試用for-earch遍歷ArrayList,調用remove方法時候,常常回報ConcurrentModificationException異常,今天來從源碼上解析這個問題.
列出來問題
代碼1:
ArrayList<String> list = new ArrayList<>();
list.add("1");
list.add("2");
list.add("2");
list.add("2");
for (String e : list) {
System.out.println(e);
if (e.equals("1")) {
list.remove(e);
}
}
output:
1
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
at java.util.ArrayList$Itr.next(ArrayList.java:851)
at com.own.learn.jdk.ref.Test.main(Test.java:17)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)
``
**代碼2:**
ArrayList<String> list = new ArrayList<>();
list.add("1");
list.add("2");
list.add("2");
list.add("2");
for (int i=0; i< list.size(); i++) {
String e = list.get(i);
System.out.println(e);
if (e.equals("2")) {
list.remove(e);
}
}
output:
1
2
2
**代碼三:**
ArrayList<String> list = new ArrayList<>();
list.add("1");
list.add("2");
list.add("2");
list.add("2");
for (Iterator<String> it = list.iterator(); it.hasNext();) {
String next = it.next();
if(next.equals("2"))
it.remove();
System.out.println(next);
}
output:
1
2
2
2
###解決問題
1,checkForComodification報錯,查看源碼:
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
我們可以清楚看到,modCount和expectedModCount不相等的時候,纔有這個問題
list自帶的reomve方法調用了fastRemove是:
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
}
modCount和expectedModCount方法不同步
而Itr中的remove方法:
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
“`
modCount和expectedModCount方法同步
到這裏ConcurrentModificationException解決了
2,代碼2中輸出少了一個數字,爲啥呢.答案在上面的fastRemove中