add logseq-java

logseq-java/pages/HashMap.md

@@ -0,0 +1,46 @@
+- [[Map]]集合中的一种实现
+- 底层结构
+	- JDK1.7前：[[数组]]+ [[链表]]
+	- JDK1.7后：[[数组]]+[[链表]]+ [[红黑二叉树]]
+- 用法
+	- ```java
+	  HashMap<String,String> datas = new HashMap<>();
+	  ```
+	- [[put]]
+	- [[putAll]]
+	- [[get]]
+	- [[getOrDefault]]
+	- 允许放[[null]]键null值
+	- [[replace]]
+	- [[remove]]
+	- [[clear]]
+	- [[keySet]]
+	- [[values]]
+- [[去重]]
+	- 依靠Hashcode和equals
+	- HashSet的底层是HashMap的key
+	- Keys要重写equals和hashCode
+- 遍历
+	- [[forEach]]
+		- ```java
+		  datas.forEach(new BiConsumer<Girl, Boy>() {
+		    @Override
+		    public void accept(Girl key, Boy value) {
+		      System.out.println("key=" + key + ",value" + value);
+		    }
+		  });
+		  ```
+	- 获得键的集合
+		- ```java
+		  Set<Girl> keys = datas.keySet(); 
+		  for (Girl e : keys) {
+		    System.out.println("key=" + e + ",value=" + datas.get(e));
+		  }
+		  ```
+	- [[entrySet]]
+		- ```java
+		  Set<Map.Entry<Girl, Boy>> entries = datas.entrySet();
+		  for (Map.Entry<Girl, Boy> e : entries  {
+		    System.out.println("key=" + e.getKey() + ",value=" + e.getValue());
+		  }
+		  ```