How to make a Java interface that extends Iterable with two different generic types?

Question

Ideally, it would look like this (the context doesn't matter):

public interface myInterface extends Iterable<Point>, Iterable<Segment> { ... }

But this is not allowed in Java. How can I achieve this behaviour?

Answer 1

Unfortunately you cannot . In Java you cannot have two methods with following signatures:

Iterator<Point> iterator();
Iterator<Segment> iterator();

in one class or interface.

Answer 2

As other said before, this is impossible. Better use delegation instead of multiple implementation like this:

public interface MyInterface {
  Iterable<Point> points();
  Iterable<Segment> segments();
}

So you can iterate using for :

MyInterface my = ...;
for (Point p : my.points()) {
  ...
}
for (Segment s : my.segments()) {
  ...
}

Answer 3

You cannot. Due to type erasure, in the bytecode, and therefore at run time, Iterable<Whatever> becomes Iterable .

So, at run time, your class' prototype would be:

public interface myInterface extends Iterable, Iterable { ... }

Considering that, how do you determine what class was meant to be iterated over?

Answer 4

As a possible workaround, you could create interfaces for the iterations you want.

public interface SegmentIterable{
    public Iterator<Segment> segmentIterator();
}

public interface PointIterable{
    public Iterator<Point> pointIterator();
}

It's not ideal, but would be passable as long as you had a limited number of things you wanted to iterate over.

Answer 5

Others have said it is impossible. They are wrong. It is possible, but probably not what you want.

public interface MyInterface<T extends Point & Segment> extends Iterable<T>
{
}

If what you are iterating extends both point and segment this will work. Otherwise Type Erasure means this won't work.

Answer 6

http://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.1.5

A class may not at the same time be a subtype of two interface types which are different invocations of the same generic interface (§9.1.2), or a subtype of an invocation of a generic interface and a raw type naming that same generic interface, or a compile-time error occurs.

Answer 7

Instead of inheriting from the iterable types, try something like this:

public interface MyInterface {
    public Iterable<Point> asPoints() { ... }
    public Iterable<Segment> asSegments() { ... }
}

Then when you want to iterate, it's simply a matter of:

for (Point p : myClass.asPoints()) {
    ...
}

This is a pretty common practice, as seen in the Java Collections class.

Answer 8

您还可以考虑为Point和Segment创建公共接口，超类或包装器，并将其用作通用参数。