Java nested generic class with only one type parameter

Question

I'm working on a project that has a generic stream interface that provides values of a type:

interface Stream<T> {
  T get();  // returns the next value in the stream
}

I have one implementation that provides single values simply from a file or whatever. It looks like this:

class SimpleStream<T> implements Stream<T> {
  // ...
}

I would also like to have another implementation that provides pairs of values (say, to provide the next two values for each call to get()). So I define a small Pair class:

class Pair<T> {
  public final T first, second;

  public Pair(T first, T second) {
    this.first = first; this.second = second;
}

and now I would like to define a second implementation of the Stream interface that only works with Pair classes, like this:

// Doesn't compile
class PairStream<Pair<T>> implements Stream<Pair<T>> {
  // ...
}

This does not compile, however.

I could do this:

class PairStream<U extends Pair<T>, T> implements Stream<U> {
  // ...
}

but is there any more elegant way? Is this the "right" way to do this?

You left out a > in the class PairStream<Pair<T>> implements Stream<Pair<T> line. — Eric Jablow
– Eric Jablow, Commented Jun 24, 2013 at 21:00

rgettman · Accepted Answer · 2013-06-24 21:01:33Z

5

The generic type parameter Pair<T> is not valid; you just need to declare <T> and use it when implementing the interface.

//         Just <T> here;          Added missing (2nd) '>'
class PairStream<T> implements Stream<Pair<T>> {
    public Pair<T> get() { /* ... */ }
}

answered Jun 24, 2013 at 21:01

rgettman

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Comments

Eric Jablow · Accepted Answer · 2013-06-24 21:04:42Z

1

All you really need is

class PairStream<T> implements Stream<Pair<T>> {
  // ...
}

This might work too:

class PairStream<U extends Pair<T>> implements Stream<U> {
    // ...
}

answered Jun 24, 2013 at 21:04

Eric Jablow

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