Java generic member initialization

Question

Suppose I have a Java class parameterized on that contains a private T _member. I want to write a default constructor (no arguments), that somehow initializes my T _member to some known, type T specific value (like -1 for Integer, Float.MAX_VALUE for Float...). Is that possible? I tried new T(), but the compiler doesn't like that. Or do I do nothing, and the default constructor is guaranteed to be called for me?

score 5 · Answer 1 · edited Aug 03 '12 at 22:12

5

Because of type erasure, at runtime "there is no T".

The way around it is to pass an instance of Class<T> into the constructor, like this:

public class MyClass<T> {

    T _member;

    public MyClass(Class<T> clazz) {
        _member = clazz.newInstance(); // catch or throw exceptions etc
    }

}

BTW, this is a very common code pattern to solve the issue of "doing something with T"

edited Aug 03 '12 at 22:12

Paul Bellora

54,340
18
130
181

answered Aug 03 '12 at 20:29

Bohemian

412,405
93
575
722

1

That clazzy clazz has razz-matazz. :D this is off topic but I couldn't resist. – Wug Aug 03 '12 at 20:32
3

You mean *type* erasure, I trust? (Would be kinda bad if the *runtime* erased itself ;-) – meriton Aug 03 '12 at 20:36
The Class instance passed as an argument is known as a [type token](http://docs.oracle.com/javase/tutorial/extra/generics/literals.html) – hertzsprung Aug 03 '12 at 20:41

score 3 · Answer 2 · answered Aug 03 '12 at 20:28

3

It's not directly possible, since there's no way to guarantee T even has a default constructor. You could conceivably do it using reflection with newInstance, however. Just be sure to catch any exceptions that may be thrown.

public class MyClass<T> {

    private T _member;

    public MyClass(Class<T> cls) {
        try {
            _member = cls.newInstance();
        } catch (InstantiationException e) {
            // There is no valid no-args constructor.
        }
    }
}

answered Aug 03 '12 at 20:28

Jon Newmuis

25,722
2
45
57

-1 You can't do anything with `T`. This answer completely won't work. See my answer for why – Bohemian Aug 03 '12 at 20:30
OMG! You totally changed your answer and copy-pasted mine over the top of it. Have you no honour? – Bohemian Aug 03 '12 at 20:35
I did not copy and paste your answer. I added an example of my second and third sentences. I've upvoted your answer; no harm intended here, my friend. – Jon Newmuis Aug 03 '12 at 20:36
2

+1 for pointing out that `T` may not have a default constructor. – Louis Wasserman Aug 04 '12 at 08:44

score 3 · Accepted Answer · answered Aug 03 '12 at 23:06

3

Every field in Java without an initializer expression is automatically initialized to a known default value. And yes, in this case it is a known value of type T -- null.

answered Aug 03 '12 at 23:06

newacct

119,665
29
163
224

score 0 · Answer 4 · edited May 23 '17 at 12:32

In fact there is a way to do this, here's a link to answer on related SO question.

Thus in fact your main question if there's a possibility to get Class of generic member at runtime? Yes, there is - using reflection.

When I came over that answer about a year ago, I tested it and investigated reflection a little bit more. In fact, there were couple of cases when it didn't work as is (be ware of cases with inheritance), but in general it's wrong to claim this is impossible.

However, I strongly do not suggest doing it this way.

EDIT. Surprisingly I found my own comment below that answer :) And in the next comment there's a link by author to the original article on generics' reflection.

Java generic member initialization

4 Answers4