Java - Bounded Type Parameters

A generic method or class that operates on numbers will used bounded type parameters to restrict the type parameters accepted.

The extends keyword, followed by one or several upper bounds.

<T (extends|super) B1 [& B2 & ... & Bn]>

where:

• B1 to Bn are class or interface that defined the bounds
• B2 to Bn are optional
• extends defined upper bounds
• super define lower bounds

In this context, extends is used in a general sense to mean either:

• extends (as in classes)
• implements (as in interfaces).

public <U extends Number> void inspect(U u)

public class NaturalNumber<T extends Integer> {

    private T n;

    public NaturalNumber(T n)  { this.n = n; }

    public boolean isEven() {
        // intValue is methods defined in the bounds
        return n.intValue() % 2 == 0;
    }

    // ...
}

• https://docs.oracle.com/javase/tutorial/java/generics/bounded.html