Map集合

Map实现类用于保存具有映射关系的数据。Map保存的每项数据都是key-value对，也就是由key和value两个值组成。Map里的key是不可重复的，key用户标识集合里的每项数据。其中最主要的实现类是HashMap和TreeMap。

Map集合的体系结构：

image

HashMap

HashMap类实现了Map接口，HashMap是一个无序的key-value集合。

有下面几个特点：

1. HashMap类是基于key来包含value的。

2. HashMap类的key值不能重复。

3. HashMap类可以包含null值，并且可以包含多个。

4. HashMap类构造函数默认的大小是16，装载因子是0.75.

构造函数：

1. HashMap() 创建一个空的HashMap。

2. HashMap(Map<? extends K,? extends V> m) 基于一个map集合来创建一个新的集合。

3. HashMap(int capacity) 创建一个指定大小的map集合。

4. HashMap(int capacity, float loadFactor) 创建一个指定大小和指定装载因子的map集合。

常用方法：

method	Description
void clear()	It is used to remove all of the mappings from this map.
boolean isEmpty()	It is used to return true if this map contains no key-value mappings.
Object clone()	It is used to return a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
Set entrySet()	It is used to return a collection view of the mappings contained in this map.
Set keySet()	It is used to return a set view of the keys contained in this map.
V put(Object key, Object value)	It is used to insert an entry in the map.
void putAll(Map map)	It is used to insert the specified map in the map.
V remove(Object key)	It is used to delete an entry for the specified key.
boolean remove(Object key, Object value)	It removes the specified values with the associated specified keys from the map.
boolean containsValue(Object value)	This method returns true if some value equal to the value exists within the map, else return false.
boolean containsKey(Object key)	This method returns true if some key equal to the key exists within the map, else return false.
boolean equals(Object o)	It is used to compare the specified Object with the Map.
V get(Object key)	This method returns the object that contains the value associated with the key.
V getOrDefault(Object key, V defaultValue)	It returns the value to which the specified key is mapped, or defaultValue if the map contains no mapping for the key.
Collection values()	It returns a collection view of the values contained in the map.
int size()	This method returns the number of entries in the map.

遍历方式

1. for循环遍历

for (Map.Entry entry : mapCollection.entrySet()) {
 // other code
}

1. Iterator遍历

Iterator iterator = mapCollection.entrySet().iterator();
while (iterator.hasNext()) {
 Map.Entry entry = (Map.Entry) iterator.next();
 // other code
}

举个例子：

import java.util.HashMap;
import java.util.Map;
import java.util.Iterator;
import java.util.Set;
public class Details {
​
 public static void main(String args[]) {
​
 /* This is how to declare HashMap */
 HashMap<Integer, String> hmap = new HashMap<Integer, String>();
​
 /*Adding elements to HashMap*/
 hmap.put(12, "Chaitanya");
 hmap.put(2, "Rahul");
 hmap.put(7, "Singh");
 hmap.put(49, "Ajeet");
 hmap.put(3, "Anuj");
​
 /* Display content using Iterator*/
 Set set = hmap.entrySet();
 Iterator iterator = set.iterator();
 while(iterator.hasNext()) {
 Map.Entry mentry = (Map.Entry)iterator.next();
 System.out.print("key is: "+ mentry.getKey() + " & Value is: ");
 System.out.println(mentry.getValue());
 }
​
 /* Get values based on key*/
 String var= hmap.get(2);
 System.out.println("Value at index 2 is: "+var);
​
 /* Remove values based on key*/
 hmap.remove(3);
 System.out.println("Map key and values after removal:");
 Set set2 = hmap.entrySet();
 Iterator iterator2 = set2.iterator();
 while(iterator2.hasNext()) {
 Map.Entry mentry2 = (Map.Entry)iterator2.next();
 System.out.print("Key is: "+mentry2.getKey() + " & Value is: ");
 System.out.println(mentry2.getValue());
 }
​
 }
}