BAEL-5946: Reading and Writing With a ConcurrentHashMap (#13163)

* consistent read unit test

* unit test to show  consistency in writing to the same key of map

* testing writes at different map key

core-java-modules/core-java-concurrency-collections-2/src/test/java/com/baeldung/concurrenthashmap/ConcurrentHashMapUnitTest.java

@@ -0,0 +1,114 @@
+package com.baeldung.concurrenthashmap;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.*;
+import java.util.concurrent.atomic.AtomicLong;
+
+import static java.util.Arrays.asList;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class ConcurrentHashMapUnitTest {
+
+    private Map<Integer, Integer> frequencyMap;
+
+    @BeforeEach
+    public void setup() {
+        frequencyMap = new ConcurrentHashMap<>();
+        frequencyMap.put(0, 0);
+        frequencyMap.put(1, 0);
+        frequencyMap.put(2, 0);
+    }
+
+    @AfterEach
+    public void teardown() {
+        frequencyMap.clear();
+    }
+
+    @Test
+    public void givenOneThreadIsWriting_whenAnotherThreadReads_thenGetCorrectValue() throws Exception {
+        ExecutorService threadExecutor = Executors.newFixedThreadPool(3);
+
+        Runnable writeAfter1Sec = () -> frequencyMap.computeIfPresent(1, (k, v) -> {
+            sleep(1);
+            return frequencyMap.get(k) + 1;
+        });
+
+        Callable<Integer> readNow = () -> frequencyMap.get(1);
+        Callable<Integer> readAfter1001Ms = () -> {
+            TimeUnit.MILLISECONDS.sleep(1001);
+            return frequencyMap.get(1);
+        };
+
+        threadExecutor.submit(writeAfter1Sec);
+        List<Future<Integer>> results = threadExecutor.invokeAll(asList(readNow, readAfter1001Ms));
+
+        assertEquals(0, results.get(0).get());
+        assertEquals(1, results.get(1).get());
+
+        if (threadExecutor.awaitTermination(2, TimeUnit.SECONDS)) {
+            threadExecutor.shutdown();
+        }
+    }
+
+    @Test
+    public void givenOneThreadIsWriting_whenAnotherThreadWritesAtSameKey_thenWaitAndGetCorrectValue() throws Exception {
+        ExecutorService threadExecutor = Executors.newFixedThreadPool(2);
+
+        Callable<Integer> writeAfter5Sec = () -> frequencyMap.computeIfPresent(1, (k, v) -> {
+            sleep(5);
+            return frequencyMap.get(k) + 1;
+        });
+
+        Callable<Integer> writeAfter1Sec = () -> frequencyMap.computeIfPresent(1, (k, v) -> {
+            sleep(1);
+            return frequencyMap.get(k) + 1;
+        });
+
+        List<Future<Integer>> results = threadExecutor.invokeAll(asList(writeAfter5Sec, writeAfter1Sec));
+
+        assertEquals(1, results.get(0).get());
+        assertEquals(2, results.get(1).get());
+
+        if (threadExecutor.awaitTermination(2, TimeUnit.SECONDS)) {
+            threadExecutor.shutdown();
+        }
+    }
+
+    @Test
+    public void givenOneThreadIsWriting_whenAnotherThreadWritesAtDifferentKey_thenNotWaitAndGetCorrectValue() throws Exception {
+        ExecutorService threadExecutor = Executors.newFixedThreadPool(2);
+
+        Callable<Integer> writeAfter5Sec = () -> frequencyMap.computeIfPresent(1, (k, v) -> {
+            sleep(5);
+            return frequencyMap.get(k) + 1;
+        });
+
+        AtomicLong time = new AtomicLong(System.currentTimeMillis());
+        Callable<Integer> writeAfter1Sec = () -> frequencyMap.computeIfPresent(2, (k, v) -> {
+            sleep(1);
+            time.set((System.currentTimeMillis() - time.get()) / 1000);
+            return frequencyMap.get(k) + 1;
+        });
+
+        threadExecutor.invokeAll(asList(writeAfter5Sec, writeAfter1Sec));
+
+        assertEquals(1, time.get());
+
+        if (threadExecutor.awaitTermination(2, TimeUnit.SECONDS)) {
+            threadExecutor.shutdown();
+        }
+    }
+
+    private static void sleep(int timeout) {
+        try {
+            TimeUnit.SECONDS.sleep(timeout);
+        } catch (InterruptedException e) {
+            throw new RuntimeException(e);
+        }
+    }
+}