Java: Casting to a type parameter

Question

I have the following two classes:

public class GenericNumberOperation {
    public GenericNumberOperation() {} 
    public  T getSomeValu         


        

Answer 1

Forget about what you're trying to use this for. We're only going to look at this from a language perspective.

The declaration

public <T extends Number> T getSomeValue (boolean tf) {

defines a new type T that is bounded by Number. That means that a caller can only bind Number or any subtype of Number to T when invoking the method. Within the method, you don't know what that type might be.

You therefore can't do

T number = new Double(1.0);

because you don't know that T is Double. If I invoked the method as

Float f = genOp.getSomeValue(true);

T should have been Float. The compiler can't guarantee type safety and therefore rejects it (the assignment within the method, if it had been allowed, a ClassCastException would have been thrown at runtime). If you use a cast, you're telling the compiler that you're sure about what you're doing. It'll warn you, but it will trust you.

Similarly, the declaration

public <T> T getSomeValue(boolean tf)

defines a new type T that is unbounded. That means that you can bind any type to T, which makes the problem even greater. I can now do

String f = genOp.getSomeValue(true);

Answer 2

As @Sotirios Delimanolis wrote, you cannot even run that code.

Try this one:

@SuppressWarnings("unchecked")
public <T extends Number> T getSomeValue(boolean tf) {
    T number;
    if (tf) {
        number = (T) new Double(1.0);
    } else {
        number = (T) new Integer(11);
    }
    return number;
}