Generic type of local variable at runtime

Question

Is there a way in Java to reflect a generic type of a local variable? I know you can do that with a field - Get generic type of java.util.List. Any idea how to solve, for instance:

public void foo(List<String> s){
  //reflect s somehow to get String
}

Or even more general:

public void foo<T>(List<T> s){
  //reflect s somehow to get T
}

what are you exactly want to accomplish with s? can you show me a short example (pseudo code doesn't matter) — Christian Vielma, Aug 03 '12 at 16:00
With generics you could do something like public void foo T> (List s), if that's what you want — Christian Vielma, Aug 03 '12 at 16:02

Johan Sjöberg · Answer 1 · 2012-08-03T15:47:46.353

7

No. Due to Java's Type Erasure, all generics are stripped during the compile process.

You can however use instanceOf or getClass on elements in the list to see if they match a specific type.

edited Aug 03 '12 at 15:47

answered Aug 03 '12 at 15:42

Johan Sjöberg

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1

But you can still do the same thing on the field, so the info is stored somewhere... – Bober02 Aug 03 '12 at 15:52
1

Fields are different because their exact generic type is stored in the class file statically. Johan is correct: you _cannot_ do this. – Louis Wasserman Aug 03 '12 at 15:55

Pshemo · Accepted Answer · 2012-08-03T15:57:34.507

Here is nice tutorial that shows how and when you can read generic types using reflection. For example to get String from your firs foo method

public void foo(List<String> s) {
    // ..
}

you can use this code

class MyClass {

    public static void foo(List<String> s) {
        // ..
    }

    public static void main(String[] args) throws Exception {
        Method method = MyClass.class.getMethod("foo", List.class);

        Type[] genericParameterTypes = method.getGenericParameterTypes();

        for (Type genericParameterType : genericParameterTypes) {
            if (genericParameterType instanceof ParameterizedType) {
                ParameterizedType aType = (ParameterizedType) genericParameterType;
                Type[] parameterArgTypes = aType.getActualTypeArguments();
                for (Type parameterArgType : parameterArgTypes) {
                    Class parameterArgClass = (Class) parameterArgType;
                    System.out.println("parameterArgClass = "
                            + parameterArgClass);
                }
            }
        }
    }
}

Output: parameterArgClass = class java.lang.String

It was possible because your explicitly declared in source code that List can contains only Strings. However in case

public <T> void foo2(List<T> s){
      //reflect s somehow to get T
}

T can be anything so because of type erasure it is impossible to retrieve info about precise T class.

Generic type of local variable at runtime

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