Which Java Collections are synchronized(thread safe), which are not?

Question

Which Java Collections are synchronized, which are not?

Example: HashSet is not synchronized

Answer 1

synchronize makes performance lower. of course, Java collection is not synchronized. but Java provides a Synchronization Wrappers to synchronize Java Collection see link

for example:


    import java.util.ArrayList;
    import java.util.Collections;
    import java.util.Iterator;
    import java.util.List;

    public class SynchronizedListExample {

        public static void main(String[] args) {

            List syncList = Collections.synchronizedList(new ArrayList());

            syncList.add("one");//no need to synchronize here
            syncList.add("two");
            syncList.add("three");
            String st = syncList.get(0); //it is ok here => no need to synchronize

            // when iterating over a synchronized list, we need to synchronize access to the synchronized list
           //because if you don't synchronize here, synchList maybe be changed during iterating over it
            synchronized (syncList) {
                Iterator iterator = syncList.iterator();
                while (iterator.hasNext()) {
                    System.out.println("item: " + iterator.next());
                }
            }

        }

    }