Jason S Jason S - 1 month ago 7
Java Question

How can I require a method argument to implement multiple interfaces?

It's legal to do this in Java:

void spew(Appendable x)
{
x.append("Bleah!\n");
}


How can I do this (syntax not legal):

void spew(Appendable & Closeable x)
{
x.append("Bleah!\n");
if (timeToClose())
x.close();
}


I would like if possible to force callers to use objects that are both Appendable and Closeable, without requiring a specific type. There are multiple standard classes that do this, e.g. BufferedWriter, PrintStream, etc.

If I define my own interface

interface AppendableAndCloseable extends Appendable, Closeable {}


that won't work since the standard classes that implement Appendable and Closeable do not implement my interface AppendableAndCloseable (unless I don't understand Java as well as I think I do... empty interfaces still add uniqueness above and beyond their superinterfaces).

The closest I can think of is to do one of the following:


  1. pick one interface (e.g. Appendable), and use runtime tests to ensure the argument is an
    instanceof
    the others. Downside: problem not caught at compile time.

  2. require multiple arguments (catches compile-time correctness but looks dorky):

    void spew(Appendable xAppend, Closeable xClose)
    {
    xAppend.append("Bleah!\n");
    if (timeToClose())
    xClose.close();
    }


Answer

You could do it with generics:

public <T extends Appendable & Closeable> void spew(T t){
    t.append("Bleah!\n");
    if (timeToClose())
        t.close();
}

Your syntax was almost right, actually.