Java容器 HashMap

简介

在数组中我们是通过数组下标来对其内容索引的，而在Map中我们通过对象来对对象进行索引，用来索引的对象叫做key，其对应的对象叫做value。HashMap通过hashcode对其内容进行快速查找，HashMap中元素的排列顺序是不固定的。 

import java.util.*; 
 
class HashMapDemo { 
  public static void main(String args[]) { 
 
    // Create a hash map.
    HashMap<String, Double> hm = new HashMap<String, Double>(); 
     
    // Put elements to the map 
    hm.put("John Doe", new Double(3434.34)); 
    hm.put("Tom Smith", new Double(123.22)); 
    hm.put("Jane Baker", new Double(1378.00)); 
    hm.put("Tod Hall", new Double(99.22)); 
    hm.put("Ralph Smith", new Double(-19.08)); 
 
    // Get a set of the entries.
    Set<Map.Entry<String, Double>> set = hm.entrySet(); 
 
    // Display the set.
    for(Map.Entry<String, Double> me : set) {
      System.out.print(me.getKey() + ": "); 
      System.out.println(me.getValue()); 
    } 
 
    System.out.println(); 
 
    // Deposit 1000 into John Doe's account.
    double balance = hm.get("John Doe"); 
    hm.put("John Doe", balance + 1000); 
 
    System.out.println("John Doe's new balance: " + 
      hm.get("John Doe")); 
  } 
}
程序运行结果:

C:/test>java   HashMapDemo
Ralph Smith: -19.08
Tom Smith: 123.22
John Doe: 3434.34
Tod Hall: 99.22
Jane Baker: 1378.0

John Doe's new balance: 4434.34