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");
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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");
}};
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1.3 使用初始块
public class MyMap {
Map<String,String> maps;
{
maps = new HashMap<>();
maps.put("key1","value1");
maps.put("key2","value2");
maps.put("key3","value3");
}
public void printMap(){
System.out.println(maps);
}
}
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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());
}
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2.2 遍历keys或values
Map<Integer, Integer> map = new HashMap<Integer, Integer>();
for (Integer key : map.keySet()) {
System.out.println("Key = " + key);
}
for (Integer value : map.values()) {
System.out.println("Value = " + value);
}
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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());
}
Iterator<String> iterator = map.keySet().iterator();
while (iterator.hasNext()) {
String key = iterator.next();
System.out.println("MyClass.testMap key=" + key);
}
Iterator<Integer> iterator2 = map.values().iterator();
while (iterator2.hasNext()){
Integer values = iterator2.next();
System.out.println("MyClass.testMap values="+values);
}
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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);
}
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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);
}
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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);
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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"));
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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"));
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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());
}
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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"));
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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());
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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());
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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());
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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);
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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);
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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());
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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());
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