BAEL 5932 - Callback Functions in Java (#13335)

core-java-modules/core-java-functional/pom.xml

@@ -14,4 +14,30 @@
         <version>0.0.1-SNAPSHOT</version>
     </parent>
 
+    <dependencies>
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-inline</artifactId>
+            <version>${mockito-inline.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <finalName>core-java-functional</finalName>
+        <resources>
+            <resource>
+                <directory>src/main/resources</directory>
+                <filtering>true</filtering>
+            </resource>
+        </resources>
+    </build>
+
+    <properties>
+        <mockito-inline.version>3.8.0</mockito-inline.version>
+        <assertj.version>3.22.0</assertj.version>
+        <commons-lang3.version>3.12.0</commons-lang3.version>
+        <vavr.version>0.10.4</vavr.version>
+    </properties>
+
 </project>

core-java-modules/core-java-functional/src/main/java/com/baeldung/callbackfunctions/ConsumerCallback.java

@@ -0,0 +1,17 @@
+package com.baeldung.callbackfunctions;
+
+import java.util.function.Consumer;
+
+public class ConsumerCallback {
+    public void getAge(int initialAge, Consumer<Integer> callback) {
+        callback.accept(initialAge);
+    }
+
+    public void increaseAge(int initialAge, int ageDifference, Consumer<Integer> callback) {
+        System.out.println("===== Increase age ====");
+
+        int newAge = initialAge + ageDifference;
+        callback.accept(newAge);
+
+    }
+}

core-java-modules/core-java-functional/src/main/java/com/baeldung/callbackfunctions/EventListener.java

@@ -0,0 +1,12 @@
+package com.baeldung.callbackfunctions;
+
+public interface EventListener {
+
+    String onTrigger();
+
+    void respondToTrigger();
+}
+
+
+
+

core-java-modules/core-java-functional/src/main/java/com/baeldung/callbackfunctions/asynchronous/AsynchronousEventConsumer.java

@@ -0,0 +1,21 @@
+package com.baeldung.callbackfunctions.asynchronous;
+
+import com.baeldung.callbackfunctions.EventListener;
+
+public class AsynchronousEventConsumer{
+
+    private EventListener listener;
+
+    public AsynchronousEventConsumer(EventListener listener) {
+        this.listener = listener;
+    }
+
+    public void doAsynchronousOperation()
+    {
+        System.out.println("Performing operation in Asynchronous Task");
+
+        new Thread(() ->  listener.onTrigger()).start();
+    }
+
+
+}

core-java-modules/core-java-functional/src/main/java/com/baeldung/callbackfunctions/asynchronous/AsynchronousEventListenerImpl.java

@@ -0,0 +1,19 @@
+package com.baeldung.callbackfunctions.asynchronous;
+
+import com.baeldung.callbackfunctions.EventListener;
+
+public class AsynchronousEventListenerImpl implements EventListener {
+
+    @Override
+    public String onTrigger()
+    {
+        respondToTrigger();
+        return "Asynchronously running callback function";
+    }
+
+    @Override
+    public void respondToTrigger(){
+        System.out.println("This is a side effect of the asynchronous trigger.");
+    }
+
+}

core-java-modules/core-java-functional/src/main/java/com/baeldung/callbackfunctions/synchronous/SynchronousEventConsumer.java

@@ -0,0 +1,22 @@
+package com.baeldung.callbackfunctions.synchronous;
+
+import com.baeldung.callbackfunctions.EventListener;
+
+public class SynchronousEventConsumer {
+
+    private final EventListener eventListener;
+
+    public SynchronousEventConsumer(EventListener listener)
+    {
+        this.eventListener = listener;
+    }
+
+    public String doSynchronousOperation()
+    {
+        System.out.println("Performing callback before synchronous Task");
+
+           return eventListener.onTrigger();
+    }
+
+
+}

core-java-modules/core-java-functional/src/main/java/com/baeldung/callbackfunctions/synchronous/SynchronousEventListenerImpl.java

@@ -0,0 +1,18 @@
+package com.baeldung.callbackfunctions.synchronous;
+
+import com.baeldung.callbackfunctions.EventListener;
+
+public class SynchronousEventListenerImpl implements EventListener {
+
+    @Override
+    public String onTrigger()
+    {
+        return "Synchronously running callback function";
+    }
+
+    @Override
+    public void respondToTrigger(){
+        System.out.println("Response to trigger");
+    }
+
+}

core-java-modules/core-java-functional/src/test/java/com/baeldung/callbackfunctions/AsynchronousCallbackUnitTest.java

@@ -0,0 +1,22 @@
+package com.baeldung.callbackfunctions;
+
+import org.junit.Test;
+import org.mockito.Mockito;
+import com.baeldung.callbackfunctions.EventListener;
+import com.baeldung.callbackfunctions.asynchronous.AsynchronousEventConsumer;
+import com.baeldung.callbackfunctions.asynchronous.AsynchronousEventListenerImpl;
+
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+
+public class AsynchronousCallbackUnitTest {
+
+    @Test
+    public void whenCallbackIsInvokedAsynchronously_shouldRunAsynchronousOperation(){
+        EventListener listener = Mockito.mock(AsynchronousEventListenerImpl.class);
+        AsynchronousEventConsumer asynchronousEventListenerConsumer = new AsynchronousEventConsumer(listener);
+        asynchronousEventListenerConsumer.doAsynchronousOperation();
+
+        verify(listener, times(1)).onTrigger();
+    }
+}

core-java-modules/core-java-functional/src/test/java/com/baeldung/callbackfunctions/ConsumerCallbackUnitTest.java

@@ -0,0 +1,20 @@
+package com.baeldung.callbackfunctions;
+
+import org.junit.jupiter.api.Test;
+import com.baeldung.callbackfunctions.ConsumerCallback;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class ConsumerCallbackUnitTest {
+
+    @Test
+    public void whenIncreasingInitialAgeByGivenValueThroughCallback_shouldIncreaseAge(){
+        ConsumerCallback consumerCallback = new ConsumerCallback();
+        consumerCallback.getAge(20, (initialAge) -> {
+            int ageDifference = 10;
+            consumerCallback.increaseAge(initialAge, ageDifference, (newAge) -> {
+                assertEquals(initialAge + ageDifference, newAge);
+            });
+        });
+    }
+}

core-java-modules/core-java-functional/src/test/java/com/baeldung/callbackfunctions/SynchronousCallbackUnitTest.java

@@ -0,0 +1,22 @@
+package callbackFunctions;
+
+import org.junit.jupiter.api.Test;
+import com.baeldung.callbackfunctions.EventListener;
+import com.baeldung.callbackfunctions.synchronous.SynchronousEventConsumer;
+import com.baeldung.callbackfunctions.synchronous.SynchronousEventListenerImpl;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+public class SynchronousCallbackUnitTest {
+
+    @Test
+    public void whenCallbackIsInvokedSynchronously_shouldRunSynchronousOperation(){
+        EventListener listener = new SynchronousEventListenerImpl();
+        SynchronousEventConsumer synchronousEventConsumer = new SynchronousEventConsumer(listener);
+        String result = synchronousEventConsumer.doSynchronousOperation();
+
+        assertNotNull(result);
+        assertEquals("Synchronously running callback function", result);
+    }
+}