100 Java Mistakes and How to Avoid Them

7 Comparing objects

This chapter covers

• Why you should not use reference equality for boxed types
• Standard classes for which the equals() method behaves unexpectedly or is not defined at all
• How to properly implement equals(), hashCode(), compare(), and compareTo()
• What can happen if you use a malformed comparison method

Object comparison is exceptionally important in Java. There are several methods associated with object comparison: equals(), hashCode(), Comparable .compareTo(), and Comparator.compare(). They are used implicitly by many algorithms (e.g., Arrays.sort()) and data structures (e.g., HashMap). Failure to implement them correctly may lead to annoying, difficult-to-debug problems.

If you are going to implement any of these ...