What is wrong with the following generics code? [duplicate]

Question

Following code is throwing compile time exception

Type mismatch: cannot convert from Integer to K

What I understand is K should handle any value that extends Number.

public <K extends Number> K getValue(){
    Integer a = new Integer(1);
    return a;//Type mismatch: cannot convert from Integer to K
}

(Note: This is just a test code to put my question without giving irrelevant details of my actual scenario.)

Answer 1

The problem here is the fact that K is a type that extends Number but it is an unknown sub class of Number that is why the compiler raises this error because Integer is only one specific sub class of Number so it cannot match with any potential target types.

Let's say that you want to cast it explicitly to make it compile with

public <K extends Number> K getValue(){
    Integer a = new Integer(1);
    return (K) a;
}

Then if you call it expecting any sub class of Number except Integer you will get a ClassCastException, that is exactly what the compiler wants to avoid:

Double d = getValue(); <-- throws ClassCastException

As workaround you could define your method using Number as returned type as next:

public Number getValue() {
    return new Integer(1);
}