public V remove(Object key) {int hash = hash(key.hashCode());return segmentFor(hash).remove(key, hash, null);}

   V remove(Object key, int hash, Object value) {lock();try {int c = count - 1;HashEntry<K,V>[] tab = table;int index = hash & (tab.length - 1);HashEntry<K,V> first = tab[index];HashEntry<K,V> e = first;while (e != null && (e.hash != hash || !key.equals(e.key)))e = e.next;V oldValue = null;if (e != null) {V v = e.value;if (value == null || value.equals(v)) {oldValue = v;// All entries following removed node can stay// in list, but all preceding ones need to be// cloned.++modCount;HashEntry<K,V> newFirst = e.next;for (HashEntry<K,V> p = first; p != e; p = p.next)newFirst = new HashEntry<K,V>(p.key, p.hash,newFirst, p.value);tab[index] = newFirst;count = c; // write-volatile
                    }}return oldValue;} finally {unlock();}}

package tij;import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;/*** Created by huaox on 2017/4/21.**/
public class CopyOnWriteArrayListDemo {private static class ReadTask implements Runnable {List<String> list;ReadTask(List<String> list) {this.list = list;}public void run() {for (String str : list) {System.out.println(str);}}}private static class WriteTask implements Runnable {List<String> list;int index;WriteTask(List<String> list, int index) {this.list = list;this.index = index;}public void run() {list.remove(index);list.add(index, "write_" + index);}}public void run() {final int NUM = 5;List<String> list = new ArrayList<String>();for (int i = 0; i < NUM; i++) {list.add("main_" + i);}ExecutorService executorService = Executors.newFixedThreadPool(NUM);for (int i = 0; i < NUM; i++) {executorService.execute(new ReadTask(list));executorService.execute(new WriteTask(list, i));}executorService.shutdown();}public static void main(String[] args) {new CopyOnWriteArrayListDemo().run();}
}

    Exception in thread "pool-1-thread-3" java.util.ConcurrentModificationException
main_0
main_1at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
main_2
main_3at java.util.ArrayList$Itr.next(ArrayList.java:851)
main_4
main_0
main_1
write_2
main_3
main_4
main_0
main_1
write_2
write_3
main_4
main_0
write_0
write_1at tij.CopyOnWriteArrayListDemo$ReadTask.run(CopyOnWriteArrayListDemo.java:22)
write_2
write_3at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
write_4at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)at java.lang.Thread.run(Thread.java:745)Process finished with exit code 0

public boolean add(E e) {final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();int len = elements.length;Object[] newElements = Arrays.copyOf(elements, len + 1);newElements[len] = e;setArray(newElements);return true;} finally {lock.unlock();}}

 public void run() {final int NUM = 5;List<String> list = new CopyOnWriteArrayList<>();for (int i = 0; i < NUM; i++) {list.add("main_" + i);}ExecutorService executorService = Executors.newFixedThreadPool(NUM);for (int i = 0; i < NUM; i++) {executorService.execute(new ReadTask(list));executorService.execute(new WriteTask(list, i));}executorService.shutdown();}

main_0
main_1
main_2
main_3
main_4
main_0
main_1
write_2
main_3
main_4
write_0
main_1
write_2
write_3
main_4
write_0
main_1
write_2
write_3
write_4
write_0
write_1
write_2
write_3
write_4Process finished with exit code 0