Covariant

[kri shan]

'Sub1' class extends 'Base' class and 'Sub2' class extends 'Base' class. getValue() in Class B and Class C gives error. Is it Covariant?

public abstract class A {
  public abstract Base getValue(Base base);
}


public class B extends A {
  @Override
  public Sub1 getValue(Sub1 sub) {
      return sub;
  }
}


public class C extends A {
  @Override
  public Sub2 getValue(Sub2 sub) {
       return sub;
  }}

[A.J. Côté]

Java supports covariance this way:

In the code you posted the parameters of the sub classes don't match the parameters of the parent class and java sees them as unrelated methods with an overloaded name so it does not work.

public abstract class A {
  public abstract Base getValue();
}
 
 
public class B extends A {
  @Override
  public Sub1 getValue() {
      return sub;
  }
}
 
 
public class C extends A {
  @Override
  public Sub2 getValue() {
       return sub;
  }
}

[Jesper de Jong]

kri shan wrote:'Sub1' class extends 'Base' class and 'Sub2' class extends 'Base' class. getValue() in Class B and Class C gives error.

Return types can be covariant, but argument types cannot.

The getValue() method in class B does not override the getValue() in class A correctly because in class B it takes a Sub1, while in class A it takes a Base.

This example explains why this is so:

A obj = new B();

Sub2 sub2 = new Sub2();

// Sub2 is a Base, so this should be possible because A.getValue() takes a Base
// But this should not be possible if obj is actually a B, because B.getValue()
// can only be called with a Sub1 and not any other subclass of Base.
obj.getValue(sub2);

[A.J. Côté]

Jesper de Jong wrote:

kri shan wrote:'Sub1' class extends 'Base' class and 'Sub2' class extends 'Base' class. getValue() in Class B and Class C gives error.

Return types can be covariant, but argument types cannot.

The getValue() method in class B does not override the getValue() in class A correctly because in class B it takes a Sub1, while in class A it takes a Base.

This example explains why this is so:

A obj = new B();

Sub2 sub2 = new Sub2();

// Sub2 is a Base, so this should be possible because A.getValue() takes a Base
// But this should not be possible if obj is actually a B, because B.getValue()
// can only be called with a Sub1 and not any other subclass of Base.
obj.getValue(sub2);

As a side note; apparently; Sather supports contravariant but I am not sure if it is what the OP was trying to do but it looks similar...

[Campbell Ritchie]

I do not think contravariance is relevant to the question here. That link however suggests that C# supports covariance, contravariance and invariance in generics.