Dictionary中的Dictionary类和Map类有什么区别？

Question

I was just seeing the difference between a HashMap and HashTable . Other than the obvious synchronized and unsynchronized difference, I noticed that HashTable extends the Dictionary class while HashMap implements Map . Both of them store <Key, Value> pair, so what's the difference. I tried Googling for it and I found few questions on SO ( 1 , 2 and 3 ). I didn't find anything satisfactory other than the fact that Dictionary class is obsolete. Is there any difference between thee two classes? If no, why is the dictionary class obsolete now?

Answer 1

Dictionary is an abstract class that contains no non-abstract methods. Modern practice strongly recommends using an interface rather than a class in such a case. Map is such an interface.

It also uses Enumeration which is also considered obsolete; Map uses Iterator which is not.

Answer 2

• Dictionary is an abstract class whereas Map is an interface, hence it is more Java-idiomatic.
• Dictionary was deprecated early in Java 1.2 and replaced by its better interface counterpart, Map . This leads to believed that Dictionary was just a bad design choice and the Java folks kept the Dictionary class for backward compatibility.

Answer 3

The main difference is that Dictionary is an abstract class and Map is a interface. Collections API uses interfaces like Map, List, Set.

Answer 4

Dictionary is an abstract base class of Hashtable. Both are still in JDK for backwards compatibility with old code. We are expected to use HashMap and other implementations of Map interface introduced in Java 1.2.