Why this generic reference can't point to this similar extended class in Java?
public class HelloWorld{
public static void main(String []args){
Interface<Integer,Object> test = new Test();
System.out.println(test.method(test));
}
}
public interface Interface<A, B extends Object> {
public A method(B param);
}
public class Test implements Interface<Integer,Test> {
private int a = 0;
public Integer method(Test param){
return param.a;
}
}
I get this error when compiling
HelloWorld.java:5: error: incompatible types
Interface<Integer,Object> test = new Test();
^
required: Interface<Integer,Object>
found: Test
1 error
The class Test implements Interface<Integer,Test> and the type is declared as Interface<Integer,Object>. This assignment is not allowed because Generics are invariant:
Interface<Integer,Test> is neither a subtype nor a supertype of Interface<Integer,Object>. So, Interface<Integer,Test> cannot be assigned to Interface<Integer,Object>.
You can change the assignment statement as below to make it work :
Interface<Integer, ? extends Test> test = new Test();