Should I override equals() and hashCode() for String key in HashMap, Java?

Question

I'm using HashMap<String, MySuperClass> collection, where the key is in String format. In general, in order to use HashMap collection, it's necessary to override equals() and hashCode() .

I want to clarify, should I override equals() and hashCode() for String , Integer , and other «primitive» classes in HashMap ?

Answer 1

不。 String s（和Integer s）已经实现了equals和hashCode ，并且您无法为它们覆盖它，因为这些类是final ，因此不可扩展。

Answer 2

When you use a class as a key for a HashMap (or HashSet , or Hashtable for that matter), you should make sure it overrides hashCode() and equals(Object) . The JDK's String , the primitive wrappers (such as Integer , Long , etc.) and many others already do this for you, so you don't need to worry about it.