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Commitea6c4e5

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Added Singleton Design pattern implementation in Java and its test
1 parent09c4cd7 commitea6c4e5

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packagesrc.main.java.com.designpatterns.singletonpattern;
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/**
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* The singleton pattern is a design pattern that restricts the instantiation of a class to one "single" instance.
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* This is useful when exactly one object is needed to coordinate actions across the system. The term comes from the
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* mathematical concept of a singleton.
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* <p>
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* The key idea in this pattern is to make the class itself responsible for controlling its instantiation (only once).
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* The hidden constructor (declared private) ensures that the class can never be instantiated from outside the class.
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* The public static operation can be accessed easily by using the class name and function name(Singleton.getInstance())
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*
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* @see <a href="https://en.wikipedia.org/wiki/Singleton_pattern">Singleton Pattern</a>
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*/
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publicclassSingleton {
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privatevolatilestaticSingletoninstance =null;
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privateSingleton() {
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}
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/**
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* A singleton implementation may use lazy initialization, where the instance is created when the static method
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* is first invoked.
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* <p>
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* If the static method might be called from multiple threads simultaneously, measures may need
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* to be taken to prevent race conditions that could result in the creation of multiple instances of the class.
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* <p>
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* The following implementation is a thread-safe sample implementation, using lazy initialization with
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* double-checked locking.
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*
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* @return the single instance of the Singleton class
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*/
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publicstaticSingletongetInstance() {
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if (instance ==null) {
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// First attempt to make thread safe
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synchronized (Singleton.class) {
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// Double Checked locking as multiple threads can reach the above step
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if (instance ==null) {
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instance =newSingleton();
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}
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}
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}
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returninstance;
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}
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}
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packagesrc.test.java.com.designpatterns.singletonpattern;
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importorg.junit.Assert;
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importorg.junit.Test;
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importsrc.main.java.com.designpatterns.singletonpattern.Singleton;
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importjava.util.ArrayList;
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importjava.util.concurrent.ExecutorService;
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importjava.util.concurrent.Executors;
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importjava.util.concurrent.TimeUnit;
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publicclassSingletonTest {
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privatestaticvolatileArrayList<Integer>hashCodeList =newArrayList<>();
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@Test
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publicvoidtestSingleton()throwsInterruptedException {
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booleantestFailed =false;
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ExecutorServicees =Executors.newCachedThreadPool();
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// Creates 15 threads and makes all of them access the Singleton class
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// Saves the hash code of the object in a static list
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for (inti =0;i <15;i++)
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es.execute(() -> {
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try {
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SingletonsingletonInstance =Singleton.getInstance();
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intsingletonInsCode =singletonInstance.hashCode();
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System.out.println(singletonInsCode);
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hashCodeList.add(singletonInsCode);
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}catch (Exceptione) {
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System.out.println("Exception is caught");
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}
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});
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es.shutdown();
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booleanfinished =es.awaitTermination(1,TimeUnit.MINUTES);
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// wait for all threads to finish
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if (finished) {
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IntegerfirstCode =hashCodeList.get(0);
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for (Integercode :hashCodeList) {
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if (!firstCode.equals(code)) {
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testFailed =true;
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}
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}
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Assert.assertFalse(testFailed);
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}
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}
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}

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