Add singleton examples with tests (#36)

Author: mperor

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/creational/singleton/LazyInitializedSingleton.java

@@ -0,0 +1,22 @@
+package pl.mperor.lab.java.design.pattern.creational.singleton;
+
+public class LazyInitializedSingleton {
+
+    private static LazyInitializedSingleton instance;
+    private final long time = System.currentTimeMillis();
+
+    private LazyInitializedSingleton() {
+    }
+
+    public static synchronized LazyInitializedSingleton getInstance() {
+        if (instance == null) {
+            instance = new LazyInitializedSingleton();
+        }
+        return instance;
+    }
+
+    public long getTime() {
+        return time;
+    }
+
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/creational/singleton/Singleton.java

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+package pl.mperor.lab.java.design.pattern.creational.singleton;
+
+public class Singleton {
+
+    private static final Singleton instance = new Singleton();
+    private final long time = System.currentTimeMillis();
+
+    private Singleton() {
+    }
+
+    public static Singleton getInstance() {
+        return instance;
+    }
+
+    public long getTime() {
+        return time;
+    }
+
+}

DesignPatterns/src/test/java/pl/mperor/lab/java/design/pattern/behavioral/iterator/IteratorTest.java

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+package pl.mperor.lab.java.design.pattern.behavioral.iterator;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+public class IteratorTest {
+
+    private final List<String> names = List.of("Adam", "Bob", "Conor");
+
+    @Test
+    public void testIteratorImperatively() {
+        findName("Bob");
+    }
+
+    public boolean findName(String n) {
+        for (int i = 0; i < names.size(); i++) {
+            if (names.get(i).equals(n)) {
+                return true;
+            }
+        }
+
+        // same as foreach
+        for (String name : names) {
+            if (name.equals(n)) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    @Test
+    public void testIteratorFunctionally() {
+        // internal iterator
+
+    }
+
+}

DesignPatterns/src/test/java/pl/mperor/lab/java/design/pattern/creational/singleton/LazyInitializedSingletonTest.java

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+package pl.mperor.lab.java.design.pattern.creational.singleton;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.CompletableFuture;
+
+public class LazyInitializedSingletonTest {
+
+    @Test
+    public void shouldOnlyAllowToLazyCreateOneInstanceOfSingleton() {
+        var first = CompletableFuture.supplyAsync(() -> {
+            System.out.println("First ...");
+            return LazyInitializedSingleton.getInstance();
+        });
+        var second = CompletableFuture.supplyAsync(() -> {
+            System.out.println("Second  ...");
+            return LazyInitializedSingleton.getInstance();
+        });
+
+        LazyInitializedSingleton firstResult = first.join();
+        LazyInitializedSingleton secondResult = second.join();
+
+        Assertions.assertSame(firstResult, secondResult);
+        Assertions.assertEquals(firstResult.getTime(), secondResult.getTime());
+    }
+}

DesignPatterns/src/test/java/pl/mperor/lab/java/design/pattern/creational/singleton/SingletonTest.java

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+package pl.mperor.lab.java.design.pattern.creational.singleton;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.TimeUnit;
+
+public class SingletonTest {
+
+    @Test
+    public void shouldOnlyAllowToCreateOneInstanceOfSingleton() throws InterruptedException {
+        var instance = Singleton.getInstance();
+        long time = instance.getTime();
+
+        TimeUnit.MILLISECONDS.sleep(50);
+        long timeAfterBreak = Singleton.getInstance().getTime();
+
+        Assertions.assertSame(instance, Singleton.getInstance());
+        Assertions.assertEquals(time, timeAfterBreak);
+    }
+
+}