Java 简明教程

Java HashMap Class

Introduction

Java HashMap 类是 Map 接口的基于哈希表实现，以下是有关 HashMap 的一些要点：

The Java HashMap class is the Hash table based implementation of the Map interface.Following are the important points about HashMap −

  1. This class makes no guarantees as to the iteration order of the map; in particular, it does not guarantee that the order will remain constant over time.

  2. This class permits null values and the null key.

Class declaration

以下是 java.util.HashMap 类的声明：

Following is the declaration for java.util.HashMap class −

public class HashMap<K,V>
   extends AbstractMap<K,V>
   implements Map<K,V>, Cloneable, Serializable

Parameters

以下是 java.util.HashMap 类的参数：

Following is the parameter for java.util.HashMap class −

  1. K − This is the type of keys maintained by this map.

  2. V − This is the type of mapped values.

Class constructors

Class methods

Methods inherited

此类从以下类中继承方法：

This class inherits methods from the following classes −

  1. java.util.AbstractMap

  2. java.util.Object

  3. java.util.Map

Example

以下程序说明了 HashMap 集合支持的几种方法 -

The following program illustrates several of the methods supported by HashMap collection −

import java.util.*;
public class HashMapDemo {

   public static void main(String args[]) {

      // Create a hash map
      HashMap hm = new HashMap();

      // Put elements to the map
      hm.put("Zara", new Double(3434.34));
      hm.put("Mahnaz", new Double(123.22));
      hm.put("Ayan", new Double(1378.00));
      hm.put("Daisy", new Double(99.22));
      hm.put("Qadir", new Double(-19.08));

      // Get a set of the entries
      Set set = hm.entrySet();

      // Get an iterator
      Iterator i = set.iterator();

      // Display elements
      while(i.hasNext()) {
         Map.Entry me = (Map.Entry)i.next();
         System.out.print(me.getKey() + ": ");
         System.out.println(me.getValue());
      }
      System.out.println();

      // Deposit 1000 into Zara's account
      double balance = ((Double)hm.get("Zara")).doubleValue();
      hm.put("Zara", new Double(balance + 1000));
      System.out.println("Zara's new balance: " + hm.get("Zara"));
   }
}

这会产生以下结果 −

This will produce the following result −

Output

Daisy: 99.22
Ayan: 1378.0
Zara: 3434.34
Qadir: -19.08
Mahnaz: 123.22

Zara's new balance: 4434.34