Add examples from chapter 21

chapter21/src/com/packt/java/references/Assignment1.java

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+package com.packt.java.references;
+
+import java.util.WeakHashMap;
+
+public class Assignment1 {
+    public static void main(String[] args) throws Exception {
+        Student harry = new Student("Harry");
+        Student jenny = new Student("Jenny");
+
+        WeakHashMap<Student, Result> testResults =
+                new WeakHashMap<>();
+
+        testResults.put(harry, new Result(harry, 23));
+        testResults.put(jenny, new Result(jenny, 25));
+
+        System.out.println("Test results: " + testResults.size());
+
+        // HARRY is dereferenced
+        harry = null;
+        System.gc();
+
+        Thread.sleep(5000);
+
+        System.out.println("Test results: " + testResults.size());
+    }
+}

chapter21/src/com/packt/java/references/Assignment2.java

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+package com.packt.java.references;
+
+import java.lang.ref.ReferenceQueue;
+import java.lang.ref.WeakReference;
+
+public class Assignment2 {
+    public static void main(String[] args) throws Exception {
+        ShoppingCart cart1 = new ShoppingCart();
+        ShoppingCart cart2 = new ShoppingCart();
+        ShoppingCart cart3 = new ShoppingCart();
+
+        ReferenceQueue<ShoppingCart> referenceQueue = new ReferenceQueue<>();
+
+        WeakReference<ShoppingCart> weakReference1 =
+                new WeakReference<>(cart1, referenceQueue);
+        WeakReference<ShoppingCart> weakReference2 =
+                new WeakReference<>(cart2, referenceQueue);
+        WeakReference<ShoppingCart> weakReference3 =
+                new WeakReference<>(cart3, referenceQueue);
+
+        countReferenceQueue(referenceQueue);
+
+        cart1 = null;
+        cart2 = null;
+        cart3 = null;
+
+        System.gc();
+
+        Thread.sleep(100);
+
+        countReferenceQueue(referenceQueue);
+    }
+
+    private static void countReferenceQueue(ReferenceQueue<ShoppingCart> referenceQueue) {
+        int count = 0;
+        while (referenceQueue.poll() != null) {
+            count++;
+        }
+
+        System.out.println("There were " + count + " items in the queue");
+    }
+}

chapter21/src/com/packt/java/references/Exercise1.java

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+package com.packt.java.references;
+
+import java.lang.ref.WeakReference;
+
+public class Exercise1 {
+
+    public static void main(String[] args) {
+        MyObject myObject = new MyObject();
+        System.out.println(myObject);
+
+        WeakReference<MyObject> weakReference = new WeakReference<>(myObject);
+        MyObject myObject2 = weakReference.get();
+        System.out.println(myObject2);
+
+        myObject = null;
+        myObject2 = null;
+
+        System.gc();
+
+        MyObject myObject3 = weakReference.get();
+        System.out.println(myObject3);
+    }
+}

chapter21/src/com/packt/java/references/Exercise2.java

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+package com.packt.java.references;
+
+import java.util.WeakHashMap;
+
+public class Exercise2 {
+    public static void main(String[] args) throws Exception {
+        Student harry = new Student("Harry");
+        Student jenny = new Student("Jenny");
+
+        WeakHashMap<Student, Integer> testResults =
+                new WeakHashMap<>();
+
+        testResults.put(harry, 23);
+        testResults.put(jenny, 25);
+
+        System.out.println("Test results: " + testResults.size());
+
+        // HARRY is dereferenced
+        harry = null;
+        System.gc();
+
+        Thread.sleep(500);
+
+        System.out.println("Test results: " + testResults.size());
+    }
+}

chapter21/src/com/packt/java/references/Exercise3.java

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+package com.packt.java.references;
+
+import java.lang.ref.ReferenceQueue;
+import java.lang.ref.WeakReference;
+
+public class Exercise3 {
+    public static void main(String[] args) throws Exception {
+        ShoppingCart shoppingCart = new ShoppingCart();
+
+        ReferenceQueue<ShoppingCart> referenceQueue = new ReferenceQueue<>();
+
+        WeakReference<ShoppingCart> weakReference =
+                new WeakReference<>(shoppingCart, referenceQueue);
+
+        System.out.println(String.format("ShoppingCart has %sbeen cleared.",
+                (referenceQueue.poll() == null ? "not " : "")));
+
+        shoppingCart = null;
+
+        System.gc();
+
+        Thread.sleep(500);
+
+        System.out.println(String.format("ShoppingCart has %sbeen cleared.",
+                (referenceQueue.poll() == null ? "not " : "")));
+
+        System.out.println(String.format("ShoppingCart reference %sexist",
+                (weakReference.get() == null ? "does not " : "")));
+    }
+}

chapter21/src/com/packt/java/references/Exercise4.java

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+package com.packt.java.references;
+
+import java.lang.ref.SoftReference;
+
+public class Exercise4 {
+    public static void main(String[] args) throws Exception {
+        ShoppingCart shoppingCart = new ShoppingCart();
+
+        SoftReference<ShoppingCart> softReference = new SoftReference<>(shoppingCart);
+
+        checkSoftReference(softReference);
+
+        shoppingCart = null;
+
+        System.gc();
+
+        Thread.sleep(5000);
+
+        checkSoftReference(softReference);
+
+        ShoppingCart rescuedCart = softReference.get();
+    }
+
+    private static void checkSoftReference(SoftReference<ShoppingCart> softReference) {
+        ShoppingCart shoppingCart = softReference.get();
+        System.out.println(String.format("ShoppingCart was %scleared.",
+                (shoppingCart == null ? "" : "not ")));
+    }
+}

chapter21/src/com/packt/java/references/MyObject.java

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+package com.packt.java.references;
+
+public class MyObject {
+    @Override
+    protected void finalize() throws Throwable {
+        super.finalize();
+        System.out.println("Finalize called.");
+    }
+}

chapter21/src/com/packt/java/references/Result.java

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+package com.packt.java.references;
+
+public class Result {
+    private Student student;
+    private Integer score;
+
+    public Result(Student student, Integer score) {
+        this.student = student;
+        this.score = score;
+    }
+}

chapter21/src/com/packt/java/references/ShoppingCart.java

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+package com.packt.java.references;
+
+public class ShoppingCart {
+    @Override
+    protected void finalize() throws Throwable {
+        super.finalize();
+        System.out.println("Finalized");
+    }
+}

chapter21/src/com/packt/java/references/Student.java

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+package com.packt.java.references;
+
+public class Student {
+    private String name;
+
+    public Student(String name) {
+        this.name = name;
+    }
+}