From 7135047ba1ee35b1cec031a7e653fd68426a0806 Mon Sep 17 00:00:00 2001
From: maenolis <emmanouil.varvarigos@gmail.com>
Date: Wed, 15 Nov 2023 08:33:33 +0200
Subject: [PATCH] Initial commit for lazy lambda article

---
 core-java-modules/core-java-21/pom.xml        | 13 +++++++--
 .../baeldung/lazylambda/LambdaSupplier.java   | 16 ++++++++++
 .../lazylambda/LazyLambdaSupplier.java        | 22 ++++++++++++++
 .../LazyLambdaThreadSafeSupplier.java         | 29 +++++++++++++++++++
 .../lazylambda/LambdaSupplierUnitTest.java    | 24 +++++++++++++++
 .../LazyLambdaSupplierUnitTest.java           | 23 +++++++++++++++
 .../LazyLambdaThreadSafeSupplierUnitTest.java | 23 +++++++++++++++
 7 files changed, 147 insertions(+), 3 deletions(-)
 create mode 100644 core-java-modules/core-java-21/src/main/java/com/baeldung/lazylambda/LambdaSupplier.java
 create mode 100644 core-java-modules/core-java-21/src/main/java/com/baeldung/lazylambda/LazyLambdaSupplier.java
 create mode 100644 core-java-modules/core-java-21/src/main/java/com/baeldung/lazylambda/LazyLambdaThreadSafeSupplier.java
 create mode 100644 core-java-modules/core-java-21/src/test/java/com/baeldung/lazylambda/LambdaSupplierUnitTest.java
 create mode 100644 core-java-modules/core-java-21/src/test/java/com/baeldung/lazylambda/LazyLambdaSupplierUnitTest.java
 create mode 100644 core-java-modules/core-java-21/src/test/java/com/baeldung/lazylambda/LazyLambdaThreadSafeSupplierUnitTest.java

diff --git a/core-java-modules/core-java-21/pom.xml b/core-java-modules/core-java-21/pom.xml
index bfe1cd2c78..98e73c89c0 100644
--- a/core-java-modules/core-java-21/pom.xml
+++ b/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>
-                        <arg>--enable-preview</arg>
-                    </compilerArgs>
+                    <compilerArgs>--enable-preview</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>
diff --git a/core-java-modules/core-java-21/src/main/java/com/baeldung/lazylambda/LambdaSupplier.java b/core-java-modules/core-java-21/src/main/java/com/baeldung/lazylambda/LambdaSupplier.java
new file mode 100644
index 0000000000..a9c4c78edd
--- /dev/null
+++ b/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();
+    }
+}
diff --git a/core-java-modules/core-java-21/src/main/java/com/baeldung/lazylambda/LazyLambdaSupplier.java b/core-java-modules/core-java-21/src/main/java/com/baeldung/lazylambda/LazyLambdaSupplier.java
new file mode 100644
index 0000000000..484fadbe90
--- /dev/null
+++ b/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();
+    }
+
+}
diff --git a/core-java-modules/core-java-21/src/main/java/com/baeldung/lazylambda/LazyLambdaThreadSafeSupplier.java b/core-java-modules/core-java-21/src/main/java/com/baeldung/lazylambda/LazyLambdaThreadSafeSupplier.java
new file mode 100644
index 0000000000..27180ced7d
--- /dev/null
+++ b/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();
+    }
+
+}
diff --git a/core-java-modules/core-java-21/src/test/java/com/baeldung/lazylambda/LambdaSupplierUnitTest.java b/core-java-modules/core-java-21/src/test/java/com/baeldung/lazylambda/LambdaSupplierUnitTest.java
new file mode 100644
index 0000000000..b2d8b91043
--- /dev/null
+++ b/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();
+    }
+
+}
diff --git a/core-java-modules/core-java-21/src/test/java/com/baeldung/lazylambda/LazyLambdaSupplierUnitTest.java b/core-java-modules/core-java-21/src/test/java/com/baeldung/lazylambda/LazyLambdaSupplierUnitTest.java
new file mode 100644
index 0000000000..37464afe2e
--- /dev/null
+++ b/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();
+    }
+
+}
diff --git a/core-java-modules/core-java-21/src/test/java/com/baeldung/lazylambda/LazyLambdaThreadSafeSupplierUnitTest.java b/core-java-modules/core-java-21/src/test/java/com/baeldung/lazylambda/LazyLambdaThreadSafeSupplierUnitTest.java
new file mode 100644
index 0000000000..a67600c974
--- /dev/null
+++ b/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();
+    }
+
+}