享元模式 - Hello Algorithm

享元（Flyweight）

介绍

享元模式（Flyweight Pattern）主要用于减少创建对象的数量，以减少内存占用和提高性能。这种类型的设计模式属于结构型模式，它提供了减少对象数量从而改善应用所需的对象结构的方式。

享元模式尝试重用现有的同类对象，如果未找到匹配的对象，则创建新对象。我们将通过创建 5 个对象来画出 20 个分布于不同位置的圆来演示这种模式。由于只有 5 种可用的颜色，所以 color 属性被用来检查现有的 Circle 对象。

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

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)

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