Most compact way to compare three objects for equality using Java?

Answer 1

You can abstract that method in a utilities method like:

public boolean allUnique(Object... objs) {
  Set<Object> set = new HashSet<Object>();
  for (Object o : objs)
    set.add(o);
  return set.size() == objs.length
}

The method may not perform well for small numbers (due to the overhead of creating the Set and the varargs array). However, it grows linearly O(n), and for large values it's better than the quadratic growth of a nested if statements.

Answer 2

boolean areDistinct(Object a, Object b, Object c) {
    return (!a.equals(b) &&
            (c == null || (!c.equals(a) && !c.equals(b))));
}

Answer 3

EDIT: I removed my first solution as it implemented the wrinkle-scenario as invalid, and it shouldn't

No need to check for null (shortest?):

if(A.equals(B) || B.equals(C) || A.equals(C))
   // not unique

and, as others have already suggested, you can put it in a method for reuse. Or a generic method if you need more reuse ;-)

EDIT: A note to first-timers (if any): the example uses a feature of equals: if its argument is null it should not throw, but return false.

Answer 4

Since I never start a Java project without using Apache commons-lang, try ObjectUtils.equals (it's null safe):

if (ObjectUtils.equals(a, b) || ObjectUtils.equals(b, c) || ObjectUtils.equals(a, c)) {
  // error condition
}

Put that logic in a generic method, and you'll do even better.

While the business logic allows C to be null, in scenarios like this, it's often better to code defensively and assume that either A or B could be null as well.