Java集合之HashMap用法详解

import java.util.Map;
import java.util.HashMap;
public class lzwCode {
  public static void main(String [] args) {
    Map<Integer, String> map = new HashMap<Integer, String>();
    map.put(1, "Barcelona");
    map.put(2, "RealMadrid");
    map.put(3, "ManchesterUnited");
    map.put(4, "AC milan");
    map.put(5, null);
    map.put(null, null);
    //map.put(null, "Chelsea"); //可以运行键值都为空(如果键相同，后者覆盖前者)
    System.out.println(map);
    System.out.print(map.keySet()+" "); //集合中所有键以Set集合形式返回
    System.out.println();
    System.out.print(map.values()+" "); //集合中所有键以Collection集合形式返回
    System.out.println();
    System.out.println("集合大小:"+map.size());
    System.out.println("是否包含该键:"+map.containsKey(2));//返回boolean
    System.out.println("是否包含该值:"+map.containsValue("Barcelona"));//返回boolean
    System.out.println(map.isEmpty()); //不包含键-值映射关系,则返回true
    map.remove(5); //删除映射关系
    System.out.println(map);
    map.clear();//清空集合
    System.out.println(map);
  }
}

import java.util.Map;
import java.util.HashMap;
import java.util.Iterator;
public class lzwCode {
  public static void main(String [] args) {
    Map<String, Integer> map = new HashMap<String, Integer>();
    map.put("Barcelona", 1);
    map.put("RealMadrid", 2);
    map.put("ManchesterUnited", 3);
    map.put("AC milan", 4);
    map.put("Chelsea", 5);
    //第一种：(效率高)
    System.out.println("第一种方法：");
    Iterator iter = map.entrySet().iterator();
    while (iter.hasNext()) {
      Map.Entry entry = (Map.Entry)iter.next();
      Object key = entry.getKey();
      Object val = entry.getValue();
      System.out.println("键:"+key+"<==>"+"值:"+val);
    }
    //第二种：(效率低)
    System.out.println("第二种方法：");
    Iterator it = map.keySet().iterator();
    while (it.hasNext()) {
      Object key = it.next();
      Object val = map.get(key);
      System.out.println("键:"+key+"<==>"+"值:"+val);
    }
    //对于keySet只是遍历了2次，一次是转为iterator，一次就从HashMap中取出key所对于的value。
    //对于entrySet只是遍历了第一次,它把key和value都放到了entry中，所以快比keySet快些。
    System.out.println("For-Each循环输出");
    //For-Each循环
    for (Map.Entry<String, Integer> entry:map.entrySet()) {
      String key = entry.getKey().toString();
      String val = entry.getValue().toString();
      System.out.println("键:"+key+"<==>"+"值:"+val);
    }
  }
}