利用Maps

Map（接口） 维持“键－值”对应干系（对），以便通过一个键查找相应的值
HashMap＊ 基于一个散列表实现（用它取代Hashtable）。针对“键－值”对的插入和检索，这种形式具有最不变的机能。可通过构建器对这一机能举办调解，以便配置散列表的“本领”和“装载因子”
ArrayMap 由一个ArrayList后推获得的Map。对重复的顺序提供了准确的节制。面向很是小的Map设计，出格是那些需要常常建设和删除的。对付很是小的Map，建设和重复所支付的价钱要比HashMap低得多。但在Map变大今后，机能也会相应地大幅度低落
TreeMap 在一个“红－黑”树的基本上实现。查察键可能“键－值”对时，它们会按牢靠的顺序分列（取决于Comparable或Comparator，稍后即会讲到）。TreeMap最大的长处就是我们获得的是已排好序的功效。TreeMap是含有subMap()要领的独一一种Map，操作它可以返回树的一部门。

Map (interface)	Maintains key-value associations (pairs), so you can look up a value using a key.
HashMap*	Implementation based on a hash table. (Use this instead of Hashtable.) Provides constant-time performance for inserting and locating pairs. Performance can be adjusted via constructors that allow you to set the capacity and load factor of the hash table.
TreeMap	Implementation based on a red-black tree. When you view the keys or the pairs, they will be in sorted order (determined by Comparable or Comparator, discussed later). The point of a TreeMap is that you get the results in sorted order. TreeMap is the only Map with the subMap() method, which allows you to return a portion of the tree.

下例包括了两套测试数据以及一个fill()要领，操作该要领可以用任何两维数组（由Object组成）填充任何Map。这些东西也会在其他Map例子顶用到。
 

//: Map1.java
// Things you can do with Maps
package c08.newcollections;
import java.util.*;

public class Map1 {
  public final static String[][] testData1 = {
    { "Happy", "Cheerful disposition" },
    { "Sleepy", "Prefers dark, quiet places" },
    { "Grumpy", "Needs to work on attitude" },
    { "Doc", "Fantasizes about advanced degree"},
    { "Dopey", "'A' for effort" },
    { "Sneezy", "Struggles with allergies" },
    { "Bashful", "Needs self-esteem workshop"},
  };
  public final static String[][] testData2 = {
    { "Belligerent", "Disruptive influence" },
    { "Lazy", "Motivational problems" },
    { "Comatose", "Excellent behavior" }
  };
  public static Map fill(Map m, Object[][] o) {
    for(int i = 0; i < o.length; i++)
      m.put(o[i][0], o[i][1]);
    return m;
  }
  // Producing a Set of the keys:
  public static void printKeys(Map m) {
    System.out.print("Size = " + m.size() +", ");
    System.out.print("Keys: ");
    Collection1.print(m.keySet());
  }
  // Producing a Collection of the values:
  public static void printValues(Map m) {
    System.out.print("Values: ");
    Collection1.print(m.values());
  }
  // Iterating through Map.Entry objects (pairs):
  public static void print(Map m) {
    Collection entries = m.entries();
    Iterator it = entries.iterator();
    while(it.hasNext()) {
      Map.Entry e = (Map.Entry)it.next();
      System.out.println("Key = " + e.getKey() +
        ", Value = " + e.getValue());
    }
  }
  public static void test(Map m) {
    fill(m, testData1);
    // Map has 'Set' behavior for keys:
    fill(m, testData1);
    printKeys(m);
    printValues(m);
    print(m);
    String key = testData1[4][0];
    String value = testData1[4][1];
    System.out.println("m.containsKey(\"" + key +
      "\"): " + m.containsKey(key));
    System.out.println("m.get(\"" + key + "\"): "
      + m.get(key));
    System.out.println("m.containsValue(\"" 
      + value + "\"): " + 
      m.containsValue(value)); 
    Map m2 = fill(new TreeMap(), testData2);
    m.putAll(m2);
    printKeys(m);
    m.remove(testData2[0][0]);
    printKeys(m);
    m.clear();
    System.out.println("m.isEmpty(): " 
      + m.isEmpty());
    fill(m, testData1);
    // Operations on the Set change the Map:
    m.keySet().removeAll(m.keySet());
    System.out.println("m.isEmpty(): " 
      + m.isEmpty());
  }
  public static void main(String args[]) {
    System.out.println("Testing HashMap");
    test(new HashMap());
    System.out.println("Testing TreeMap");
    test(new TreeMap());
  }
} ///:~

#p#分页标题#e#

printKeys()，printValues()以及print()要领并不可是有用的东西，它们也清楚地展现了一个Map的Collection“情形”的发生进程。keySet()要了解发生一个Set，它由Map中的键后推得来。在这儿，它只被看成一个Collection看待。values()也获得了雷同的看待，它的浸染是发生一个List，个中包括了Map中的所有值（留意键必需是唯一无二的，而值可以有反复）。由于这些Collection是由Map后推获得的，所以一个Collection中的任何改变城市在相应的Map中反应出来。
print()要领的浸染是收集由entries发生的Iterator（重复器），并用它同时打印出每个“键－值”对的键和值。措施剩余的部门提供了每种Map操纵的简朴示例，并对每种范例的Map举办了测试。
当建设本身的类，将其作为Map中的一个键利用时，必需留意到和以前的Set沟通的问题。