Passing Collection into a function expecting Collection fails in Java

Question

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Most efficient way to cast List<SubClass> to List<BaseClass>
why polymorphism doesn't treat generic collections and plain arrays the same way?

If I have an abstract base class BaseClass and I write a function expecting Collection<BaseClass> as its argument, calls to that function with a Collection<SubClass> of a class SubClass extends BaseClass fail to compile.

In the base class:

public void addLots(Collection<BaseClass> collection) {
    for(BaseClass yourbase : collection) {
        us.add(yourbase) //what you say!!
    }
}

And in the subclass:

public void addMoreLots(Collection<SubClass> collection) {
    addLots(collection); //FAILS TO COMPILE
}

Now I think I can see why this is: Collection<SubClass> is NOT a subclass of Collection<BaseClass>. What is the correct method of making this call?

Thanks—I couldn't find it though, and I think this addresses a different question (more focused on "how do I do this", less focused on "why are arrays different than collections"). This presumable applies to any generic, for example `WeakReference`. — Andrew Wyld, Sep 03 '12 at 17:07

score 2 · Answer 1 · answered Sep 03 '12 at 21:34

2

In a pinch, Collections.unmodifiableCollection(Collection<SubClass>) can return a Collection<BaseClass>. (It makes the collection unmodifiable, of course, but that's what makes it safe to treat a Collection<SubClass> as a Collection<BaseClass>.)

answered Sep 03 '12 at 21:34

Louis Wasserman

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score 1 · Answer 2 · answered Sep 03 '12 at 17:00

1

<? extends BaseClass> should replace <BaseClass>. The correct syntax for addLots is:

public void addLots(Collection<? extends BaseClass> collection) {
    for(BaseClass yourbase : collection) {
        us.add(yourbase) //what you say!!
    }
}

answered Sep 03 '12 at 17:00

Andrew Wyld

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score 1 · Accepted Answer · answered Sep 03 '12 at 17:01

1

Collections are checked only during the compilation time, not during the run time, so this is done in order to protect a collection taking in the wrong type of object in.

Try it this way...

public <T extends BaseClass> void addLots(Collection<T> collection){ }

OR

public void addLots(Collection<? extends BaseClass> collection) {}

answered Sep 03 '12 at 17:01

Kumar Vivek Mitra

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I already worked it out and posted my own answer, but thank you :) – Andrew Wyld Sep 03 '12 at 17:05
1

@AndrewWyld, nothing is more interesting and encouraging than finding an answer by ourself...... Keep it up... But please see i have mention both the ways to do it..... – Kumar Vivek Mitra Sep 03 '12 at 17:06

score 1 · Answer 4 · answered Sep 03 '12 at 17:04

1

You can use

public void addLots(Collection<? extends BaseClass> collection) {
}

instead of

public void addLots(Collection<BaseClass> collection) {
}

which denotes a Collection of types which extends the base class can be a parameter.

answered Sep 03 '12 at 17:04

Bharat Sinha

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I already worked it out, see below, but thank you :) – Andrew Wyld Sep 03 '12 at 17:05

score 0 · Answer 5 · edited Sep 03 '12 at 22:56

0

public void addLots(Collection<? extends BaseClass> collection) {
}

but with this you cannot call add to collection (i.e. you cannot do collection.add()),
but in this

public void addLots(Collection<? super SubClass> collection) {
}

you can call add (i.e. you can do collection.add()).

edited Sep 03 '12 at 22:56

Piotr Sobiegraj

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answered Sep 03 '12 at 18:02

Kailash

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Passing Collection into a function expecting Collection fails in Java

5 Answers5