1.声明及初始化Map

1.1 先定义,后赋值

Map<String , Object> map = new HashMap<String , Object>();
map.put("key1", "value1");
map.put("key2", "value2");
map.put("keyN", "valueN");

1.2 双括号初始化法

Map<String , String> map = new HashMap<String , Object>(){{
put("key1", "value1");
put("key2", "value2");
put("keyN", "valueN");
}};

1.3 使用初始块

public class MyMap {
Map<String,String> maps;
// 使用初始块定义
{
maps = new HashMap<>();
maps.put("key1","value1");
maps.put("key2","value2");
maps.put("key3","value3");
}

public void printMap(){
System.out.println(maps);
// 输出:{key1=value1, key2=value2, key3=value3}
}
}

2.遍历Map

2.1 key=>value

Map<Integer, Integer> map = new HashMap<Integer, Integer>(); 
for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
}

2.2 遍历keys或values

Map<Integer, Integer> map = new HashMap<Integer, Integer>(); 
//遍历map中的键
for (Integer key : map.keySet()) {
System.out.println("Key = " + key);
}
//遍历map中的值
for (Integer value : map.values()) {
System.out.println("Value = " + value);
}

2.3 使用Iterator遍历

2.3.1 使用泛型

Map<Integer, Integer> map = new HashMap<Integer, Integer>(); 
Iterator<Map.Entry<Integer, Integer>> entries = map.entrySet().iterator();
while (entries.hasNext()) {
Map.Entry<Integer, Integer> entry = entries.next();
System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
}


// 在keySet上

Iterator<String> iterator = map.keySet().iterator();
while (iterator.hasNext()) {
String key = iterator.next();
System.out.println("MyClass.testMap key=" + key);
}


//在values上

Iterator<Integer> iterator2 = map.values().iterator();
while (iterator2.hasNext()){
Integer values = iterator2.next();
System.out.println("MyClass.testMap values="+values);
}

2.3.2 不使用泛型

Map map = new HashMap(); 
Iterator entries = map.entrySet().iterator();
while (entries.hasNext()) {
Map.Entry entry = (Map.Entry) entries.next();
Integer key = (Integer)entry.getKey();
Integer value = (Integer)entry.getValue();
System.out.println("Key = " + key + ", Value = " + value);
}

2.4 通过键找值遍历(效率低)

Map<Integer, Integer> map = new HashMap<Integer, Integer>(); 
for (Integer key : map.keySet()) {
Integer value = map.get(key);
System.out.println("Key = " + key + ", Value = " + value);
}

3.map接口方法使用

3.1 void clear():删除该Map对象中的所有key-value对

clear():删除该Map对象中的所有key-value对

Map<String,String>  maps = new HashMap<>();
maps.put("key1","value1");
maps.put("key2","value2");
System.out.println("clear前:" + maps);
maps.clear();
System.out.println("clear后:" + maps);

/**
* 输出:
clear前:{key1=value1, key2=value2}
clear后:{}
*/

3.2 boolean containsKey(Object key)

containsKey(Object key):查询Map中是否包含指定的key,如果包含则返回true

Map<String,String>  maps = new HashMap<>();
maps.put("key1","value1");
maps.put("key2","value2");
System.out.println("判断key1是否存在:" + maps.containsKey("key1"));
System.out.println("判断key8是否存在:" + maps.containsKey("key8"));
/**
* 输出:
判断key1是否存在:true
判断key8是否存在:false
*/

3.3 boolean containsValue(Object value)

查询Map中是否包含一个或多个value,如果包含则返回true。

Map<String,String>  maps = new HashMap<>();
maps.put("key1","value1");
maps.put("key2","value2");
System.out.println("判断value1是否存在:" + maps.containsValue("value1"));
System.out.println("判断value8是否存在:" + maps.containsValue("value8"));

/**
* 输出:
判断value1是否存在:true
判断value8是否存在:false
*/

3.4 Set entrySet()

Set entrySet():返回Map中包含的key-value对所组成的Set集合,每个集合元素都是Map.Entry (Entry是Map的内部类)对象。

Map<Integer, Integer> map = new HashMap<Integer, Integer>(); 
for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
}

3.5 Object get(Object key)

返回指定key所对应的value;如果此Map中不包含该key,则返回null。

Map<String,String>  maps = new HashMap<>();
maps.put("key1","value1");
maps.put("key2","value2");
System.out.println("key1的值:" + maps.get("key1"));
System.out.println("key2的值:" + maps.get("key2"));

/**
* 输出:
key1的值:value1
key2的值:value2
*/

3.6 boolean isEmpty()

查询该Map是否为空(即不包含任何key-value对),如果为空则返回true。

Map<String,String>  map1 = new HashMap<>();
map1.put("key1","value1");
map1.put("key2","value2");
Map<String,String> map2 = new HashMap<>();
System.out.println("map1是否为空:" + map1.isEmpty());
System.out.println("map2是否为空:" + map2.isEmpty());

/**
* 输出:
map1是否为空:false
map2是否为空:true
*/

3.7 Set keySet()

返回该Map中所有key组成的Set集合。

Map<String,String>  map1 = new HashMap<>();
map1.put("key1","value1");
map1.put("key2","value2");
System.out.println("keySet结果:" + map1.keySet());

// 输出:keySet结果:[key1, key2]

3.8 Object put(Object key, Object value):

添加一个key-value对,如果当前Map中已有一个与该key相等的key-value对,则新的key-value对会覆盖原来的key-value对。

Map<String,String>  map1 = new HashMap<>();
map1.put("key1","value1");
map1.put("key1","value11");
System.out.println("keySet结果:" + map1.keySet());

//输出:map1结果:{key1=value11, key2=value2}

3.9 void putAll(Map m)

将指定Map中的key-value对复制到本Map中。

Map<String,String>  map1 = new HashMap<>();
map1.put("key1","value1");
map1.put("key2","value2");
Map<String,String> map2 = new HashMap<>();
map2.put("a","php");
map2.put("key2","java");
map1.putAll(map2);
System.out.println("map1结果:" + map1);

//输出:{key1=value1, key2=java, a=php}

3.10 Object remove(Object key)

删除指定key所对应的key-value对,返回被删除key所关联的value,如果该key不存在,则返回null。

Map<String,String>  map1 = new HashMap<>();
map1.put("key1","value1");
map1.put("key2","value2");
map1.put("key3","value3");
map1.remove("key2");
System.out.println("map1结果:" + map1);

//输出:map1结果:{key1=value1, key3=value3}

3.11 int size()

返回该Map里的key-value对的个数。

Map<String,String>  map1 = new HashMap<>();
map1.put("key1","value1");
map1.put("key2","value2");
map1.put("key3","value3");
System.out.println("map1的key-value对的个数:" + map1.size());

//输出:map1的key-value对的个数:3

3.12 Collection values()

返回该Map里所有value组成的Collection。

Map<String,String>  map1 = new HashMap<>();
map1.put("key1","value1");
map1.put("key2","value2");
map1.put("key3","value3");
System.out.println("map1的value集合:" + map1.values());
//输出:map1的value集合:[value1, value2, value3]