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

Question

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

Answer 1

Official definition

From the reference documents Comparable.compareTo (T) :

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

The developer must provide sgn (x.compareTo (y)) == -sgn (y.compareTo (x)) for all x and y. (This means that x.compareTo (y) should throw an exception; iff y.compareTo (x) throws an exception.)

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

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

It is highly 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. Recommended language: "Note: this class has a natural order that is incompatible with peers."

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

My version

In short:

 this.compareTo(that) 

returns

• negative int if this <what
• 0 if it == that
• positive int if it> what

where the implementation of this method determines the actual semantics of < > and == (I do not mean == in the sense of the identification operator of a Java object)

Examples

 "abc".compareTo("def") 

will give something less than 0, since abc is in alphabetical order before def .

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

will give something more than 0 because 2 is greater than 1.

Some additional points

Note. . Good practice for a class implementing Comparable to declare semantics of the compareTo () method in javadocs.

Note: at least one of the following values ​​should be read:

• Object Ordering section under Trail Collection in Sun Java Tutorial
• Effective Java by Joshua Bloch, especially clause 12: Consider implementing Comparable
• Java Generics and Collections by Maurice Naphthalene, Philip Wadler, Chapter 3.1: Comparable

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

Answer 2

I use this mnemonic:

 a.compareTo(b) < 0 // a < b a.compareTo(b) > 0 // a > b a.compareTo(b) == 0 // a == b 

You keep signs and always compare the result of compareTo() with 0

Answer 3

Answer briefly: (find your situation)

• 1 .compareTo ( 0 ) (return: 1 )
• 1 .compareTo ( 1 ) (return: 0 )
• 0 .comapreTo ( 1 ) (return: -1 )

Answer 4

let's take an example if we want to compare "a" and "b", i.e. ("a" == this)

• negative int if a <b
• if a == b
• Positive int if a> b

Answer 5

It can be used for sorting, and 0 means "equal" while -1, and 1 means "less" and "more (more)".

Any return value less than 0 means that the left operand is less, and if the value is greater than 0, then the left operand is greater.

Answer 6

 int x = thisObject.compareTo(anotherObject); 

The compareTo() method returns an int with the following characteristics:

• negative If thisObject < anotherObject
• zero If thisObject == anotherObject
• positive If thisObject > anotherObject