Subtyping of Generics

As you already know, it's possible to assign an object of one type to an object of another type provided that the types are compatible. For example, you can assign an Integer to an Object, since Object is one of Integer's supertypes:
Object someObject = new Object();
Integer someInteger = new Integer(10);
someObject = someInteger; // OK
In object-oriented terminology, this is called an "is a" relationship. Since an Integer is a kind of Object, the assignment is allowed. But Integer is also a kind of Number, so the following code is valid as well:
public void someMethod(Number n){
    // method body omitted
}
someMethod(new Integer(10));  // OK
someMethod(new Double(10.1)); // OK
The same is also true with generics. You can perform a generic type invocation, passing Number as its type argument, and any subsequent invocation of add will be allowed if the argument is compatible with Number:
Box<Number> box = new Box<Number>();
box.add(new Integer(10));  // OK
box.add(new Double(10.1)); // OK

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Posted in: Generics