to Animal

Define an abstract class Animal. Add the abstract void sound(); signature. Write class Dog extends Animal; write the public void sound() method with a body that runs System.out.println("Woof woof");. In main call new Dog().sound().

Show example solution

abstract class Animal {
    abstract void sound();
}
class Dog extends Animal {
    public void sound() {
        System.out.println("Woof woof");
    }
}
class Main {
    public static void main(String[] args) {
        new Dog().sound();
    }
}

Test yourself

Match the abstract method features

abstract

Bodyless keyword
;

Signature end
override

Rewrite
void sound()

Signature example
What must a class be to contain an abstract method?
- final
- static
- sealed
- abstract
Show answer

abstract — An abstract method belongs only in an abstract class.

Complete the abstract method signature

abstract class Animal {
    ____ void sound();
}

Show answer

abstract class Animal {
    abstract void sound();
}

Next lesson: Abstract and concrete

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