add ConcurrentNavigableMap tests (#1083)

* add concurrentmap tests

* fix ConcurrentMapTests#givenConcurrentMap_whenKeyWithSameHashCode_thenPerformanceDegrades

* add ConcurrentNavigableMap tests

core-java/src/test/java/com/baeldung/java/concurrentmap/ConcurrentNavigableMapTests.java

@@ -0,0 +1,79 @@
+package com.baeldung.java.concurrentmap;
+
+import org.junit.Test;
+
+import java.util.Iterator;
+import java.util.NavigableMap;
+import java.util.TreeMap;
+import java.util.concurrent.*;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static org.testng.Assert.*;
+
+public class ConcurrentNavigableMapTests {
+
+    @Test
+    public void givenSkipListMap_whenAccessInMultiThreads_thenOrderingStable() throws InterruptedException {
+        NavigableMap<Integer, String> skipListMap = new ConcurrentSkipListMap<>();
+
+        updateMapConcurrently(skipListMap, 4);
+
+        Iterator<Integer> skipListIter = skipListMap
+          .keySet()
+          .iterator();
+        int previous = skipListIter.next();
+        while (skipListIter.hasNext()) {
+            int current = skipListIter.next();
+            assertTrue(previous < current);
+        }
+    }
+
+    private void updateMapConcurrently(NavigableMap<Integer, String> navigableMap, int concurrencyLevel) throws InterruptedException {
+        ExecutorService executorService = Executors.newFixedThreadPool(concurrencyLevel);
+        for (int i = 0; i < concurrencyLevel; i++) {
+            executorService.execute(() -> {
+                ThreadLocalRandom random = ThreadLocalRandom.current();
+                for (int j = 0; j < 10000; j++) {
+                    navigableMap.put(random.nextInt(), "test");
+                }
+            });
+        }
+        executorService.shutdown();
+        executorService.awaitTermination(1, TimeUnit.MINUTES);
+    }
+
+    @Test
+    public void givenSkipListMap_whenNavConcurrently_thenCountCorrect() throws InterruptedException {
+        NavigableMap<Integer, Integer> skipListMap = new ConcurrentSkipListMap<>();
+        int count = countMapElementByPollingFirstEntry(skipListMap, 10000, 4);
+        assertEquals(10000 * 4, count);
+    }
+
+    @Test
+    public void givenTreeMap_whenNavConcurrently_thenCountError() throws InterruptedException {
+        NavigableMap<Integer, Integer> treeMap = new TreeMap<>();
+        int count = countMapElementByPollingFirstEntry(treeMap, 10000, 4);
+        assertNotEquals(10000 * 4, count);
+    }
+
+    private int countMapElementByPollingFirstEntry(NavigableMap<Integer, Integer> navigableMap, int elementCount, int concurrencyLevel) throws InterruptedException {
+        for (int i = 0; i < elementCount * concurrencyLevel; i++) {
+            navigableMap.put(i, i);
+        }
+        AtomicInteger counter = new AtomicInteger(0);
+        ExecutorService executorService = Executors.newFixedThreadPool(concurrencyLevel);
+        for (int j = 0; j < concurrencyLevel; j++) {
+            executorService.execute(() -> {
+                for (int i = 0; i < elementCount; i++) {
+                    if (navigableMap.pollFirstEntry() != null) {
+                        counter.incrementAndGet();
+                    }
+                }
+            });
+        }
+        executorService.shutdown();
+        executorService.awaitTermination(1, TimeUnit.MINUTES);
+        return counter.get();
+    }
+
+}