How to Use Comparable and Comparator in Java

See Java: Tips and Tricks for similar articles.

The ordering of a tree set in Java can be dictated using implementations of either the Comparable or Comparator interface. For example, we might want to build a set of integers that we wish to store in numerical order. Such an ordering is considered a "natural order" in Java. Natural ordering can be accomplished by storing items that belong to a class type that implements the Comparable interface. The Integer class, for instance, implements this interface. Therefore, integer variables that are stored in a set are automatically placed in numerical order. The Comparable interface specifies one method, compareTo, that determines how the objects are to be sequenced. This method returns a negative integer, zero, or positive integer to indicate the current object is less than, equal to, or greater than the object passed to method. For example, consider a garden tools class that has two properties: SKU and description. The natural order might be to store the tools in ascending SKU sequence.

If you want to sequence the set in a different order, you can use the Comparator interface. This interface contains one method; compare. For example, we might want to build a set that stores garden tools in ascending description order. Rather than modify the compareTo method (and potentially upset applications that rely on that sequence), we can build a separate class that implements the Comparator interface.

To learn how to use the Comparable and Comparator interfaces, follow these seven steps.

  1. Open your text editor and type in the following Java statements:Java Source for Garden ToolNotice that the class implements Comparable. The class therefore must implement the compareTo method. Also notice the presence of an equals method. In Java, a set cannot have duplicates. The equals method is called when an object is added to a set to determine if an equivalent object already exists on the set.
  2. Save your file as
  3. Open a command prompt and navigate to the directory containing your Java program. Then type in the command to compile the source and hit Enter.Compile Source for Garden Tools
  4. Next you will create the program to create various sets using Comparable and Comparator. In your text editor, type in the following Java statements:Java Source for Testing Garden ToolsThe program creates three tree sets. The first set stores integers (line 5). The Integer class implements the Comparable interface and therefore the integers will be stored in a natural order, i.e., 1, 2, 3. The next set (created at line 15) stores GardenTool objects. As noted earlier, the GardenTool class implements Comparable and specifies an order of SKU in the compareTo method. The final set is created with a Comparator implementation passed to the constructor (line 24). The implementation class, GardenToolComparator, is contained in the source code. The compare method dictates that objects will be sequenced in description order.
  5. Save your file as
  6. Open a command prompt and navigate to the directory containing your Java program. Then type in the command to compile the source and hit Enter.Compile Source for Testing Garden Tools
  7. Type in the command to run your program and hit Enter.Run for Test Garden ToolsNotice in the output that the integer set is listed in ascending numerical sequence. The next set is listed in SKU sequence, and the final set of garden tools is listed in description order.

Related Articles

  1. How to Check Object Type in Java
  2. How to Create a Jar File in Java
  3. How to Compile Packages in Java
  4. How to Throw an Exception in Java
  5. How to Create an Exception Class in Java
  6. How to Use the super Keyword to Call a Base Class Constructor in Java
  7. How to Use the Comparator.comparing Method in Java 8
  8. How to Use in Java
  9. How to Call an Interface Method in Java
  10. How to Add a Time Zone in the Java 8 Date/Time API
  11. How to Rethrow an Exception in Java
  12. How to Use the instanceof Operator with a Generic Class in Java
  13. How to Instantiate an Object in Java
  14. How to Filter Distinct Elements from a Collection in Java 8
  15. How to Create a Derived Class in Java
  16. How to Skip Elements with the Skip Method in Java 8
  17. How to Create a Java Bean
  18. How to Implement an Interface in Java
  19. How to Compare Two Objects with the equals Method in Java
  20. How to Set PATH from JAVA_HOME
  21. How to Prevent Race Conditions in Java 8
  22. How to Write a Block of Code in Java
  23. How to Display the Contents of a Directory in Java
  24. How to Group and Partition Collectors in Java 8
  25. How to Create a Reference to an Object in Java
  26. How to Reduce the Size of the Stream with the Limit Method in Java 8
  27. How to Write an Arithmetic Expression in Java
  28. How to Format Date and Time in the Java 8 Date/Time API
  29. How to Use Comparable and Comparator in Java (this article)
  30. How to Break a Loop in Java
  31. How to Use the this Keyword to Call Another Constructor in Java
  32. How to Write a Unit Test in Java
  33. How to Declare Variables in Java
  34. How to Override Base Class Methods with Derived Class Methods in Java
  35. How to Use Serialized Objects in Java
  36. How to Write Comments in Java
  37. How to Implement Functional Interfaces in Java 8
  38. How to Write Type Parameters with Multiple Bounds in Java
  39. How to Add Type and Repeating Annotations to Code in Java 8
  40. How to Use Basic Generics Syntax in Java
  41. How to Map Elements Using the Map Method in Java 8
  42. How to Work with Properties in Java
  43. How to Write while and do while Loops in Java
  44. How to Use the finally Block in Java
  45. How to Write for-each Loops in Java
  46. How to Create a Method in Java
  47. How to Continue a Loop in Java
  48. How to Handle Java Files with Streams
  49. How to Create an Interface Definition in Java
  50. How Default Base Class Constructors Are Used with Inheritance