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:
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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);
}
}
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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
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/**
* 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();
}
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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
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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) {
}
}
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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
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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();
}
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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:
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/**
* The head of the doubly linked list.
*/
/定义了链头
private transient Entry<K,V> header;
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初始化链表的方法:
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void init() {
header = new Entry<K,V>(- 1 , null , null , null );
header.before = header.after = header;
}
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TreeMap:
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//定义根节点
private transient Entry<K,V> root = null ;
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再看他的put方法,是不是很面熟(二叉排序树的插入操作)
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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 ;
}
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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
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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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原文链接:https://blog.csdn.net/kyi_zhu123/article/details/52769469