Java Construct Generic Instance From Class

Question

I am trying to construct an instance of a generic class, with a type parameter. For example if have ArrayList.class and I want to initialize it with class String.class, how do I do that? I understand I can use ArrayList.class.newInstance() to create a new instance, but I can't pass in a type parameter. I want to do new ArrayList<String>, but instead of for that specific example, any class that has a type parameter.

Specifically, I am trying to implement testing of different implementations of an interface, through subclassing of a JUnit test.

possible duplicate of [Instantiating a generic class in Java](http://stackoverflow.com/questions/1090458/instantiating-a-generic-class-in-java) — Robby Cornelissen, Oct 29 '14 at 03:21
Basically, you can't because of [type erasure](http://docs.oracle.com/javase/tutorial/java/generics/erasure.html). — Elliott Frisch, Oct 29 '14 at 03:23
@RobbyCornelissen I don't think it is. That is asking how to initialize the type within a generic class. I am asking how to initialize a generic class with a type. — saul.shanabrook, Oct 29 '14 at 03:23
Since type information is erased at runtime and only useful for the static typecheck during compilation, why do you want to do this? In particular, there is *no difference* at runtime between an instance of `ArrayList` and an instance of `ArrayList`. — Thilo, Oct 29 '14 at 03:23
Because then I know when I get, say, an item from my newly created ArrayList then I can use all the String methods on it — saul.shanabrook, Oct 29 '14 at 03:25
Basically doing this: http://stackoverflow.com/questions/6724401/how-to-write-junit-tests-for-interfaces#comment28660208_6724555 ill update to show my example — saul.shanabrook, Oct 29 '14 at 03:27

score 0 · Accepted Answer · answered Oct 29 '14 at 03:40

0

I realized that can just return an initialization of the generic class, without the type paremeter and just cast it to that paremeter.

For example:

private Class<? extends List> implementation = ArrayList.class;

private <E> List<E> newList(Class<E> itemClass) {
    try {
        return (List)implementation.newInstance();
    } catch (Throwable x) {
       return null;
    }
}

@Test
public void test1() {
    List<String> testInstance = newList(String.class);
    testInstance.add('hey');
    assertEquals(3, testInstance.get(0).lenght());
}

answered Oct 29 '14 at 03:40

saul.shanabrook

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You shouldn't catch Throwable. Otherwise, yes you can do this this way. I would personally cast to `(List)` and use a temp variable to annotate the assignment with `@SuppressWarnings("unchecked")`. – Radiodef Oct 29 '14 at 03:49
you'll get a warning for this, though, about unchecked conversion. – Thilo Oct 29 '14 at 03:49
@Thilo It's safe though as long as you don't put something in the List while it's raw. – Radiodef Oct 29 '14 at 03:52
2

that's right. But I am assuming that List is only an example. This may or may not be safe with the real interface (and implementations) that are being tested here. – Thilo Oct 29 '14 at 03:53
Of course, triggering run time errors (like ClassCastException) is exactly what JUnit is for. So that should not be a problem. – Thilo Oct 29 '14 at 03:55

Java Construct Generic Instance From Class

1 Answers1