How To Instantiate a java.util.ArrayList with Generic Class Using Reflection

Question

How To Instantiate a java.util.ArrayList with Generic Class Using Reflection? I am writing a method that sets java.util.List on target object. A target object and a generic type of list is knowing in runtime:

public static void initializeList(Object targetObject, PropertyDescriptor prop, String gtype) {
    try {
        Class clazz = Class.forName("java.util.ArrayList<"+gtype+">");
        Object newInstance = clazz.newInstance();
        prop.getWriteMethod().invoke(targetObject, newInstance);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

score 15 · Answer 1 · answered Jan 27 '11 at 15:23

15

An object doesn't know about its generic type at execution time. Just create a new instance of the raw type (java.util.ArrayList). The target object won't know the difference (because there isn't any difference).

Basically Java generics is a compile-time trick, with metadata in the compiled classes but just casting at execution time. See the Java generics FAQ for more information.

answered Jan 27 '11 at 15:23

Jon Skeet

1,261,211
792
8,724
8,929

just for not creating duplicate answers, using reflection is just like doing `new ArrayList – Yanick Rochon Jan 27 '11 at 15:27

alpian · Answer 2 · 2011-01-28T22:55:37.783

7

There's no need to do any reflection for creating your List. Just pass in some additional type information (usually done by passing a class of the correct type).

public static <T> List<T> createListOfType(Class<T> type) {
    return new ArrayList<T>();
}

Now you have a list of the required type you can presumably/hopefully set it directly on your targetObject without any reflection.

edited Jan 28 '11 at 22:55

answered Jan 27 '11 at 15:43

alpian

4,398
16
18

score 2 · Answer 3 · answered Jan 27 '11 at 15:24

2

Generics are a compile-time only "trick".

Reflection is runtime-only.

Basically, you can't - you can only create a "raw" ArrayList. If you need to pass it into methods that take generic parameters, casting it directly after construction will be safe (regardless of the "unchecked" warning). In this example, there's no compile-time type safety anyway due to using general Objects, so no casting is needed.

answered Jan 27 '11 at 15:24

Andrzej Doyle

97,637
30
185
225

1

Generics is not completely a compile-time only thing. Concrete type parameters in field, method and class definitions are actually present in the bytecode and can be acquired at runtime via reflection. – Michael Borgwardt Jan 27 '11 at 15:29

score 0 · Answer 4 · answered Jan 27 '11 at 15:25

0

Generics only work at compile time, therefore if you're using reflection, they won't be available.

See more about this here.

answered Jan 27 '11 at 15:25

biziclop

46,403
12
73
97

score 0 · Answer 5 · answered Jan 27 '11 at 15:33

0

An object doesn't know about its generic type at execution time. Just create a new instance of the raw type (java.util.ArrayList). The target object won't know the difference (because there isn't any difference).

answered Jan 27 '11 at 15:33

ram singh

1
4

score 0 · Answer 6 · answered Jan 27 '11 at 15:40

Generics is largely a compile time feature. What you are trying to do is the same as

public static void initializeList(Object targetObject, PropertyDescriptor prop, String gtype) {
    prop.getWriteMethod().invoke(targetObject, new ArrayList());
}

Note: This could change with Types in Java 7.

How To Instantiate a java.util.ArrayList with Generic Class Using Reflection

6 Answers6

Linked