#10 effective java: item 3

effective-java/src/main/java/com/example/effectivejava/chapter01/item03/field/Concert.java

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+package com.example.effectivejava.chapter01.item03.field;
+
+public class Concert {
+
+    private boolean lightsOn;
+
+    private boolean mainStateOpen;
+
+    private IElvis elvis;
+
+    public Concert(IElvis elvis) {
+        this.elvis = elvis;
+    }
+
+    public void perform() {
+        mainStateOpen = true;
+        lightsOn = true;
+        elvis.sing();
+    }
+
+    public boolean isLightsOn() {
+        return lightsOn;
+    }
+
+    public boolean isMainStateOpen() {
+        return mainStateOpen;
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item03/field/Elvis.java

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+package com.example.effectivejava.chapter01.item03.field;
+
+import java.io.Serial;
+import java.io.Serializable;
+
+// 코드 3-1 public static final 필드 방식의 싱글턴 (23쪽)
+public class Elvis implements IElvis, Serializable {
+
+    /**
+     * 싱글톤 오브젝트
+     */
+    public static final Elvis INSTANCE = new Elvis();
+    private static boolean created;
+
+    private Elvis() {
+        if (created) {
+            throw new UnsupportedOperationException("can't be created by constructor.");
+        }
+
+        created = true;
+    }
+
+    public void leaveTheBuilding() {
+        System.out.println("Whoa baby, I'm outta here!");
+    }
+
+    public void sing() {
+        System.out.println("I'll have a blue~ Christmas without you~");
+    }
+
+    // 이 메서드는 보통 클래스 바깥(다른 클래스)에 작성해야 한다!
+    public static void main(String[] args) {
+        Elvis elvis = Elvis.INSTANCE;
+        elvis.leaveTheBuilding();
+    }
+
+    @Serial
+    private Object readResolve() {
+        return INSTANCE;
+    }
+
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item03/field/ElvisReflection.java

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+package com.example.effectivejava.chapter01.item03.field;
+
+import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationTargetException;
+
+// 생성자로 여러 인스턴스 만들기
+public class ElvisReflection {
+
+    public static void main(String[] args) {
+        try {
+            Constructor<Elvis> defaultConstructor = Elvis.class.getDeclaredConstructor();
+            defaultConstructor.setAccessible(true);
+            Elvis elvis1 = defaultConstructor.newInstance();
+            Elvis elvis2 = defaultConstructor.newInstance();
+
+            Elvis.INSTANCE.sing();
+        } catch (InvocationTargetException | NoSuchMethodException | InstantiationException | IllegalAccessException e) {
+            e.printStackTrace();
+        }
+    }
+
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item03/field/ElvisSerialization.java

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+package com.example.effectivejava.chapter01.item03.field;
+
+import java.io.*;
+
+// 역직렬화로 여러 인스턴스 만들기
+public class ElvisSerialization {
+
+    public static void main(String[] args) {
+        try (ObjectOutput out = new ObjectOutputStream(new FileOutputStream("elvis.obj"))) {
+            out.writeObject(Elvis.INSTANCE);
+        } catch (IOException e) {
+            e.printStackTrace();
+        }
+
+        try (ObjectInput in = new ObjectInputStream(new FileInputStream("elvis.obj"))) {
+            Elvis elvis3 = (Elvis) in.readObject();
+            System.out.println(elvis3 == Elvis.INSTANCE);
+        } catch (IOException | ClassNotFoundException e) {
+            e.printStackTrace();
+        }
+    }
+
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item03/field/IElvis.java

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+package com.example.effectivejava.chapter01.item03.field;
+
+public interface IElvis {
+
+    void leaveTheBuilding();
+
+    void sing();
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item03/staticfactory/Concert.java

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+package com.example.effectivejava.chapter01.item03.staticfactory;
+
+import java.util.function.Supplier;
+
+public class Concert {
+
+    public void start(Supplier<Singer> singerSupplier) {
+        Singer singer = singerSupplier.get();
+        singer.sing();
+    }
+
+    public static void main(String[] args) {
+        Concert concert = new Concert();
+        concert.start(Elvis::getInstance);
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item03/staticfactory/Elvis.java

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+package com.example.effectivejava.chapter01.item03.staticfactory;
+
+// 코드 3-2 정적 팩터리 방식의 싱글턴 (24쪽)
+public class Elvis implements Singer {
+    private static final Elvis INSTANCE = new Elvis();
+    private Elvis() { }
+    public static Elvis getInstance() { return INSTANCE; }
+
+    public void leaveTheBuilding() {
+        System.out.println("Whoa baby, I'm outta here!");
+    }
+
+    // 이 메서드는 보통 클래스 바깥(다른 클래스)에 작성해야 한다!
+    public static void main(String[] args) {
+        Elvis elvis = Elvis.getInstance();
+        elvis.leaveTheBuilding();
+
+        System.out.println(Elvis.getInstance());
+        System.out.println(Elvis.getInstance());
+    }
+
+    @Override
+    public void sing() {
+        System.out.println("my way~~~");
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item03/staticfactory/MetaElvis.java

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+package com.example.effectivejava.chapter01.item03.staticfactory;
+
+// 코드 3-2 제네릭 싱글톤 팩토리 (24쪽)
+public class MetaElvis<T> {
+
+    private static final MetaElvis<Object> INSTANCE = new MetaElvis<>();
+
+    private MetaElvis() { }
+
+    @SuppressWarnings("unchecked")
+    public static <E> MetaElvis<E> getInstance() { return (MetaElvis<E>) INSTANCE; }
+
+    public void say(T t) {
+        System.out.println(t);
+    }
+
+    public void leaveTheBuilding() {
+        System.out.println("Whoa baby, I'm outta here!");
+    }
+
+    public static void main(String[] args) {
+        MetaElvis<String> elvis1 = MetaElvis.getInstance();
+        MetaElvis<Integer> elvis2 = MetaElvis.getInstance();
+        System.out.println(elvis1);
+        System.out.println(elvis2);
+        elvis1.say("hello");
+        elvis2.say(100);
+    }
+
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item03/staticfactory/Singer.java

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+package com.example.effectivejava.chapter01.item03.staticfactory;
+
+public interface Singer {
+
+    void sing();
+}

effective-java/src/test/java/com/example/effectivejava/chapter01/item03/field/ConcertTest.java

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+package com.example.effectivejava.chapter01.item03.field;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class ConcertTest {
+
+    @Test
+    void perform() {
+        Concert concert = new Concert(new MockElvis());
+        concert.perform();
+
+        assertTrue(concert.isLightsOn());
+        assertTrue(concert.isMainStateOpen());
+    }
+}

effective-java/src/test/java/com/example/effectivejava/chapter01/item03/field/MockElvis.java

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+package com.example.effectivejava.chapter01.item03.field;
+
+public class MockElvis implements IElvis {
+    @Override
+    public void leaveTheBuilding() {
+
+    }
+
+    @Override
+    public void sing() {
+        System.out.println("You ain't nothin' but a hound dog.");
+    }
+}