How to check that a method returns Collection in Java?

Question

I wish to a check if a method exists in an interface based on its signatures.

The signature that the method should have is:

Collection<Foo> methodName(Spam arg0, Eggs arg1, ...)

I can find the methods via Class.getMethods() then find the name, parameters and return type respectively with method.getName(), method.getParameterTypes() and method.getReturnType().

But what do I compare the return type to in order to ensure that only methods that return Collection<Foo> are chosen, and not other collections?

method.getReturnType().equals(Collection.class)

Since the above will be true for all methods that return a collection, not just for those that return a Foo Collection.

See http://www.artima.com/weblogs/viewpost.jsp?thread=208860 — Artefacto, Aug 20 '10 at 14:48
duplicate of http://stackoverflow.com/questions/3520057/how-can-i-learn-actual-type-argument-of-an-generic-class — Thierry Roy, Aug 20 '10 at 14:50
This isn't a duplicate - the question is how to inspect the parameterized return type, not instantiate an instance of the parameterized type — Jon Freedman, Aug 20 '10 at 14:54

Colin Hebert · Accepted Answer · 2010-08-20T15:44:13.067

8

There is a method named public Type getGenericReturnType() which can return (if it's the case) a ParameterizedType.

A ParameterizedType can give you more informations on a generic type such as Collection<Foo>.

In particular with the getActualTypeArguments() method you can get the actual type for each parameter.

Here, ParameterizedType represents Collection and getActualTypeArguments() represents an array containing Foo

You can try this to list the parameters of your generic type :

Type returnType = method.getGenericReturnType();
if (returnType instanceof ParameterizedType) {
    ParameterizedType type = (ParameterizedType) returnType;
    Type[] typeArguments = type.getActualTypeArguments();
    for (Type typeArgument : typeArguments) {
        Class typeArgClass = (Class) typeArgument;
        System.out.println("typeArgClass = " + typeArgClass);
    }
}

Sources : http://tutorials.jenkov.com/

edited Aug 20 '10 at 15:44

answered Aug 20 '10 at 14:54

Colin Hebert

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Thanks Colin. That did it and the article explains a little bit more about how and why. – brice Aug 20 '10 at 15:02
I'm wondering, would [method.getTypeParameters()](http://download.oracle.com/javase/6/docs/api/java/lang/reflect/GenericDeclaration.html) work too? – brice Aug 20 '10 at 15:11
1

Method.getTypeParameters() give generic informations on the method itself for example Collections.unmodifiableList : `public static List unmodifiableList(List extends T> list) {` The result will be an array containing "T" – Colin Hebert Aug 20 '10 at 16:00

score 2 · Answer 2 · answered Aug 20 '10 at 14:51

2

See http://download.oracle.com/javase/tutorial/reflect/member/methodType.html

answered Aug 20 '10 at 14:51

Jon Freedman

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score 0 · Answer 3 · answered Aug 20 '10 at 14:47

0

Generic type parameters are not retained at runtime (i.e. it's a compile-time only feature) in Java so you cannot do this.

answered Aug 20 '10 at 14:47

Raoul Duke

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How to check that a method returns Collection in Java?

3 Answers3