Comparable

The Comparable interface is used to define how a class is to be sorted. It is not to be confused with the Comparator interface, which is implemented in a separate class. The Comparable interface is implemented in the class to be sorted.

class MyClass implements Comparable<MyClass> {
  // Class body.
  ...
  @Override public int compareTo(MyClass value)
  {
      // Comparison Logic
      ...
      return result;
  }
}

Applying the Comparable interface to MyClass requires the implements keyword (e.g., Comparable<MyClass>). This interface has a single .compareTo() method that returns an int value based on whether the value of a current class instance (referenced by this) can be logically sorted with the value of another instance of the same class.

This method determines how items are sorted by methods such as Arrays.sort() and Collections.sort().

The following example defines an Employee class that sorts based on lastName and then on firstName.

Implementing the class:

import java.util.*;

// Employee.java
public class Employee implements Comparable<Employee> {
  String firstName;
  String lastName;

  // Constructor sets firstName and lastName
  public Employee(String first, String last)
  {
    this.firstName = first;
    this.lastName = last;
  }

  // User-friendly output when printed.
  public String toString()
  {
    return "( " + lastName + ", " + firstName + " )";
  }

  // Implement the Comparable interface
  @Override public int compareTo(Employee value)
  {
    if (this.lastName.compareTo(value.lastName) != 0) {
      // If lastNames are different, compare lastName
      return this.lastName.compareTo(value.lastName);
    } else {
      // If lastNames are the same, compare firstName
      return this.firstName.compareTo(value.firstName);
    }
  }
}

This last snippet demonstrates the Comparable interface:

import java.util.*;

// SortExample.java
public class SortExample {
  public static void main(String[] args)
  {
    // Set up array with a few Employee classes
    Employee a[] = new Employee[5];
    a[0] = new Employee("Kirk","Douglas");
    a[1] = new Employee("Mel","Brooks");
    a[2] = new Employee("Jane","Fonda");
    a[3] = new Employee("Henry","Fonda");
    a[4] = new Employee("Michael","Douglas");

    // The .sort() method uses the Comparable interface.
    Arrays.sort(a);

    // Print out the sorted Employees
    for (int i=0; i < a.length; i++) {
      System.out.println(a[i]);
    }
  }
}

This example results in the following output:

( Brooks, Mel )
( Douglas, Kirk )
( Douglas, Michael )
( Fonda, Henry )
( Fonda, Jane )