描述
大家在工作中应该都会遇到从List集合中删除某一个或多个元素的业务场景
相信大家都会避开在循环里面删除元素，使用其他方式处理
很多面试官也都会问为什么循环里面不能删除元素？
示例
JAVA复制代码
public static void main(String[] args) { List<String> list = new ArrayList<>(); list.add("test0"); list.add("test1"); list.add("test2"); list.add("test3"); list.add("test4"); list.add("test5"); System.out.println(list); list.remove(3); System.out.println(list); list.remove("test1"); System.out.println(list); for (int i = 0; i < list.size(); i++) { if (i == 2) { list.remove(i); } } System.out.println(list); Iterator<String> it = list.iterator(); while (it.hasNext()) { if (it.next().equals("test2")) { it.remove(); } } System.out.println(list); for (String s : list) { if ("test5".equals(s)) { list.remove(s); } } System.out.println(list); } 打印结果 [test0, test1, test2, test3, test4, test5] [test0, test1, test2, test4, test5] [test0, test2, test4, test5] [test0, test2, test5] [test0, test5] Exception in thread "main" java.util.ConcurrentModificationException at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909) at java.util.ArrayList$Itr.next(ArrayList.java:859) at com.fc.store.Test.main(Test.java:46)
从打印结果可以看到
- 根据索引删除 --正常
- 根据元素删除 --正常
- 循环根据索引删除 --正常
- 迭代删除原始 --正常
- 循环根据元素删除 --不正常
执行过程
JAVA复制代码
List<String> list = new ArrayList<>(); 集合初始化时 transient Object[] elementData; 空数组 private int size; 长度为0 protected transient int modCount = 0; 修改次数为0 添加元素 list.add("test0"); elementData[0] = "test0"; size = 1; modCount = 1; list.add("test1"); elementData[0] = "test0"; elementData[1] = "test1"; size = 2; modCount = 2; list.add("test2"); elementData[0] = "test0"; elementData[1] = "test1"; elementData[2] = "test2"; size = 3; modCount = 3; list.add("test3"); elementData[0] = "test0"; elementData[1] = "test1"; elementData[2] = "test2"; elementData[3] = "test3"; size = 4; modCount = 4; list.add("test4"); elementData[0] = "test0"; elementData[1] = "test1"; elementData[2] = "test2"; elementData[3] = "test3"; elementData[4] = "test4"; size = 5; modCount = 5; list.add("test5"); elementData[0] = "test0"; elementData[1] = "test1"; elementData[2] = "test2"; elementData[3] = "test3"; elementData[4] = "test4"; elementData[5] = "test5"; size = 6; modCount = 6; 可以发现每添加一个元素，集合size会增加1， 修改次数会增加1 根据索引删除 list.remove(3); elementData[0] = "test0"; elementData[1] = "test1"; elementData[2] = "test2"; elementData[4] = "test4"; elementData[5] = "test5"; size = 5; modCount = 7; 根据元素删除 list.remove("test1"); elementData[0] = "test0"; elementData[1] = "test2"; elementData[2] = "test4"; elementData[5] = "test5"; size = 4; modCount = 8; 循环根据索引删除 for (int i = 0; i < list.size(); i++) { if (i == 2) { list.remove(i); } } elementData[0] = "test0"; elementData[1] = "test2"; elementData[5] = "test5"; size = 3; modCount = 9; 循环根据索引删除 Iterator<String> it = list.iterator(); while (it.hasNext()) { if (it.next().equals("test2")) { it.remove(); } } elementData[0] = "test0"; elementData[5] = "test5"; size = 2; modCount = 10; 可以发现每删除一个元素后，集合size会减1，修改次数会增加1 循环根据元素删除 for (String s : list) { if ("test2".equals(s)) { list.remove(s); } } 就抛异常了，因为Iterator的next方法会校验是否修改 此时：expectedModCount = 10， modCount = 11 public E next() { checkForComodification(); } final void checkForComodification() { if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } }
源码解析
List 的add和remove方法每次操作都会对modCount++
JAVA复制代码
public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } public E remove(int index) { rangeCheck(index); modCount++; E oldValue = elementData(index); 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 return oldValue; } public boolean remove(Object o) { if (o == null) { for (int index = 0; index < size; index++) { if (elementData[index] == null) { fastRemove(index); return true; } } } else { for (int index = 0; index < size; index++) { if (o.equals(elementData[index])) { fastRemove(index); return true; } } } return false; } 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 } private void ensureCapacityInternal(int minCapacity) { ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); } private void ensureExplicitCapacity(int minCapacity) { modCount++; }
Iterator 会继承List的modCount，并赋值给expectedModCount
JAVA复制代码
private class Itr implements Iterator<E> { int cursor; // index of next element to return int lastRet = -1; // index of last element returned; -1 if no such int expectedModCount = modCount; }
Iterator 的next方法会校验modCount和expectedModCount 是否相同
JAVA复制代码
public E next() { checkForComodification(); int i = cursor; if (i >= size) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) throw new ConcurrentModificationException(); cursor = i + 1; return (E) elementData[lastRet = i]; } final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); }
由于在Iterator的便利中使用了List的remove方法，导致modCount增加了 所以在下次next方法中判断modCount和expectedModCount不一致就直接抛出了异常
解决方案
第一种使用迭代器删除
JAVA复制代码
Iterator<String> it = list.iterator(); while (it.hasNext()) { if (it.next().equals("test2")) { it.remove(); } }
第二种for循环删除后要立即退出
JAVA复制代码
for (String s : list) { if ("test5".equals(s)) { list.remove(s); return; } } JAVA8语法 list.removeIf("test5"::equals);
Set，Map 同理
同样Set，Map循环里面删除报错也是同样的原理
往往大家在遇到问题后，都只是找到了主要原因，但是并没有找到根本原因。
只有通过深入分析找到根本原因，制定预防措施（加入CR清单，添加扫码规则，制定奖惩措施），才能够真正避免问题