IdentityHashMapDemonstrator class

java-collections-maps-3/src/main/java/com/baeldung/map/identity/IdentityHashMapDemonstrator.java

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+package com.baeldung.map.identity;
+
+import java.util.*;
+
+public class IdentityHashMapDemonstrator {
+    public static void main(String[] args) {
+        IdentityHashMap<String, String> identityHashMap = createWithSimpleData();
+        System.out.println("Map details: " + identityHashMap);
+        IdentityHashMap<String, String> copiedMap = createFromAnotherMap(identityHashMap);
+
+        updateWithNewValue(copiedMap);
+        iterateIdentityHashMap(copiedMap);
+        addNullKeyValue();
+        demoHashMapVsIdentityMap(identityHashMap);
+        demoMutableKeys();
+
+        Map<String, String> synchronizedMap = getSynchronizedMap();
+        //Do multithreaded operations on synchronizedMap
+    }
+
+    private static void addNullKeyValue() {
+        IdentityHashMap<String, String> identityHashMap = new IdentityHashMap<>();
+        identityHashMap.put(null, "Null Key Accepted");
+        identityHashMap.put("Null Value Accepted", null);
+        System.out.println(identityHashMap);
+    }
+
+    private static void iterateIdentityHashMap(IdentityHashMap<String, String> identityHashMap) {
+        // Iterating using entrySet
+        System.out.println("Iterating values: ");
+        Set<Map.Entry<String, String>> entries = identityHashMap.entrySet();
+        for (Map.Entry<String, String> entry: entries) {
+            System.out.println(entry.getKey() + ": " + entry.getValue());
+        }
+
+        // Iterating using keySet
+        System.out.println("Iterating values using keySet: ");
+        for (String key: identityHashMap.keySet()) {
+            System.out.println(key + ": " + identityHashMap.get(key));
+        }
+
+        // Throws error if we modify while iterating
+        System.out.println("This iteration throws error: ");
+        try {
+            for (Map.Entry<String, String> entry: entries) {
+                System.out.println(entry.getKey() + ": " + entry.getValue());
+                identityHashMap.remove("title");
+            }
+        } catch (ConcurrentModificationException ex) {
+            System.out.println("This exception will raise for sure, if we modify while iterating");
+        }
+    }
+
+    private static class Book {
+        String title;
+        int year;
+
+        Book() {
+            // nothing to do
+        }
+
+        Book(String title, int year) {
+            this.title = title;
+            this.year = year;
+        }
+
+        @Override
+        public boolean equals(Object o) {
+            if (this == o) return true;
+            if (o == null || getClass() != o.getClass()) return false;
+            Book book = (Book) o;
+            return year == book.year && title.equals(book.title);
+        }
+
+        @Override
+        public int hashCode() {
+            return Objects.hash(title, year);
+        }
+
+        @Override
+        public String toString() {
+            return "Book{" +
+                    "title='" + title + '\'' +
+                    ", year=" + year +
+                    '}';
+        }
+    }
+
+    private static void demoMutableKeys() {
+        Book book1 = new Book("A Passage to India", 1924);
+        Book book2 = new Book("Invisible Man", 1953);
+        HashMap<Book, String> hashMap = new HashMap<>(10);
+        IdentityHashMap<Book, String> identityHashMap = new IdentityHashMap<>(10);
+
+        hashMap.put(book1, "A great work of fiction");
+        identityHashMap.put(book1, "A great work of fiction");
+
+        hashMap.put(book2, "won the US National Book Award");
+        identityHashMap.put(book2, "won the US National Book Award");
+
+        book2.year = 1952;
+        System.out.println("Book2 from HashMap: " + hashMap.get(book2));
+        System.out.println("Book2 from IdentityHashMap: " + identityHashMap.get(book2));
+        System.out.println("HashMap: " + hashMap);
+        System.out.println("IdentityHashMap: " + identityHashMap);
+    }
+
+    private static void demoHashMapVsIdentityMap(IdentityHashMap<String, String> identityHashMap) {
+        HashMap<String, String> hashMap = new HashMap<>(identityHashMap);
+        hashMap.put(new String("genre"), "Drama");
+        identityHashMap.put(new String("genre"), "Drama");
+        System.out.println("HashMap size: " + hashMap.size());
+        System.out.println("IdentityHashMap size: " + identityHashMap.size());
+        System.out.println("HashMap content: " + hashMap);
+        System.out.println("IdentityHashMap content: " + identityHashMap);
+    }
+
+    private static Map<String, String> getSynchronizedMap() {
+        Map<String, String> synchronizedMap = Collections.synchronizedMap(new IdentityHashMap<String, String>());
+        return synchronizedMap;
+    }
+
+    private static IdentityHashMap<String, String> createFromAnotherMap(Map<String, String> otherMap) {
+        IdentityHashMap<String, String> identityHashMap = new IdentityHashMap<>(otherMap);
+        return identityHashMap;
+    }
+
+    private static void updateWithNewValue(IdentityHashMap<String, String> identityHashMap) {
+        String oldTitle = identityHashMap.put("title", "Harry Potter and the Deathly Hallows");
+        System.out.println("Old Title: " + oldTitle);
+        System.out.println("Updated Title: " + identityHashMap.get("title"));
+    }
+
+    public static void addValue(IdentityHashMap<String, String> identityHashMap, String key, String value) {
+        identityHashMap.put(key, value);
+    }
+
+    public static void addAllValues(IdentityHashMap<String, String> identityHashMap, Map<String, String> otherMap) {
+        identityHashMap.putAll(otherMap);
+    }
+
+    public static IdentityHashMap<String, String> createWithSimpleData() {
+        IdentityHashMap<String, String> identityHashMap = new IdentityHashMap<>();
+        identityHashMap.put("title", "Harry Potter and the Goblet of Fire");
+        identityHashMap.put("author", "J. K. Rowling");
+        identityHashMap.put("language", "English");
+        identityHashMap.put("genre", "Fantasy");
+        return identityHashMap;
+    }
+}

java-collections-maps-3/src/test/java/com/baeldung/map/identity/IdentityHashMapDemonstratorUnitTest.java

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+package com.baeldung.map.identity;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.IdentityHashMap;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class IdentityHashMapDemonstratorUnitTest {
+
+    @Test
+    public void givenIdentityHashMap_whenNewObjectWithSameKey_thenAddsAsNewValue() {
+        IdentityHashMap<String, String> identityHashMap = IdentityHashMapDemonstrator.createWithSimpleData();
+        String newGenreKey = new String("genre");
+        identityHashMap.put(newGenreKey, "Drama");
+
+        assertEquals(5, identityHashMap.size());
+        assertEquals("Fantasy", identityHashMap.get("genre"));
+        assertEquals("Drama", identityHashMap.get(newGenreKey));
+    }
+}