14. Comparable Intro

A class implements the Comparable interface when there is a way to decide whether one object of the class is greater than another. The compareTo method is the way to find out how the two classes compare.

The first.compareTo(second) method call gives a positive number when the first object is greater than the second. Summary of meaning of results:

• positive: first is greater than second
• negative: first is less than second
• zero: first equals second

Important Examples

A good example to remember is comparing Integer objects.

 Integer a = new Integer(5);
 Integer b = new Integer(12);
 int c = a.compareTo(b); // results in 5-12 = -7

Another example is String objects, where strings that are later in the alphabet are like larger numbers. In the code below, do you expect the result to be positive, negative, or zero? Why?

 int result = "bog".compareTo("aardvark);

Details

Technically, the Comparable interface has a class parameter <T> which indicates to which type you can compare your class. This parameter should just be the type of the class you are writing. Examples:

  public class Student implements Comparable<Student> {
      // ...
      public int compareTo(Student other) { ... }
  }

  public class Time implements Comparable<Time> {
      // ...
      public int compareTo(Time other) { ... }
  }