Question

I’m looking through EntityManager.java and I see

public <T extends Object> T find(Class<T> type, Object o, LockModeType lmt, Map<String, Object> map);

I’ve never seen <X> type declarations within a class method definition. What does this mean?

public <T extends Object> T find( seems like it’s defining that T will extend another class. Am I understanding that this requires you to extend another object?

Answer 1

public <T extends Number> T find()

This means that you define a generic function, which has a type parameter denoted by T for the sake of definition. T has to be a subclass (extends) of Number. The function returns an object of type T.

Then you can call the function like that:

Double d = find();
Integer i = find();

but you the following is a compile time error:

String s = find();  //error

because String doesn’t extend Number.