www百科

  1. 首页 > 生活知识 >

Map.entry案例详解 Java

生活百科

Java,Map.entry案例详解,博智网带你了解详细信息 。
   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= "https://www.yf-zs.com/redian/+ 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= "https://www.yf-zs.com/redian/+ 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= "https://www.yf-zs.com/redian/+ 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= "https://www.yf-zs.com/redian/+ v);} }
下面是HashMap的源代码：
【Map.entry案例详解 Java】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 = https://www.yf-zs.com/redian/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();}

推荐阅读


上一篇：北京回河北需要什么手续

下一篇：JavaScript实现九宫格点击变色效果