Java Method Signatures and Interface

Question

We know that method signatures include only method name and parameter lists but not method return types. So why am I getting compiler error for the following code since java does not differentiate between methods with same signature.

 public class InterfaceTest implements I2{

     public void hello(){  }

     public void world(){  }
}

interface I1{
    public int hello();
}

interface I2 extends I1{
  public void world();
}

Answer 1

You have not overloading here, you are overriding and also hidding methods but not in a correct way.... there are 2 possibilities to solve your problem:

public class InterfaceTest implements I2{

    public void hello(int a){  }  // overloaded method

    @Override
    public int hello(){ return 1; }  // overriden method

    public void world(){  }   // this hides I1 method
}

Point is if you try this in a single class:

public void hello() {}
public int hello() {return 1;}

You will get Duplicate method hello() in type YourClass error, because to overloading a method you must change the FormalParameterListopt of the signature:

If two methods of a class [...] have the same name but signatures that are not override-equivalent, then the method name is said to be overloaded.

Last but not Least point:

method signatures include only method name and parameter lists but not method return types

According JSL §8.4, when you declare a method:

MethodDeclaration:
    MethodHeader MethodBody

MethodHeader:
     MethodModifiersopt TypeParametersopt Result MethodDeclarator Throwsopt

MethodDeclarator:
    Identifier ( FormalParameterListopt )

So when you do this:

  public int hellow(int number) throws Exception;
//|      |   |      |           └ throwing an exception (Throwsopt)
//|      |   |      └──────────── receiving one int argument (MethodDeclarator FormalParameterListopt )
//|      |   └─────────────────── name hellow (MethodDeclarator Identifier)
//|      └─────────────────────── returning an int (Result)
//└────────────────────────────── is a public method (MethodModifiersopt)

Answer 2

You are referring method overriding here not overloading. The return type must be the same as, or a subtype, of the return type declared in the original overridden method in the superclass/interface.