add a demo for HashSet

fishercoder1534 · fishercoder1534 · commit 75ac9050a155 · 2024-02-05T08:41:28.000-08:00
diff --git a/src/main/java/java_collections/HashSetDemo.java b/src/main/java/java_collections/HashSetDemo.java
@@ -0,0 +1,32 @@
+package java_collections;
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class HashSetDemo {
+
+    private static final int NUMBER_OF_SET_ENTRIES = 2;
+
+    public static void main(String... args) {
+        System.out.println("Program started.");
+        HashSetDemo hashSetDemo = new HashSetDemo();
+        hashSetDemo.understandHashSetInternalWorkings();
+        System.out.println("Program finished.");
+    }
+
+    private void understandHashSetInternalWorkings() {
+        /**
+         * 1. Internally, Java uses a HashMap to implement HashSet, it just inserts a dummy object as value into the map: private static final Object PRESENT = new Object();
+         *      you can step into the java.util.HashSet library to see this:
+         *      public boolean add(E e) {
+         *         return map.put(e, PRESENT)==null;
+         *     }
+         * 2. https://medium.com/javarevisited/internal-working-of-hashset-in-java-interview-question-129bdd31fc60 for more references/
+         *     */
+        Set<String> set = new HashSet<>();
+        for (int i = 0; i < NUMBER_OF_SET_ENTRIES; i++) {
+            set.add(i + "");
+        }
+        System.out.println("Method finishes.");
+    }
+}
