implementing abstract methods/classes in java

Question

Can I implement abstract methods in an abstract base class A in java?

If the answer is yes and there is an implemented abstract method in a base class A and there is a derived class B from A (B is not abstract). Does B still has to implement that base abstract method?

Solution 1

If I understand your question correctly, Yes.

public abstract class TopClass {
  public abstract void methodA();
  public abstract void methodB();
}

public abstract class ClassA extends TopClass {
  @Override
  public void methodA() {
    // Implementation
  }
}

public class ClassB extends ClassA {
  @Override
  public void methodB() {
    // Implementation
  }
}

In this example, ClassB will compile. It will use it's own implementation of methodB(), and ClassA's implementation of methodA(). You could also override methodA() in ClassB if desired.

Solution 2

You could have two abstract classes, X and Y, where Y extends X. In that case it could make sense for Y to implement an abstract method of X, while still being abstract. Another non-abstract class Z could extend Y. However, in your example, for A to implement its own abstract methods is a contradiction, the point of making them abstract is so it doesn't provide implementations, it just specifies what the method signatures should look like.

Solution 3

If you implement an abstract method it's not really abstract any more, so no.

Solution 4

Abstract classes can have regular methods. If you want to implement some of the methods of class A and leave rest of the methods abstract, you can do this. However, abstract methods cannot have a body, therefore if you mark a method as abstract, then it has to be implemented by a subclass, you can't implement them in the abstract class. However, you can have an abstract class without abstract methods, then subclass only needs to extend it.

Solution 5

Yes, you can implement abstract methods in a class which is declared as abstract. If a class is declared abstract that does not mean all its method must be abstract.

For a concrete sub class, it is not mandatory to implement the abstract methods that are already implemented by one of their super class.

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