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Difference between HashMap and ConcurrentHashMap in Java
Following are the notable differences between HashMap and ConcurrentHashMap classes in Java.
| HashMap | ConcurrentHashMap | |
|---|---|---|
| Synchronized | HashMap is not synchronized. | ConcurrentHashMap is synchronized. |
| Thread Safe | HashMap is not thread safe. | ConcurrentHashMap is thread safe. |
| Iterator type | HashMap iterator is fail-fast and ArrayList throws ConcurrentModificationException if concurrent modification happens during iteration. | ConcurrentHashMap is fail-safe and it will never throw ConcurrentModificationException during iteration. |
| Null values | HashMap allows key and value to be null. | ConcurrentHashMap does not allow null key/value. It will throw NullPointerException. |
| Performance | HashMap is faster. | ConcurrentHashMap is slower than HashMap. |
| Since Java Version | 1.2 | 1.5 |
Example
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
public class Tester {
public static void main(String[] args) {
List<String> arrayList = new ArrayList<>();
arrayList.add("A");
arrayList.add("B");
arrayList.add("C");
Iterator<String> iterator = arrayList.iterator();
System.out.println(arrayList);
while (iterator.hasNext()) {
if (iterator.next().equals("C")) {
//removal is allowed.
iterator.remove();
}
}
System.out.println(arrayList);
List<String> copyOnWriteArrayList = new CopyOnWriteArrayList<>();
copyOnWriteArrayList.add("A");
copyOnWriteArrayList.add("B");
copyOnWriteArrayList.add("C");
Iterator<String> iterator1 = copyOnWriteArrayList.iterator();
System.out.println(copyOnWriteArrayList);
while (iterator1.hasNext()) {
if (iterator1.next().equals("C")) {
try{
iterator1.remove();
}catch(UnsupportedOperationException e){
System.out.println("Removal not allowed.");
}
}
}
System.out.println(copyOnWriteArrayList);
}
}Output
[A, B, C] [A, B] [A, B, C] Removal not allowed. [A, B, C]
Updated on: 21-Jun-2020
4K+ Views
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