Comparable Interface

In general, an interface consists of constants and method declarations. A class that implements an interface must implement each of the methods listed in the interface. The Comparable interface consists of just one method:

int compareTo( T obj )  // Compare this object with obj.
                        // Return a negative integer, zero, or a postive integer,
                        // when this object is less than, equal, or greater than obj.

In the above, "T" stands for the type of the objects. For example, if the objects are Strings, then "T" is String.

If some objects are instances of a class that implements Comparable, then each object is less than, equal, or greater than any object of that class. compareTo() returns an integer to show which of these three relations hold.

	Relation		objectA.compareTo( objectB )
objectA	Less Than	objectB	Negative Integer
objectA	Equal	objectB	Zero
objectA	Greater Than	objectB	Positive Integer

For example,

"apple".compareTo("orange")	Negative Integer
"apple".compareTo("plum")	Negative Integer
"apple".compareTo("apple")	Zero
"orange".compareTo("orange")	Zero
"orange".compareTo("apple")	Positive Integer

Only the sign of the returned integer matters if the return value is not zero. The magnitude of a returned integer does not signify anything.

Integer minusTen = new Integer( -10 );
Integer minusFive = new Integer( -5 );
Integer five = new Integer( 5 );
Integer ten = new Integer( 10 );
Integer fifteen = new Integer( 15 );