In Java, is there a way to specify that an argument implements two interfaces

Question

I am tempted to do such kind of code, using jGraphT

/*
  interface DirectedGraph<V,E> { ...}
  interface WeightedGraph<V,E> { ...}
*/

public class SteinerTreeCalc  {

    public SteinerTreeCalc( < ??? implements DirectedGraph<V,E>, WeightedGraph<V,E> > graph )  {
     ......
    }


}

I want to create a constructor that ask for an object implementing two interfaces.

Update :

In my goal, there are already chosen classes for Vertex and Edges (V and E), but thanks a lot to people who come up with :

public class SteinerTreeCalc <V, E, T extends DirectedGraph<V, E> & WeightedGraph<V, E>>  
{ 
   ....
}

I assume you have a good reason for not naming the interface that is both a `DirectedGraph` and a `WeightedGraph`, so that you can get all the bits of the contract that the user of the `SteinerTreeCalc` is required to support in one place? — Donal Fellows, Sep 16 '10 at 09:40
A good one ! JgraphT doesn't provide such interface that is both a DirectedGraph and a WeightedGraph, and the user might not want to use my custom interface/classes. I don't really understand why they didn't provide this interface. — jwinandy, Sep 16 '10 at 10:24

score 21 · Accepted Answer · answered Sep 16 '10 at 08:58

21

Yes, it's possible:

public class SteinerTreeCalc<T extends DirectedGraph<V,E> & WeightedGraph<V,E>> {
  public SteinerTreeCalc(T graph) {
    ......
  }
}

answered Sep 16 '10 at 08:58

amorfis

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What's V and E? Shouldn't it be SteinerTreeCalc & WeightedGraph> – bruno conde Sep 16 '10 at 09:02
2

I assume it's some inner class, where `V` and `E` are known. It's absent also in original question. – amorfis Sep 16 '10 at 09:05
This makes the class generic though. – aioobe Sep 16 '10 at 09:05

score 9 · Answer 2 · answered Sep 16 '10 at 09:02

9

Should work like this, but this is complexer generics logic, hope you can adapt:

public static interface DirectedGraph<V, E> {
}

public static interface WeightedGraph<V, E> {
}

public <V, E, T extends DirectedGraph<V, E> & WeightedGraph<V, E>> SteinerTreeCalc(T bothInterfaces) {
    // do it
}

These are the interfaces and the constructor like asked in your question.

answered Sep 16 '10 at 09:02

GHad

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+1: Very similar to what I was coding. However, wouldn't `V` and `E` reside on the class itself and not on the constructor? I suppose it depends on the author's goals. – Adam Paynter Sep 16 '10 at 09:05

score 6 · Answer 3 · edited May 23 '17 at 12:07

6

This could be what you want:
Hidden Features of Java

edited May 23 '17 at 12:07

Community

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answered Sep 16 '10 at 08:58

Enno Shioji

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Yes it is. I will take a deeper look to hidden features ;) – jwinandy Sep 16 '10 at 09:35

score 2 · Answer 4 · answered Sep 16 '10 at 08:56

2

you can use extends instead of implements in above code

answered Sep 16 '10 at 08:56

jmj

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score 0 · Answer 5 · answered Sep 16 '10 at 09:45

If V and E are concrete classes rather than type parameters, then you could create a new interface as follows:

public interface DirectedWeightedGraph extends 
    DirectedGraph<V,E>, WeightedGraph<V,E> {
}

then

public class SteinerTreeCalc  {

    public SteinerTreeCalc(DirectedWeightedGraph graph)  {
       ...
    }
}

The problem is that the actual argument must implement the DirectedWeightedGraph interface. A type that just implements DirectedGraph<V,E> and WeightedGraph<V,E> is not sufficient.

In Java, is there a way to specify that an argument implements two interfaces

5 Answers5

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