How wrap a generic class in java

Question

If I am writing a wrapper for a generic class, which I have tried to embed the type into at construction (as in this SO question):

class Foo<T> {
    private T value;
    private Class<T> type;

    public Foo(Class<T> type, T value) {
        this.value = value;
        this.type = type;
    }

    public T getValue() {
        return value;
    }

    public Class<T> getType() {
        return type;
    }
}

I have a list of instances of Foo, which I want to transform into a list of FooWrappers, along these lines:

List<Foo<?>> someListOfFoos = ...
List<FooWrapper<?>> fooWrappers = someListOfFoos
    .stream()
    .map(foo -> FooWrapper.from(foo))
    .collect(Collectors.toList());

Is there any way to recover the type of each element in someListOfFoos when building each FooWrapper? Something along these lines:

class FooWrapper<T> {
    private Foo<T> foo;

    public static FooWrapper<?> from(Foo<?> toWrap) {
        Class<E> type = toWrap.getType(); // i know this is wrong
        return new FooWrapper<type>(toWrap); // this is very wrong
    }

    private FooWrapper(Foo<T> foo) {
        this.foo = foo;
    }
}

Is there a reason for using raw types for `someListOfFoos` and `fooWrappers`? — Tom, Nov 06 '18 at 10:36
See this answer about typesafe heterogenous containers: https://stackoverflow.com/a/416611/2513200 - basically, you can only recover the type if you know it and can cast to it. — Hulk, Nov 06 '18 at 10:47
What do you gain by correctly inferring the type, only to immediately throw it away when adding it to the list? — Boris van Katwijk, Nov 06 '18 at 10:51

score 4 · Accepted Answer · answered Nov 06 '18 at 10:49

4

You just have to modify your FooWrapper#from slightly, by introducing a generic:

public static <E> FooWrapper<E> from(Foo<E> toWrap) {
    return new FooWrapper<E>(toWrap);
}

answered Nov 06 '18 at 10:49

user1983983

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How wrap a generic class in java

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