Question

As per The Java Programming Language(by Arnold Gosling) (ref pg. 275) if we use abstract explicitly before a method declaration in an interface, We can

skip the method method in the implementation class.

As per the example:

interface Sheet{
  public double computeArea();
  public abstract double computePerimter();
} 

class Sphere implements Sheet{
  // Some data members and constructors.

public double computeArea() { ... }

  // No implementation of computePerimeter  
}

But when i tried creating an object of the Sphere class, i showed up an error that the computePermeter method hasnt been overloaded(that should has been the case).

But as per the context, you could skip the method if it has been explicitly declared as abstract.However there has to be a class implementating that method. Arent the methods in an interface implicitly abstract?? Or am i interpreting it wrongly?? The explicit abstract has confused me a bit.. please help..

Answer 1

The keyword abstract makes no difference. The only thing the specification says is:

For compatibility with older versions of the Java platform, it is permitted but discouraged, as a matter of style, to redundantly specify the abstract modifier for methods declared in interfaces.

A quick test shows that the keyword indeed makes no difference. These two are both valid:

abstract class A implements B { }
interface B {
    public void meth();
}

abstract class A implements B { }
interface B {
    public abstract void meth();
}

and these two are both invalid:

class A implements B { }
interface B {
    public void meth();
}

class A implements B { }
interface B {
    public abstract void meth();
}

A.java:1: A is not abstract and does not override abstract method meth() in B
class A implements B { }
^
1 error