Casting collection to collection [duplicate]

In Java, having two interfaces:

public interface SomeAbstractResult {} public interface SomeInterface { Collection<SomeAbstractResult> someMethod(); SomeAbstractResult someMethod2(); }

I want to have concrete implementation of these interfaces like this:

public class SomeConcreteResult implements SomeAbstractResult{} public class SomeClass implements SomeInterface { SomeConcreteResult abc; Collection<SomeConcreteResult> xyz; @Override public Collection<SomeAbstractResult> someMethod() { return this.xyz; //this doesn't work } @Override public SomeAbstractResult someMethod2() { return this.abc; // this works fine } }

The problem is that compiler says:

incompatible types: Collection<SomeConcreteResult> cannot be converted to Collection<SomeAbstractResult>

Why is this the case?