What do the return values of Comparable.compareTo mean in Java?

Question

What is the difference between returning 0, returning 1 and returning -1 in compareTo() in Java?

Answer 1

Official Definition

From the reference docs of Comparable.compareTo(T):

Compares this object with the specified object for order. Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

The implementor must ensure sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y. (This implies that x.compareTo(y) must throw an exception iff y.compareTo(x) throws an exception.)

The implementor must also ensure that the relation is transitive: (x.compareTo(y)>0 && y.compareTo(z)>0) implies x.compareTo(z)>0.

Finally, the implementor must ensure that x.compareTo(y)==0 implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)), for all z.

It is strongly recommended, but not strictly required that (x.compareTo(y)==0) == (x.equals(y)). Generally speaking, any class that implements the Comparable interface and violates this condition should clearly indicate this fact. The recommended language is "Note: this class has a natural ordering that is inconsistent with equals."

In the foregoing description, the notation sgn(expression) designates the mathematical signum function, which is defined to return one of -1, 0, or 1 according to whether the value of expression is negative, zero or positive.

My Version

In short:

this.compareTo(that) 

returns

• a negative int if this < that
• 0 if this == that
• a positive int if this > that

where the implementation of this method determines the actual semantics of < > and == (I don't mean == in the sense of java's object identity operator)

Examples

"abc".compareTo("def") 

will yield something smaller than 0 as abc is alphabetically before def.

Integer.valueOf(2).compareTo(Integer.valueOf(1)) 

will yield something larger than 0 because 2 is larger than 1.

Some additional points

Note: It is good practice for a class that implements Comparable to declare the semantics of it's compareTo() method in the javadocs.

Note: you should read at least one of the following:

• the Object Ordering section of the Collection Trail in the Sun Java Tutorial
• Effective Java by Joshua Bloch, especially item 12: Consider implementing Comparable
• Java Generics and Collections by Maurice Naftalin, Philip Wadler, chapter 3.1: Comparable

Warning: you should never rely on the return values of compareTo being -1, 0 and 1. You should always test for x < 0, x == 0, x > 0, respectively.