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Java Map.entry Java Map.entry案例详解

屌丝男士zkc   2021-08-16 我要评论
想了解Java Map.entry案例详解的相关内容吗屌丝男士zkc在本文为您仔细讲解Java Map.entry的相关知识和一些Code实例欢迎阅读和指正我们先划重点:Java,Map.entry,Java,Map.entry详解下面大家一起来学习吧。

   Map.entrySet() 这个方法返回的是一个Set<Map.Entry<K,V>>Map.Entry 是Map中的一个接口他的用途是表示一个映射项(里面有Key和Value)而Set<Map.Entry<K,V>>表示一个映射项的Set。Map.Entry里有相应的getKey和getValue方法即JavaBean让我们能够从一个项中取出Key和Value。

下面是遍历Map的四种方法:

public static void main(String[] args) {
 
 
  Map<String, String> map = new HashMap<String, String>();
  map.put("1", "value1");
  map.put("2", "value2");
  map.put("3", "value3");
  
  //第一种:普遍使用二次取值
  System.out.println("通过Map.keySet遍历key和value:");
  for (String key : map.keySet()) {
   System.out.println("key= "+ key + " and value= " + map.get(key));
  }
  
  //第二种
  System.out.println("通过Map.entrySet使用iterator遍历key和value:");
  Iterator<Map.Entry<String, String>> it = map.entrySet().iterator();
  while (it.hasNext()) {
   Map.Entry<String, String> entry = it.next();
   System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
  }
  
  //第三种:推荐尤其是容量大时
  System.out.println("通过Map.entrySet遍历key和value");
  for (Map.Entry<String, String> entry : map.entrySet()) {
   System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
  }
 
  //第四种
  System.out.println("通过Map.values()遍历所有的value但不能遍历key");
  for (String v : map.values()) {
   System.out.println("value= " + v);
  }
 }

下面是HashMap的源代码:

首先HashMap的底层实现用的时候一个Entry数组

/** 
     * The table, resized as necessary. Length MUST Always be a power of two. 
     */  
   transient Entry[] table; //声明了一个数组  
   ........  
   public HashMap() {  
        this.loadFactor = DEFAULT_LOAD_FACTOR;  
        threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);  
        table = new Entry[DEFAULT_INITIAL_CAPACITY];//初始化数组的大小为DEFAULT_INITIAL_CAPACITY(这里是16)  
        init();  
    }

再来看一下Entry是在什么地方定义的继续上源码,我们在HashMap的源码的674行发现了它的定义原来他是HashMap的一个内部类并且实现了Map.Entry接口以下有些地方是转载

static class Entry<K,V> implements Map.Entry<K,V> {  
    final K key;  
    V value;  
    Entry<K,V> next;  
    final int hash;  
  
    /** 
     * Creates new entry. 
     */  
    Entry(int h, K k, V v, Entry<K,V> n) {  
        value = v;  
        next = n;  
        key = k;  
        hash = h;  
    }  
  
    public final K getKey() {  
        return key;  
    }  
  
    public final V getValue() {  
        return value;  
    }  
  
    public final V setValue(V newValue) {  
 V oldValue = value;  
        value = newValue;  
        return oldValue;  
    }  
  
    public final boolean equals(Object o) {  
        if (!(o instanceof Map.Entry))  
            return false;  
        Map.Entry e = (Map.Entry)o;  
        Object k1 = getKey();  
        Object k2 = e.getKey();  
        if (k1 == k2 || (k1 != null && k1.equals(k2))) {  
            Object v1 = getValue();  
            Object v2 = e.getValue();  
            if (v1 == v2 || (v1 != null && v1.equals(v2)))  
                return true;  
        }  
        return false;  
    }  
  
    public final int hashCode() {  
        return (key==null   ? 0 : key.hashCode()) ^  
               (value==null ? 0 : value.hashCode());  
    }  
  
    public final String toString() {  
        return getKey() + "=" + getValue();  
    }  
  
    /** 
     * This method is invoked whenever the value in an entry is 
     * overwritten by an invocation of put(k,v) for a key k that's already 
     * in the HashMap. 
     */  
    void recordAccess(HashMap<K,V> m) {  
    }  
  
    /** 
     * This method is invoked whenever the entry is 
     * removed from the table. 
     */  
    void recordRemoval(HashMap<K,V> m) {  
    }  
}  

既然这样那我们再看一下Map.Entry这个接口是怎么定义的原来他是Map的一个内部接口并且定义了一些方法

  interface Entry<K,V> {  
    /** 
 * Returns the key corresponding to this entry. 
 * 
 * @return the key corresponding to this entry 
        * @throws IllegalStateException implementations may, but are not 
        *         required to, throw this exception if the entry has been 
        *         removed from the backing map. 
 */  
K getKey();  
  
    /** 
 * Returns the value corresponding to this entry.  If the mapping 
 * has been removed from the backing map (by the iterator's 
 * <tt>remove</tt> operation), the results of this call are undefined. 
 * 
 * @return the value corresponding to this entry 
        * @throws IllegalStateException implementations may, but are not 
        *         required to, throw this exception if the entry has been 
        *         removed from the backing map. 
 */  
V getValue();  
  
    /** 
 * Replaces the value corresponding to this entry with the specified 
 * value (optional operation).  (Writes through to the map.)  The 
 * behavior of this call is undefined if the mapping has already been 
 * removed from the map (by the iterator's <tt>remove</tt> operation). 
 * 
        * @param value new value to be stored in this entry 
        * @return old value corresponding to the entry 
        * @throws UnsupportedOperationException if the <tt>put</tt> operation 
        *         is not supported by the backing map 
        * @throws ClassCastException if the class of the specified value 
        *         prevents it from being stored in the backing map 
        * @throws NullPointerException if the backing map does not permit 
        *         null values, and the specified value is null 
        * @throws IllegalArgumentException if some property of this value 
        *         prevents it from being stored in the backing map 
        * @throws IllegalStateException implementations may, but are not 
        *         required to, throw this exception if the entry has been 
        *         removed from the backing map. 
        */  
V setValue(V value);  
  
/** 
 * Compares the specified object with this entry for equality. 
 * Returns <tt>true</tt> if the given object is also a map entry and 
 * the two entries represent the same mapping.  More formally, two 
 * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping 
 * if<pre> 
        *     (e1.getKey()==null ? 
        *      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  && 
        *     (e1.getValue()==null ? 
        *      e2.getValue()==null : e1.getValue().equals(e2.getValue())) 
        * </pre> 
 * This ensures that the <tt>equals</tt> method works properly across 
 * different implementations of the <tt>Map.Entry</tt> interface. 
 * 
 * @param o object to be compared for equality with this map entry 
 * @return <tt>true</tt> if the specified object is equal to this map 
 *         entry 
        */  
boolean equals(Object o);  
  
/** 
 * Returns the hash code value for this map entry.  The hash code 
 * of a map entry <tt>e</tt> is defined to be: <pre> 
 *     (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^ 
 *     (e.getValue()==null ? 0 : e.getValue().hashCode()) 
        * </pre> 
 * This ensures that <tt>e1.equals(e2)</tt> implies that 
 * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries 
 * <tt>e1</tt> and <tt>e2</tt>, as required by the general 
 * contract of <tt>Object.hashCode</tt>. 
 * 
 * @return the hash code value for this map entry 
 * @see Object#hashCode() 
 * @see Object#equals(Object) 
 * @see #equals(Object) 
 */  
int hashCode();  
   }  

看到这里的时候大伙儿估计都明白得差不多了为什么HashMap为什么要选择Entry数组来存放key-value对了吧因为Entry实现的Map.Entry接口里面定义了getKey(),getValue()setKey(),setValue()等方法相当于一个javaBean对键值对进行了一个封装便于后面的操作从这里我们其实也可以联想到不光是HashMap譬如LinkedHashMap,TreeMap 等继承自map的容器存储key-value对都应该使用的是Entry只不过组织Entry的形式不一样HashMap用的是数组加链表的形式LinkedHashMap用的是链表的形式TreeMap应该使用的二叉树的形式不信的话上源码

LinkedHashMap:

/** 
    * The head of the doubly linked list. 
    */  
/定义了链头  
   private transient Entry<K,V> header;  

初始化链表的方法:

void init() {  
    header = new Entry<K,V>(-1, null, null, null);  
    header.before = header.after = header;  
}  

TreeMap:

//定义根节点  
 private transient Entry<K,V> root = null;  

再看他的put方法是不是很面熟(二叉排序树的插入操作)

public V put(K key, V value) {  
    Entry<K,V> t = root;  
    if (t == null) {  
 // TBD:  
 // 5045147: (coll) Adding null to an empty TreeSet should  
 // throw NullPointerException  
 //  
 // compare(key, key); // type check  
        root = new Entry<K,V>(key, value, null);  
        size = 1;  
        modCount++;  
        return null;  
    }  
    int cmp;  
    Entry<K,V> parent;  
    // split comparator and comparable paths  
    Comparator<? super K> cpr = comparator;  
    if (cpr != null) {  
        do {  
            parent = t;  
            cmp = cpr.compare(key, t.key);  
            if (cmp < 0)  
                t = t.left;  
            else if (cmp > 0)  
                t = t.right;  
            else  
                return t.setValue(value);  
        } while (t != null);  
    }  
    else {  
        if (key == null)  
            throw new NullPointerException();  
        Comparable<? super K> k = (Comparable<? super K>) key;  
        do {  
            parent = t;  
            cmp = k.compareTo(t.key);  
            if (cmp < 0)  
                t = t.left;  
            else if (cmp > 0)  
                t = t.right;  
            else  
                return t.setValue(value);  
        } while (t != null);  
    }  
    Entry<K,V> e = new Entry<K,V>(key, value, parent);  
    if (cmp < 0)  
        parent.left = e;  
    else  
        parent.right = e;  
    fixAfterInsertion(e);  
    size++;  
    modCount++;  
    return null;  
}  

ok明白了各种Map的底层存储key-value对的方式后再来看看如何遍历map吧这里用HashMap来演示吧

Map提供了一些常用方法如keySet()、entrySet()等方法keySet()方法返回值是Map中key值的集合;entrySet()的返回值也是返回一个Set集合此集合的类型为Map.Entry。

so,很容易写出如下的遍历代码

1.  Map map = new HashMap();  
  
         Irerator iterator = map.entrySet().iterator();  
  
         while(iterator.hasNext()) {  
  
                 Map.Entry entry = iterator.next();  
  
                 Object key = entry.getKey();  
  
                 //  
  
         }  
  
     2.Map map = new HashMap();   
  
         Set  keySet= map.keySet();  
  
         Irerator iterator = keySet.iterator;  
  
         while(iterator.hasNext()) {  
  
                 Object key = iterator.next();  
  
                 Object value = map.get(key);  
  
                 //  
  
         }  
另外还有一种遍历方法是单纯的遍历value值Map有一个values方法返回的是value的Collection集合。通过遍历collection也可以遍历value,如
[java] view plain copy
Map map = new HashMap();  
  
Collection c = map.values();  
  
Iterator iterator = c.iterator();  
  
while(iterator.hasNext()) {  
  
       Object value = iterator.next();   

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