在JAVA的集合(LIST,SET)中通过iterator.hasNext() 或者iterator.next()遍历集合中的数据时,如果这时候add或者remove集合中的元素,会发生ConcurrentModificationException异常.
本文简单说明一下这种现象和解决方法.
如下面的代码,不管是remove还是add都会发生ConcurrentModificationException异常.
public static void testWrongAccess2() {关于remove的解决办法是:利用iterator的remove方法.
Collection<String> myCollection = new ArrayList<String>(10);
myCollection.add("1");
myCollection.add("2");
myCollection.add("3");
myCollection.add("4");
Iterator itr = myCollection.iterator();
while (itr.hasNext()) {
String myObject = (String) itr.next();
System.out.println(myObject);
if ("2".equals(myObject)) {
myCollection.remove(myObject); // cause ConcurrentModificationException
//myCollection.add("4"); // cause ConcurrentModificationException
}
}
}
//1.5之后的写法也是会有异常的
for (String myObject: myCollection) {
System.out.println(myObject);
if ("2".equals(myObject)) {
myCollection.remove(myObject);
}
}
public static void testRightAccess() {对于在集合满足某种条件的情况下add元素的解决:先将要add的元素放在另外一个集合里,在判断遍历完成后在加入.
Collection<String> myCollection = new ArrayList<String>(10);
myCollection.add("1");
myCollection.add("2");
myCollection.add("3");
myCollection.add("4");
Iterator itr = myCollection.iterator();
while (itr.hasNext()) {
String myObject = (String) itr.next();
System.out.println(myObject);
if ("2".equals(myObject)) {
itr.remove();
}
}
}
public static void testRightAccess2() {这里碰到一个有趣的现象:下面的代码没有发生ConcurrentModificationException,在一个只有3个元素的集合中,remove第二个元素,没有ConcurrentModificationException发生而且最后集合里只有1和3,remove 1 和 3 都会有异常,add元素也会有异常,难道是JDK的bug??总之,在remove元素的时候应该用iterator的remove方法.
Collection<String> myCollection = new ArrayList<String>(10);
myCollection.add("1");
myCollection.add("2");
myCollection.add("3");
myCollection.add("4");
Collection<String> tempCollection = new ArrayList<String>(10);
Iterator itr = myCollection.iterator();
while (itr.hasNext()) {
String myObject = (String) itr.next();
if ("1".equals(myObject)) {
tempCollection.add("1 Exists");
}
if ("2".equals(myObject)) {
tempCollection.add("2 Exists");
}
}
myCollection.addAll(tempCollection);
}
public static void testWrongAccess1() {Collection<String> myCollection = new ArrayList<String>(10);myCollection.add("1");myCollection.add("2");myCollection.add("3");Iterator itr = myCollection.iterator(); while (itr.hasNext()) {String myObject = (String) itr.next();System.out.println(myObject);if ("2".equals(myObject)) {myCollection.remove(myObject); // doesn't cause ConcurrentModificationException//myCollection.add("4"); // cause ConcurrentModificationException}}}如果想在读取的同时进行修改,可以用CopyOnWriteArrayList或者CopyOnWriteArraySet
public class CopyOnWriteDemo {public static void main(String args[]) {String[] ss = { "aa", "bb", "cc" };List list1 = new CopyOnWriteArrayList(Arrays.asList(ss));List list2 = new ArrayList(Arrays.asList(ss));Iterator itor1 = list1.iterator();Iterator itor2 = list2.iterator();list1.add("New");list2.add("New");try {printAll(itor1);//不会异常} catch (ConcurrentModificationException e) {System.err.println("Shouldn't get here");e.printStackTrace();}try {printAll(itor2);//ConcurrentModificationException 异常} catch (ConcurrentModificationException e) {System.err.println("Will get ConcurrentModificationException");e.printStackTrace();}//这里会把new打印出来Iterator itor3 = list1.iterator();try {printAll(itor3);} catch (ConcurrentModificationException e) {System.err.println("Will get ConcurrentModificationException");e.printStackTrace();}}private static void printAll(Iterator itor) {while (itor.hasNext()) {System.out.println(itor.next());}}}