Java等效C＃隐式

Question

I have an object A

class A {
    B contained = new B();
}

class B
{
}

and a collection of A's

Queue<A> q1 = new PriorityQueue<A>();  

what I want to do

Queue<B> q2 = new PriorityQueue<B>(q1);

in c# I can use the implicit keyword, is there an equivalent?

Answer 1

No, you need to have

Queue<B> q2 = new PriorityQueue<B>();
for(A a: q1) q2.add(a.contained);

I can't image you would want to do this very often. Is this a very useful feature?

Answer 2

First of all, if you use PriorityQueue in Java, then the classes needs to implement comparable interface. Next thing to do, is to think what kind of equality contract do you want between A and B . Keyword implicit in C# seems to be a simple cast transform.

I imagine equivalent in Java would look like:

    Queue<A> q1 = new PriorityQueue<A>(Arrays.asList(new A(), new A(), new A()));
    Queue<B> q2 = new PriorityQueue<B>(Collections2.transform(q1, A.toB));

Class A would probably look like:

static class A implements Comparable<A> {
    B contained = new B();

    public static final Function<A, B> toB = new Function<A, B>() {
        @Override public B apply(A a) {
            return a.contained;
        }
    };

    @Override public int compareTo(A a) {
        return contained.compareTo(a.contained);
    }
}