It is an interface present in java.lang package.
It contains only one method that is compareTo().

The general form of this method is:

obj1.compareTo(obj2);

• • this method returning three values that is any +ve value, any -ve value, or 0.
• • When obj1 is coming before obj2, then compareTo() returns -ve value.
• • When obj1 is coming after obj2, then compareTo() returns +ve value.
• • When obj1 and obj2 are equal, then compareTo() returns 0.
• • When obj1 is comparing with null, then compareTo() returns NullPointerException.

Let's see a demo program of compareTo() method:

ComparableDemo.java

import java.util.*;
class ComparableDemo
{
	public static void main(String args[])
	{
		System.out.println("A".compareTo("Z"));
		System.out.println("Z".compareTo("B"));
		System.out.println("A".compareTo("A"));
		System.out.println("A".compareTo(null));
	}
}

Output

-25
24
0
Exception in thread "main" java.lang.NullPointerException
at java.base/java.lang.String.compareTo(String.java:1196)
at ComparableDemo.main(ComparableDemo.java:9)