用Java替换过时的Hashtable类

Question

When I tried to use the hashtable class, Netbeans gave me an error saying:

While still supported, these classes were made obsolete by the JDK1.2 collection classes, and should probably not be used in new development.

However, I can't seem to find an example online on a better replacement for Hashtable . Any suggestions?

Answer 1

The most direct replacement of a Hashtable is a HashMap .

One difference that could be important is that all relevant methods of Hashtable are synchronized while they are not synchronized on HashMap .

Answer 2

A better replacement for Hashtable is HashMap .

As for being obsolete, I have no reference to it, but the Javadoc states:

As of the Java 2 platform v1.2, this class was retrofitted to implement the Map interface, making it a member of the Java Collections Framework .

Hashtable is synchronized unlike HashMap .

Answer 3

You should use HashMap, but it is not designed for concurrent environments, where you may use ConcurrentHashMap .

OR

Map<K,V> myMap = Collections.synchronizedMap(/*any map instance*/);

Answer 4

There are cases when a method outside your control requires an Hashtable . Java's own javax.management.ObjectName is such an example ^[1] .

When the external interface requires it you have to use Hashtable . In netbeans, you can use:

@SupresssWarnings("UseOfObsoleteCollectionType")

to suppress the warning locally.

See

• Java: complete list of @SuppressWarnings(...) parameters (in Netbeans)?
• Suppress deprecated import warning in Java

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1: I wonder why they used Hashtable and not HashMap , since ObjectName is available since 1.5 and Hashtable is obsolete since 1.2

Answer 5

您可以使用Hashmap：

Map<String, Integer> mymap = new HashMap<String, Integer>();

Answer 6

Summary

The closest replacement is HashMap (usually via the Map interface).

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But note that Hashtable is thread-safe while HashMap is not. This is not a problem in most cases and it was intentional to make most Java collections non-thread-safe to avoid performance penalty for most common scenarios. But if you relied on Hashtable thread-safety and now need a replacement that would also be thread-safe then you have two options:

• If you need a simple synchronization wrapper for an existing Map - use Collections.synchronizedMap(...)
• If you need a class carefully and optimally designed for concurrent access - use ConcurrentHashMap (usually via the ConcurrentMap interface that extends the Map interface with additional concurrency features).