如何在Java中使用方法参数来实现多个接口？

Question

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, eg 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 (eg 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 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.