操纵Maps[Java编程]

Map（接口） 保持“键－值”对应关系（对）,以便通过一个键查找呼应的值
HashMap＊ 基于一个散列表实现（用它替换Hashtable）.针对“键－值”对的插入和检索,这种情势具有最安定的性能.可通过构建器对这一性能举行调整,以便设置散列表的“本领”和“装载因子”
ArrayMap 由一个ArrayList后推得到的Map.对反复的次序供应了切确的掌握.面向非常小的Map计划,分外是那些需求常常成立和删除的.关于非常小的Map,成立和反复所付出的代价要比HashMap低得多.但在Map变大今后,性能也会呼应地大幅度降低
TreeMap 在一个“红－黑”树的底子上实现.查看键大概“键－值”对时,它们会按固定的次序布列（取决于Comparable或Comparator,稍后即会讲到）.TreeMap最大的好处就是我们得到的是已排好序的后果.TreeMap是含有subMap()办法的唯一一种Map,操纵它可以返回树的一部份.

下例包含了两套测试数据以及一个fill()办法,操纵该办法可以用任何两维数组（由Object构成）填充当何Map.这些工具也会在其他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());
  }
} ///:~