Fun with Java generics

问题

Anybody knows how to write the piece of code below using generics AND avoiding compiler warnings ? (@SuppressWarnings("unchecked") is considered cheating).

And, maybe, checking via generics that the type of "left" is the same as the type of "right" ?

public void assertLessOrEqual(Comparable left, Comparable right) {
    if (left == null || right == null || (left.compareTo(right) > 0)) {
        String msg = "["+left+"] is not less than ["+right+"]";
        throw new RuntimeException("assertLessOrEqual: " + msg);
    }
}

回答1:

This works with subclasses of Comparable types too:

public <T extends Comparable<? super T>> void assertLessOrEqual(T left, T right) {
  if (left == null || right == null || left.compareTo(right) > 0) {
    String msg = "["+left+"] is not less than ["+right+"]";
    throw new RuntimeException("assertLessOrEqual: " + msg);
  }
}

回答2:

How about this:

public <T extends Comparable<T>> void assertLessOrEqual(T left, T right) {
  if (left == null || right == null || (left.compareTo(right) > 0)) {
    String msg = "["+left+"] is not less than ["+right+"]";
    throw new RuntimeException("assertLessOrEqual: " + msg);
  }
}

It could probably be made a little bit more general, but only by making it more complicated as well :)

回答3:

You cannot via generics check if the type of 'left' is the same as the type of 'right' at runtime. Java generics is implemented via type erasure, so that the information about the generic type parameters is lost at runtime.

public <T extends Comparable<T>> void assertLessOrEqual(T left, T right) {
    if (left == null || right == null || (left.compareTo(right) > 0)) {
        String msg = "["+left+"] is not less than ["+right+"]";
        throw new RuntimeException("assertLessOrEqual: " + msg);
    }
}

来源：https://stackoverflow.com/questions/1438028/fun-with-java-generics