BAEL-6428 Make a Method Execute Only Once in Java

core-java-modules/core-java-methods/README.md

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+=========
+
+## Core Java Methods
+
+### Relevant Articles: 
+- [Execute a Method Only Once in Java](https://www.baeldung.com/execute-a-method-only-once-in-java)

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

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>core-java-methods</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>core-java-methods</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung.core-java-modules</groupId>
+        <artifactId>core-java-modules</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>${assertj.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <finalName>core-java-methods</finalName>
+        <resources>
+            <resource>
+                <directory>src/main/resources</directory>
+                <filtering>true</filtering>
+            </resource>
+        </resources>
+    </build>
+
+    <properties>
+        <assertj.version>3.22.0</assertj.version>
+    </properties>
+
+</project>

core-java-modules/core-java-methods/src/main/java/com/baeldung/executeonce/AtomicBooleanInitializer.java

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+package com.baeldung.executeonce;
+
+import java.util.concurrent.atomic.AtomicBoolean;
+
+final class AtomicBooleanInitializer {
+
+    private final AtomicBoolean isInitialized = new AtomicBoolean(false);
+    int callCount = 0;
+
+    void initialize() {
+        if (isInitialized.compareAndSet(false, true)) {
+            initializationLogic();
+        }
+    }
+
+    private void initializationLogic() {
+        callCount++;
+    }
+}

core-java-modules/core-java-methods/src/main/java/com/baeldung/executeonce/StaticInitializer.java

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+package com.baeldung.executeonce;
+
+final class StaticInitializer {
+
+    static int CALL_COUNT = 0;
+
+    static {
+        initializationLogic();
+    }
+
+    private static void initializationLogic() {
+        CALL_COUNT++;
+    }
+}

core-java-modules/core-java-methods/src/main/java/com/baeldung/executeonce/SynchronizedInitializer.java

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+package com.baeldung.executeonce;
+
+final class SynchronizedInitializer {
+
+    private static volatile boolean isInitialized = false;
+    int callCount = 0;
+
+    synchronized void initialize() {
+        if (!isInitialized) {
+            initializationLogic();
+            isInitialized = true;
+        }
+    }
+
+    private void initializationLogic() {
+        callCount++;
+    }
+}

core-java-modules/core-java-methods/src/test/java/com/baeldung/executeonce/AtomicBooleanInitializerUnitTest.java

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+package com.baeldung.executeonce;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+final class AtomicBooleanInitializerUnitTest {
+
+    @Test
+    void givenAtomicBooleanInitializer_whenRepeatedlyCallingInitialize_thenCallCountIsOne() {
+        AtomicBooleanInitializer atomicBooleanInitializer = new AtomicBooleanInitializer();
+        assertEquals(0, atomicBooleanInitializer.callCount);
+
+        atomicBooleanInitializer.initialize();
+        atomicBooleanInitializer.initialize();
+        atomicBooleanInitializer.initialize();
+
+        assertEquals(1, atomicBooleanInitializer.callCount);
+    }
+}

core-java-modules/core-java-methods/src/test/java/com/baeldung/executeonce/StaticInitializerUnitTest.java

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+package com.baeldung.executeonce;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+final class StaticInitializerUnitTest {
+
+    @Test
+    void whenLoadingStaticInitializer_thenCallCountIsOne() {
+        assertEquals(1, StaticInitializer.CALL_COUNT);
+    }
+
+    @Test
+    void whenInitializingStaticInitializer_thenCallCountStaysOne() {
+        StaticInitializer staticInitializer = new StaticInitializer();
+        assertEquals(1, StaticInitializer.CALL_COUNT);
+    }
+}

core-java-modules/core-java-methods/src/test/java/com/baeldung/executeonce/SynchronizedInitializerUnitTest.java

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+package com.baeldung.executeonce;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+final class SynchronizedInitializerUnitTest {
+
+    @Test
+    void givenSynchronizedInitializer_whenRepeatedlyCallingInitialize_thenCallCountIsOne() {
+        SynchronizedInitializer synchronizedInitializer = new SynchronizedInitializer();
+        assertEquals(0, synchronizedInitializer.callCount);
+
+        synchronizedInitializer.initialize();
+        synchronizedInitializer.initialize();
+        synchronizedInitializer.initialize();
+
+        assertEquals(1, synchronizedInitializer.callCount);
+    }
+}