#10 effective java: item 7

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/executor/ExecutorsExample.java

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+package com.example.effectivejava.chapter01.item07.executor;
+
+import java.util.concurrent.*;
+
+public class ExecutorsExample {
+
+    public static void main(String[] args) throws ExecutionException, InterruptedException {
+        ExecutorService service = Executors.newFixedThreadPool(10);
+
+        Future<String> submit = service.submit(new Task());
+
+        System.out.println(Thread.currentThread() + " hello");
+
+        System.out.println(submit.get());
+
+        service.shutdown();
+    }
+
+    static class Task implements Callable<String> {
+
+        @Override
+        public String call() throws Exception {
+            Thread.sleep(2000L);
+            return Thread.currentThread() + " world";
+        }
+    }
+
+
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/optional/Channel.java

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+package com.example.effectivejava.chapter01.item07.optional;
+
+import java.util.Optional;
+
+public class Channel {
+
+    private int numOfSubscribers;
+
+    public Optional<MemberShip> defaultMemberShip() {
+        if (this.numOfSubscribers < 2000) {
+            return Optional.empty();
+        } else {
+            return Optional.of(new MemberShip());
+        }
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/optional/MemberShip.java

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+package com.example.effectivejava.chapter01.item07.optional;
+
+public class MemberShip {
+
+    public String hello() {
+        return "hello";
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/reference/BigObject.java

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+package com.example.effectivejava.chapter01.item07.reference;
+
+public class BigObject {
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/reference/BigObjectReference.java

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+package com.example.effectivejava.chapter01.item07.reference;
+
+import java.lang.ref.PhantomReference;
+import java.lang.ref.ReferenceQueue;
+
+public class BigObjectReference<BigObject> extends PhantomReference<BigObject> {
+    
+    public BigObjectReference(BigObject referent, ReferenceQueue<? super BigObject> q) {
+        super(referent, q);
+    }
+
+    public void cleanUp() {
+        System.out.println("clean up");
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/reference/PhantomReferenceExample.java

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+package com.example.effectivejava.chapter01.item07.reference;
+
+import java.lang.ref.Reference;
+import java.lang.ref.ReferenceQueue;
+
+public class PhantomReferenceExample {
+
+    public static void main(String[] args) throws InterruptedException {
+        BigObject strong = new BigObject();
+        ReferenceQueue<BigObject> rq = new ReferenceQueue<>();
+
+        BigObjectReference<BigObject> phantom = new BigObjectReference<>(strong, rq);
+        strong = null;
+
+        System.gc();
+        Thread.sleep(3000L);
+
+        // 사라지진 않고 큐에 들어갑니다.
+        System.out.println(phantom.isEnqueued());
+
+        Reference<? extends BigObject> reference = rq.poll();
+        BigObjectReference bigObjectCleaner = (BigObjectReference) reference;
+        bigObjectCleaner.cleanUp();
+        reference.clear();
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/reference/SoftReferenceExample.java

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+package com.example.effectivejava.chapter01.item07.reference;
+
+import java.lang.ref.SoftReference;
+
+public class SoftReferenceExample {
+
+    public static void main(String[] args) throws InterruptedException {
+        Object strong = new Object();
+        SoftReference<Object> soft = new SoftReference<>(strong);
+        strong = null;
+
+        System.gc();
+        Thread.sleep(3000L);
+
+        // 메모리가 충분해서.. 굳이 제거할 필요가 없어서 잘 안지워짐;;
+        System.out.println(soft.get());
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/reference/WeakReferenceExample.java

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+package com.example.effectivejava.chapter01.item07.reference;
+
+import java.lang.ref.WeakReference;
+
+public class WeakReferenceExample {
+
+    public static void main(String[] args) throws InterruptedException {
+        Object strong = new Object();
+        WeakReference<Object> weak = new WeakReference<>(strong);
+        strong = null;
+
+        System.gc();
+        Thread.sleep(3000L);
+
+        System.out.println(weak.get());
+    }
+}

effective-java/src/test/java/com/example/effectivejava/chapter01/item07/optional/ChannelTest.java

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+package com.example.effectivejava.chapter01.item07.optional;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Optional;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class ChannelTest {
+
+    @Test
+    void npe() {
+        Channel channel = new Channel();
+        Optional<MemberShip> optional = channel.defaultMemberShip();
+        optional.ifPresent(MemberShip::hello);
+    }
+}