Java type error with generics in Constructor

Question

I have a (probably simple) question regarding genrics in Java. I have the following class:

public class ValueCollection<Y> implements Collection<Y>
{
    private Set<Entry<?, Y>> entries;

    public ValueCollection(Set<Entry<?, Y>> entries)
    {
        this.entries = entries;
    }
    ...
}

When I call the constructor like this:

return new ValueCollection<V>(entries);

I get the following compiler error:

The constructor ValueCollection<V>(Set<Map.Entry<K,V>>) is undefined

If I change my class to this:

public class ValueCollection<X, Y> implements Collection<Y>
{
    private Set<Entry<X, Y>> entries;

    public ValueCollection(Set<Entry<X, Y>> entries)
    {
        this.entries = entries;
    }
    ...
}

and my constructor call to this:

return new ValueCollection<K, V>(this.entries());

the compile error goes away. I'm just wondering why this is the case. Thanks for the help!

What if you just call the constructor with new ValueCollection<V>(entries) in the first case? Your first class only has one type parameter. — millimoose
– millimoose, Commented Jul 26, 2012 at 18:54

Ben Schulz · Accepted Answer · 2012-07-26 20:16:41Z

2

A Set<Entry<?, V>> is either a set of entries with any key type and value type V or a set of entries with some specific but unknown key type K and value type V. Due to the latter the compiler rejects your original constructor invocation.

A Set<? extends Entry<?, V>> is a set of entries which are entries with any key type and value type V. This is exactly what you want, redefine your constructor parameter type to be Set<? extends Entry<?, V>>.

You can assign the parameter to your field using either this.entries = Collections.unmodifiableSet(entries) or this.entries = new HashSet<Entry<?, V>>(entries). Both right hand sides produce a Set<Entry<?, V>> in a way that convinces the compiler it means a set of entries with any key type and value type V.

answered Jul 26, 2012 at 20:16

Ben Schulz

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1 Comment

tristan Over a year ago

Thanks a lot! This is exactly what I wanted to know. Your explanation makes a lot of sense.

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