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notes/设计模式 - 享元.md

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+## 享元（Flyweight）
+
+### Intent
+
+
+
+利用共享的方式来支持大量细粒度的对象，这些对象一部分内部状态是相同的。
+
+### Class Diagram
+
+- Flyweight：享元对象
+- IntrinsicState：内部状态，享元对象共享内部状态
+- ExtrinsicState：外部状态，每个享元对象的外部状态不同
+
+<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/5f5c22d5-9c0e-49e1-b5b0-6cc7032724d4.png"/> </div><br>
+
+### Implementation
+
+```java
+public interface Flyweight {
+    void doOperation(String extrinsicState);
+}
+```
+
+```java
+public class ConcreteFlyweight implements Flyweight {
+
+    private String intrinsicState;
+
+    public ConcreteFlyweight(String intrinsicState) {
+        this.intrinsicState = intrinsicState;
+    }
+
+    @Override
+    public void doOperation(String extrinsicState) {
+        System.out.println("Object address: " + System.identityHashCode(this));
+        System.out.println("IntrinsicState: " + intrinsicState);
+        System.out.println("ExtrinsicState: " + extrinsicState);
+    }
+}
+```
+
+```java
+public class FlyweightFactory {
+
+    private HashMap<String, Flyweight> flyweights = new HashMap<>();
+
+    Flyweight getFlyweight(String intrinsicState) {
+        if (!flyweights.containsKey(intrinsicState)) {
+            Flyweight flyweight = new ConcreteFlyweight(intrinsicState);
+            flyweights.put(intrinsicState, flyweight);
+        }
+        return flyweights.get(intrinsicState);
+    }
+}
+```
+
+```java
+public class Client {
+
+    public static void main(String[] args) {
+        FlyweightFactory factory = new FlyweightFactory();
+        Flyweight flyweight1 = factory.getFlyweight("aa");
+        Flyweight flyweight2 = factory.getFlyweight("aa");
+        flyweight1.doOperation("x");
+        flyweight2.doOperation("y");
+    }
+}
+```
+
+```html
+Object address: 1163157884
+IntrinsicState: aa
+ExtrinsicState: x
+Object address: 1163157884
+IntrinsicState: aa
+ExtrinsicState: y
+```
+
+### JDK
+
+Java 利用缓存来加速大量小对象的访问时间。
+
+- java.lang.Integer#valueOf(int)
+- java.lang.Boolean#valueOf(boolean)
+- java.lang.Byte#valueOf(byte)
+- java.lang.Character#valueOf(char)
+
+
+
+
+
+
+<div align="center"><img width="320px" src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/githubio/公众号二维码-2.png"></img></div>