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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
-
java] view plain copy
-
<pre name=
"code"
class=
"java">
/**
-
* 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();
-
}</pre><br>
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:
-
[
java] view plain copy
-
//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
-
[java] view plain copy
-
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();
原文转至：https://blog.csdn.net/kyi_zhu123/article/details/52769469
发布者：全栈程序员-用户IM，转载请注明出处：https://javaforall.cn/190964.html原文链接：https://javaforall.cn
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