🎨 update

src/main/doc/学习笔记/Map四种遍历方法.md

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+```java
+    //准备
+    Map<String, String> map = new HashMap<String, String>();
+    map.put("key1", "value1");
+    map.put("key2", "value2");
+    map.put("key3", "value3");
+```
+```java
+    //第一种：普遍使用，二次取值
+    System.out.println("通过Map.keySet遍历key和value：");
+    System.out.println("map.keySet()："+map.keySet()); //[key1, key2, key3]
+    for (String key : map.keySet()) {
+        System.out.println("key= "+ key + " and value= " + map.get(key));
+    }
+```
+```java
+    //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();
+    System.out.println("map.entrySet()："+map.entrySet()); //[key1=value1, key2=value2, key3=value3]
+    while (it.hasNext()) {
+        Map.Entry<String, String> entry = it.next();
+        System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
+    }
+```
+```java
+    //第三种：推荐，尤其是容量大时
+    System.out.println("通过Map.entrySet遍历key和value");
+    System.out.println("map.entrySet()："+map.entrySet()); //[key1=value1, key2=value2, key3=value3]
+    for (Map.Entry<String, String> entry : map.entrySet()) {
+        System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
+    }
+```
+```java
+    //第四种
+    System.out.println("通过Map.values()遍历所有的value，但不能遍历key");
+    System.out.println("map.values()："+map.values()); //[value1, value2, value3]
+    for (String v : map.values()) {
+        System.out.println("value= " + v);
+    }
+```