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

问题

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();
   }
   

回答1:

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.

来源：https://stackoverflow.com/questions/1500827/how-can-i-require-a-method-argument-in-java-to-implement-multiple-interfaces