Initial commit for lazy lambda article

core-java-modules/core-java-21/pom.xml

@@ -22,9 +22,7 @@
                     <target>21</target>
                     <!-- Needed due to a bug with JDK 21, described here: https://issues.apache.org/jira/browse/MCOMPILER-546?page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel&focusedCommentId=17767513 -->
                     <debug>false</debug>
-                    <compilerArgs>
+                    <compilerArgs>--enable-preview</compilerArgs>
-                        <arg>--enable-preview</arg>
-                    </compilerArgs>
                 </configuration>
             </plugin>
             <plugin>
@@ -38,6 +36,15 @@
         </plugins>
     </build>
 
+    <dependencies>
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-core</artifactId>
+            <version>5.7.0</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
     <properties>
         <maven.compiler.source.version>21</maven.compiler.source.version>
         <maven.compiler.target.version>21</maven.compiler.target.version>

core-java-modules/core-java-21/src/main/java/com/baeldung/lazylambda/LambdaSupplier.java

@@ -0,0 +1,16 @@
+package com.baeldung.lazylambda;
+
+import java.util.function.Supplier;
+
+public class LambdaSupplier<T> {
+
+    private final Supplier<T> expensiveData;
+
+    public LambdaSupplier(Supplier<T> expensiveData) {
+        this.expensiveData = expensiveData;
+    }
+
+    public T getData() {
+        return expensiveData.get();
+    }
+}

core-java-modules/core-java-21/src/main/java/com/baeldung/lazylambda/LazyLambdaSupplier.java

@@ -0,0 +1,22 @@
+package com.baeldung.lazylambda;
+
+import java.util.function.Supplier;
+
+public class LazyLambdaSupplier<T> {
+
+    private final Supplier<T> expensiveData;
+
+    private T data;
+
+    public LazyLambdaSupplier(Supplier<T> expensiveData) {
+        this.expensiveData = expensiveData;
+    }
+
+    public T getData() {
+        if (data != null) {
+            return data;
+        }
+        return data = expensiveData.get();
+    }
+
+}

core-java-modules/core-java-21/src/main/java/com/baeldung/lazylambda/LazyLambdaThreadSafeSupplier.java

@@ -0,0 +1,29 @@
+package com.baeldung.lazylambda;
+
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Supplier;
+
+public class LazyLambdaThreadSafeSupplier<T> {
+
+    private final Supplier<T> expensiveData;
+
+    private final AtomicReference<T> data;
+
+    public LazyLambdaThreadSafeSupplier(Supplier<T> expensiveData) {
+        this.expensiveData = expensiveData;
+        data = new AtomicReference<>();
+    }
+
+    public T getData() {
+        if (data.get() == null) {
+            synchronized (data) {
+                if (data.get() == null) {
+                    data.set(expensiveData.get());
+                }
+            }
+        }
+
+        return data.get();
+    }
+
+}

core-java-modules/core-java-21/src/test/java/com/baeldung/lazylambda/LambdaSupplierUnitTest.java

@@ -0,0 +1,24 @@
+package com.baeldung.lazylambda;
+
+import java.util.function.Supplier;
+
+import org.junit.Test;
+import org.mockito.Mockito;
+
+public class LambdaSupplierUnitTest {
+
+    @Test
+    public void whenCalledMultipleTimes_thenShouldBeCalledMultipleTimes() {
+        @SuppressWarnings("unchecked")
+        Supplier<String> mockedExpensiveFunction = Mockito.mock(Supplier.class);
+        Mockito.when(mockedExpensiveFunction.get()).thenReturn("expensive call");
+        LambdaSupplier<String> testee = new LambdaSupplier<>(mockedExpensiveFunction);
+        Mockito.verify(mockedExpensiveFunction, Mockito.never()).get();
+        testee.getData();
+        testee.getData();
+        testee.getData();
+        testee.getData();
+        Mockito.verify(mockedExpensiveFunction, Mockito.times(4)).get();
+    }
+
+}

core-java-modules/core-java-21/src/test/java/com/baeldung/lazylambda/LazyLambdaSupplierUnitTest.java

@@ -0,0 +1,23 @@
+package com.baeldung.lazylambda;
+
+import java.util.function.Supplier;
+
+import org.junit.Test;
+import org.mockito.Mockito;
+
+public class LazyLambdaSupplierUnitTest {
+
+    @Test
+    public void whenCalledMultipleTimes_thenShouldBeCalledOnlyOnce() {
+        @SuppressWarnings("unchecked") Supplier<String> mockedExpensiveFunction = Mockito.mock(Supplier.class);
+        Mockito.when(mockedExpensiveFunction.get()).thenReturn("expensive call");
+        LazyLambdaSupplier<String> testee = new LazyLambdaSupplier<>(mockedExpensiveFunction);
+        Mockito.verify(mockedExpensiveFunction, Mockito.never()).get();
+        testee.getData();
+        testee.getData();
+        testee.getData();
+        testee.getData();
+        Mockito.verify(mockedExpensiveFunction, Mockito.times(1)).get();
+    }
+
+}

core-java-modules/core-java-21/src/test/java/com/baeldung/lazylambda/LazyLambdaThreadSafeSupplierUnitTest.java

@@ -0,0 +1,23 @@
+package com.baeldung.lazylambda;
+
+import java.util.function.Supplier;
+
+import org.junit.Test;
+import org.mockito.Mockito;
+
+public class LazyLambdaThreadSafeSupplierUnitTest {
+
+    @Test
+    public void whenCalledMultipleTimes_thenShouldBeCalledOnlyOnce() {
+        @SuppressWarnings("unchecked") Supplier<String> mockedExpensiveFunction = Mockito.mock(Supplier.class);
+        Mockito.when(mockedExpensiveFunction.get()).thenReturn("expensive call");
+        LazyLambdaThreadSafeSupplier<String> testee = new LazyLambdaThreadSafeSupplier<>(mockedExpensiveFunction);
+        Mockito.verify(mockedExpensiveFunction, Mockito.never()).get();
+        testee.getData();
+        testee.getData();
+        testee.getData();
+        testee.getData();
+        Mockito.verify(mockedExpensiveFunction, Mockito.times(1)).get();
+    }
+
+}