Map集合的遍历方式
先看下Map集合的部分源码：
package java.util;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Function;
import java.io.Serializable;
public interface Map<K,V> {
/**
* Returns the number of key-value mappings in this map.
* @return the number of key-value mappings in this map
*/
//返回的是集合的大小
int size();
/**
* Returns <tt>true</tt> if this map contains no key-value mappings.
*/
// 判断集合中是否包含键值对 的元素
boolean isEmpty();
/**
* Returns <tt>true</tt> if this map contains a mapping for the specified
* key.
*/
// 是否包含这个键
boolean containsKey(Object key);
/**
* Returns <tt>true</tt> if this map maps one or more keys to the
* specified value.
*/
// 是否包含这个值
boolean containsValue(Object value);
/**
* Returns the value to which the specified key is mapped,
*
*/
// 返回的是包含这个键的值
V get(Object key);
// Modification Operations
/**
* Associates the specified value with the specified key in this map
* (optional operation).
*/
// 将键值对添加到Map集合中
V put(K key, V value);
/**
* Removes the mapping for a key from this map if it is present
* (optional operation).
*/
// 删除集合中键对应的值
V remove(Object key);
// Bulk Operations
/**
* Copies all of the mappings from the specified map to this map
* (optional operation). The effect of this call is equivalent to that
* of calling {@link #put(Object,Object) put(k, v)} on this map once
* for each mapping from key <tt>k</tt> to value <tt>v</tt> in the
* specified map. The behavior of this operation is undefined if the
* specified map is modified while the operation is in progress.
*
* @param m mappings to be stored in this map
* @throws UnsupportedOperationException if the <tt>putAll</tt> operation
* is not supported by this map
* @throws ClassCastException if the class of a key or value in the
* specified map prevents it from being stored in this map
* @throws NullPointerException if the specified map is null, or if
* this map does not permit null keys or values, and the
* specified map contains null keys or values
* @throws IllegalArgumentException if some property of a key or value in
* the specified map prevents it from being stored in this map
*/
void putAll(Map<? extends K, ? extends V> m);
/**
* Removes all of the mappings from this map (optional operation).
* The map will be empty after this call returns.
*
* @throws UnsupportedOperationException if the <tt>clear</tt> operation
* is not supported by this map
*/
void clear();
// Views
/**
* @return a set view of the keys contained in this map
*/
// 返回的是
Set<K> keySet();
/**
* Returns a {@link Collection} view of the values contained in this map.
* @return a collection view of the values contained in this map
*/
// 返回的是Map集合中的值
Collection<V> values();
/**
*
*
* @return a set view of the mappings contained in this map
*/
// 返回的是map集合中的键值对的映射，也就是键值对
Set<Map.Entry<K, V>> entrySet();
/**
* A map entry (key-value pair).
*/
// 获取的是键值对
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).
*
* @
*/
V setValue(V value);
/**
* Compares the specified object with this entry for equality.
*/
boolean equals(Object o);
/**
* Returns the hash code value for this map entry.
*/
int hashCode();
/**
* Returns a comparator that compares {@link Map.Entry} in natural order on key.
*
* <p>The returned comparator is serializable and throws {@link
* NullPointerException} when comparing an entry with a null key.
*
* @param <K> the {@link Comparable} type of then map keys
* @param <V> the type of the map values
* @return a comparator that compares {@link Map.Entry} in natural order on key.
* @see Comparable
* @since 1.8
*/
public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K,V>> comparingByKey() {
return (Comparator<Map.Entry<K, V>> & Serializable)
(c1, c2) -> c1.getKey().compareTo(c2.getKey());
}
/**
* Returns a comparator that compares {@link Map.Entry} in natural order on value.
*
* <p>The returned comparator is serializable and throws {@link
* NullPointerException} when comparing an entry with null values.
*
* @param <K> the type of the map keys
* @param <V> the {@link Comparable} type of the map values
* @return a comparator that compares {@link Map.Entry} in natural order on value.
* @see Comparable
* @since 1.8
*/
public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K,V>> comparingByValue() {
return (Comparator<Map.Entry<K, V>> & Serializable)
(c1, c2) -> c1.getValue().compareTo(c2.getValue());
}
Map<Integer,String> map = new HashMap<>();
map.put(1, "shitian");
map.put(2, "wangca");
map.put(3, "sss");① 遍历方式一：通过Map的key的值获取value值，使用的是 V get(Object key)，获取键的值
// 集合的键是一个Set集合,通过map.get(key)获取集合的值
Set<Integer> keys = map.keySet();
for (Integer key : keys) {
System.out.println("键是 ：" + key + "值是：" + map.get(key));
}② 遍历方式二：通过调用Map集合的values()获取所有键的值
Collection<String> values = map.values();
for (String s : values) {
System.out.println("值：" + s);
}③ 通过迭代器遍历整个键值对的集合
// map.entrySet()获取键值对的集合，然后通过迭代器进行遍历
// 键值对的集合是通过Map.Entry<Integer,String>来接收的
// Map中的内部类Entry，内部类里面有getKey()和getValue()的方法
Iterator<Map.Entry<Integer, String>> iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
Map.Entry<Integer,String> maps = iterator.next();
int key = maps.getKey();
String value = maps.getValue();
System.out.println(key + "----" + value);Map.Entry是Map的一个内部接口。对于Map中提供的一些常用方法，如map.keySet()、map.entrySet()、map.values()等方法大多数人都不是很熟悉。map.keySet()方法返回值是Map中key值的集合；map.entrySet()的返回值是一个Set集合，Map.Entry是Map声明的一个内部接口，此接口为泛型，定义为Entry<K,V>。它表示的是Map中的一个实体（key-value）。接口中有我们常用的getKey(),getValue方法。
④ 通过返回键值对的映射，Set进行接收，增强for循环进行遍历
Set<Map.Entry<Integer, String>> set = map.entrySet();
for (Map.Entry<Integer, String> entry : set) {
System.out.println(entry.getKey() + "--" + entry.getValue());