Java 数据结构之Map总结

/**
     * Returns a {@code Set} containing all of the mappings in this {@code Map}. Each mapping is
     * an instance of {@link Map.Entry}. As the {@code Set} is backed by this {@code Map},
     * changes in one will be reflected in the other.
     *
     * @return a set of the mappings
     */
    public Set<Map.Entry<K,V>> entrySet(); 

/**
     * Returns a set of the keys contained in this {@code Map}. The {@code Set} is backed by
     * this {@code Map} so changes to one are reflected by the other. The {@code Set} does not
     * support adding.
     *
     * @return a set of the keys.
     */
    public Set<K> keySet();

/**
     * Returns a {@code Collection} of the values contained in this {@code Map}. The {@code Collection}
     * is backed by this {@code Map} so changes to one are reflected by the other. The
     * {@code Collection} supports {@link Collection#remove}, {@link Collection#removeAll},
     * {@link Collection#retainAll}, and {@link Collection#clear} operations,
     * and it does not support {@link Collection#add} or {@link Collection#addAll} operations.
     * <p>
     * This method returns a {@code Collection} which is the subclass of
     * {@link AbstractCollection}. The {@link AbstractCollection#iterator} method of this subclass returns a
     * "wrapper object" over the iterator of this {@code Map}'s {@link #entrySet()}. The {@link AbstractCollection#size} method
     * wraps this {@code Map}'s {@link #size} method and the {@link AbstractCollection#contains} method wraps this {@code Map}'s
     * {@link #containsValue} method.
     * <p>
     * The collection is created when this method is called at first time and
     * returned in response to all subsequent calls. This method may return
     * different Collection when multiple calls to this method, since it has no
     * synchronization performed.
     *
     * @return a collection of the values contained in this map.
     */
    public Collection<V> values();

 /**
     * {@code Map.Entry} is a key/value mapping contained in a {@code Map}.
     */
    public static interface Entry<K,V> {<span style="font-family: Tahoma, 'Microsoft Yahei', Simsun;">}</span>

entry用来迭代map
Map<String, String> map = new HashMap<String, String>();
		map.put("111", "aaa");
		map.put("222", "bbb");
		for (Entry<String, String> entry : map.entrySet()) {
			System.out.println(entry.getKey());
			System.out.println(entry.getValue());
		}

HashMap是无序的，TreeMap的顺序是插入的先后顺序Eg:JAVA 自带的Map接口是没有排序功能。即使你按照一定的顺序输入，但是输出结果也往往是随机的，对一些特殊的应用很不爽。这时候，可以使用TreeMap类进行转换一下就可以了。如果需要排序的功能，最好在new Map对象的时候，使用TreeMap. 但是如果对已经存在的Map进行按值排序，则需进行转换一下。

eg:

1、基本概述

Set<Map.Entry<K,V>> entrySet()  返回此映射中包含的映射关系的 set 视图。

Set<K>              keySet()      返回此映射中包含的键的 set 视图。

2、效率分析

对于keySet其实是遍历了2次，一次是转为iterator，一次就从hashmap中取出key所对于的value。而entryset只是遍历了第一次，他把key和value都放到了entry中，所以就快了。

3、使用举例

Map<String, String> maps = new HashMap<String, String>();   
  //方法一: 用entrySet()   
  Iterator<Entry<String,String>> it = maps.entrySet().iterator();   
  while(it.hasNext()){   
   Map.Entry<String,String> m = it.next();   
   String key = m.getKey();
   String value= m.getValue(); 
  }   
  // 方法二：jdk1.5支持，用entrySet()和For-Each循环()   
  for (Map.Entry<String, String> m : maps.entrySet()) {   
   String key = m.getKey();
   String value= m.getValue();    
  }   
  // 方法三：用keySet()   
  Iterator<String> it2 = maps.keySet().iterator();   
  while (it2.hasNext()){   
   String key = it2.next();
   String value= maps.get(key);
  }   
  // 方法四：jdk1.5支持，用keySet()和For-Each循环   
  for(String m: maps.keySet()){   
   String key = m;
   String value= maps.get(m);
  }

foreach和while的效率几乎是差不多的,而for则相对较慢一些。foreach可以替代掉for吗？显然不是。
foreach的内部原理其实还是 Iterator，但它不能像Iterator一样可以人为的控制,而且也不能调用iterator.remove()，更不能使用下标来方便的访问元素。因此foreach这种循环一般只适合做数组的遍历，提取数据显示等，不适合用于增加删除和使用下标等复杂的操作。

//写入值 标签<string name="">value</string>
	public static boolean saveSearchRecord(Context context,String key,String time){
		SharedPreferences sp = context.getSharedPreferences("search_record", 0);
		Editor e = sp.edit();
		//map 映射的总数，大于20条时，末尾插入一条数据
		int count = sp.getAll().size();
		if(count <20){
			e.putString(key, time);
			//e.putInt("count", count+1);
		}
		else{
			e.putString(key, time);
			Iterator iter = sp.getAll().entrySet().iterator();
			int cunIter = 1;
			Boolean flag =true;
			while (iter.hasNext()) {
				if(cunIter < sp.getAll().size()){
					cunIter++;
				}else{
					//迭代器iter.next(); 返回迭代的下一个对象，
					Map.Entry entry = (Map.Entry) iter.next();
					Log.v("Boolean", ""+iter.hasNext());
					Log.v("key", (String) entry.getKey());
					if(flag){
						e.remove((String) entry.getKey());
						flag= false;
					}
					
				}	
			}
				
		}
		return e.commit();
	}

//存HashMap所有值
	public static void saveHotNewsStatus(Context context,HashMap<String, String> map){
			SharedPreferences sp = context
				.getSharedPreferences("hotnews_status", 0);
			Editor e = sp.edit();
			
		
			Iterator iter = map.entrySet().iterator();
			
			while (iter.hasNext()) {
				Map.Entry entry = (Map.Entry) iter.next();
				String key = (String) entry.getKey();
				String val = (String) entry.getValue();
				
				e.putString(key, val);
			}
			e.commit();
			//　效率高,以后一定要使用此种方式！
	}