Add proxy example with test

Common/src/main/java/pl/mperor/lab/common/TestUtils.java

@@ -47,6 +47,10 @@ default E getThird() {
             return getNext(2);
         }
 
+        default E getForth() {
+            return getNext(3);
+        }
+
         private E getNext(int index) {
             if (this.isEmpty() && index >= size()) {
                 throw new NoSuchElementException();

Common/src/test/java/pl/mperor/lab/common/UtilsTest.java → Common/src/test/java/pl/mperor/lab/common/ReadableOutputTest.java

@@ -6,7 +6,7 @@
 import java.util.NoSuchElementException;
 import java.util.stream.Stream;
 
-class UtilsTest {
+class ReadableOutputTest {
 
     @Test
     public void testReadableOutputReturnsReadableList() {

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/structural/proxy/ExecutableService.java

@@ -0,0 +1,5 @@
+package pl.mperor.lab.java.design.pattern.structural.proxy;
+
+public interface ExecutableService {
+    void execute();
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/structural/proxy/ServiceImpl.java

@@ -0,0 +1,13 @@
+package pl.mperor.lab.java.design.pattern.structural.proxy;
+
+class ServiceImpl implements ExecutableService {
+
+    ServiceImpl() {
+        System.out.println("Service has been initialized!");
+    }
+
+    @Override
+    public void execute() {
+        System.out.println("Method has been executed!");
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/structural/proxy/ServiceProvider.java

@@ -0,0 +1,13 @@
+package pl.mperor.lab.java.design.pattern.structural.proxy;
+
+import java.util.function.Supplier;
+
+public enum ServiceProvider implements Supplier<ExecutableService> {
+    INSTANCE;
+
+    @Override
+    public ExecutableService get() {
+        return new ServiceProxy();
+    }
+
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/structural/proxy/ServiceProxy.java

@@ -0,0 +1,20 @@
+package pl.mperor.lab.java.design.pattern.structural.proxy;
+
+class ServiceProxy implements ExecutableService {
+
+    private ExecutableService service;
+
+    @Override
+    public void execute() {
+        System.out.println("Before executing service method!");
+        lazyLoadingService();
+        service.execute();
+        System.out.println("After executing service method!");
+    }
+
+    private void lazyLoadingService() {
+        if (service == null) {
+            service = new ServiceImpl();
+        }
+    }
+}

DesignPatterns/src/test/java/pl/mperor/lab/java/design/pattern/structural/proxy/ServiceProviderTest.java

@@ -0,0 +1,27 @@
+package pl.mperor.lab.java.design.pattern.structural.proxy;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+import org.junit.platform.commons.util.StringUtils;
+import pl.mperor.lab.common.TestUtils;
+
+public class ServiceProviderTest {
+
+    @Test
+    public void testServiceUsingProxyUnderneath() {
+        var out = TestUtils.setTempSystemOut();
+        var service = ServiceProvider.INSTANCE.get();
+        Assertions.assertTrue(StringUtils.isBlank(out.all()),
+                "Lazy loading service should not be initialized!");
+        Assertions.assertInstanceOf(ServiceProxy.class, service);
+
+        service.execute();
+        var outLines = out.lines();
+        Assertions.assertEquals(outLines.getFirst(), "Before executing service method!");
+        Assertions.assertEquals(outLines.getSecond(), "Service has been initialized!");
+        Assertions.assertEquals(outLines.getThird(), "Method has been executed!");
+        Assertions.assertEquals(outLines.getForth(), "After executing service method!");
+        TestUtils.resetSystemOut();
+    }
+
+}