ConcurrentHashMap#computeIfAbsent

java-concurrent-hm-compute-if-absent/pom.xml

@@ -0,0 +1,22 @@
+<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>
+
+    <groupId>com.pivovarit</groupId>
+    <version>1.0</version>
+    <artifactId>java-concurrent-hm-compute-if-absent</artifactId>
+
+    <name>java-concurrent-hm-compute-if-absent</name>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>21</source>
+                    <target>21</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+</project>

java-concurrent-hm-compute-if-absent/src/main/java/com/pivovarit/chm/ComputeIfAbsentExample.java

@@ -0,0 +1,35 @@
+package com.pivovarit.chm;
+
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.stream.Stream;
+
+class ComputeIfAbsentExample {
+
+    public static void main(String[] args) {
+        var map = new ConcurrentHashMap<>(1024);
+        var counter = new AtomicInteger();
+        Stream.of(1, 2)
+          .map(tid -> Thread.ofPlatform().start(() -> {
+              for (int i = 0; i < 10; i++) {
+                  int key = ThreadLocalRandom.current().nextInt(10);
+                  map.computeIfAbsent(key, s -> {
+                      if (counter.incrementAndGet() > 1) {
+                          System.out.println("I told you so!");
+                      }
+                      doWork();
+                      counter.decrementAndGet();
+                      return key;
+                  });
+              }
+          })).forEach(t -> {});
+    }
+
+    private static void doWork() {
+        try {
+            Thread.sleep(100);
+        } catch (InterruptedException ignored) {
+        }
+    }
+}

pom.xml

@@ -24,6 +24,7 @@
         <module>java-archunit</module>
         <module>java-sealed-classes</module>
         <module>java-completable-future-timeouts</module>
+        <module>java-concurrent-hm-compute-if-absent</module>
         <module>java-event-sourcing</module>
         <module>java-completable-future-allof</module>
         <module>java-advanced-groupingby</module>