Knowing type of generic in Java

Question

I have a generic class, says :

MyClass<T>

Inside a method of this class, I would like to test the type of T, for example :

void MyMethod()
{

    if (T == String)
        ...
    if (T == int)
        ...
}

how can I do that ?

Thanks for your help

As an aside, why would you want to do this ? – Steve De Caux Dec 11 '09 at 08:50 — Steve De Caux, Dec 11 '09 at 08:50

score 12 · Accepted Answer · answered Dec 11 '09 at 08:47

12

You can't, normally, due to type erasure. See Angelika Langer's Java Generics FAQ for more details.

What you can do is pass a Class<T> into your constructor, and then check that:

public MyClass<T>
{
    private final Class<T> clazz;

    public MyClass(Class<T> clazz)
    {
        this.clazz = clazz;
    }

    public void myMethod()
    {
        if (clazz == String.class)
        {
           ...
        }
    }
}

Note that Java does not allow primitives to be used for type arguments though, so int is out...

answered Dec 11 '09 at 08:47

Jon Skeet

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4

Ew. Java is gross for this kind of thing. – Conrad Meyer Dec 11 '09 at 08:50
4

which honestly you shouldn't need if you design your classes properly. – Bozho Dec 11 '09 at 09:02
I understand, maybe my desing is bad. I have a view that can be applyed to 2 models, except for some little things depending on the type of the object model. I don't want to create 2 views that are quite the same, but they are not working exactly the same. – Tim Dec 11 '09 at 09:19
1

With all respect for "proper design", I dislike it when a language tells me what "proper design" is. Notwithstanding that this might be bad design, I find "Java does not need to do this because you designed it improperly" is possibly not the best reply. – Cornelius May 19 '11 at 12:02

score 8 · Answer 2 · answered Dec 11 '09 at 08:58

Because of type erasure you can't... mostly. But there is one exception to that. Consider:

class A {
  List<String> list;
}

public class Main {
  public static void main(String args[]) {
    for (Field field : A.class.getDeclaredFields()) {
      System.out.printf("%s: %s%n", field.getName(), field.getGenericType());
    }
  }
}

Output:

list: java.util.List<java.lang.String>

If you need the class object, this is how you generally handle it:

public <T> T createObject(Class<T> clazz) {  
  return clazz.newInstance();
}

ie by passing the class object around and deriving the generic type from that class.

score 3 · Answer 3 · answered Dec 11 '09 at 12:13

3

Additionally to cletus one exception I've mine: super type tokens. The super type token will preserve the type information.

new Type<Set<Integer>>() {}

The type information can be retrieved with Class.getGenericSuperClass.

answered Dec 11 '09 at 12:13

Thomas Jung

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score 1 · Answer 4 · answered Dec 11 '09 at 09:04

1

if (object instanceof String)
   System.out.println("object is a string");

answered Dec 11 '09 at 09:04

oliver31

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2

I think that's actually a reasonable point given the question. – Tom Hawtin - tackline Dec 11 '09 at 11:14

score 0 · Answer 5 · answered Dec 11 '09 at 09:33

0

As it was already stated you can get only generics-related information available at the static byte code level.

It's possible to resolve type arguments values and check if one type may be used in place of another then.

answered Dec 11 '09 at 09:33

denis.zhdanov

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score 0 · Answer 6 · answered Dec 11 '09 at 10:19

0

if you have subclass B extends A that should match, too, the approach clazz == A.class. Doesn't work. You should then use A.class.isInstance(b) where b is an object of type B.

answered Dec 11 '09 at 10:19

bertolami

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score 0 · Answer 7 · answered Dec 11 '09 at 10:32

0

If you want to do different things for different types would it still be generic?

answered Dec 11 '09 at 10:32

fastcodejava

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Knowing type of generic in Java

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