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package com.workit.demo.array;import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.function.Predicate;/*** @author:* @Date: 2020/6/9* @Description:*/
public class ArrayListDelete {public static void main(String[] args) {Predicate<String> predicate = p -> p.equals("a") || p.equals("b");// 可以正常删除结果正确deleteByIterator(getList(), predicate);// 可以正常删除结果正确deleteByReverseOrder(getList(), predicate);// 可以删除 结果不正确deleteByOrder(getList(), predicate);// 不能删除 报错java.util.ConcurrentModificationExceptiondeleteByArrayList(getList(), predicate);// 不能删除 报错java.util.ConcurrentModificationExceptiondeleteByForeach(getList(), predicate);//不能删除 报错 java.util.ConcurrentModificationExceptiondeleteByEnhancedForLoop(getList(), predicate);// 正常删除数据       deleteAll(getList(), predicate);}public static  List<String> getList() {List<String> list = new ArrayList<>();list.add("a");list.add("b");list.add("c");return list;}/*** 普通for循环倒序删除* 可以正常删除  结果正确* @param list* @param predicate*/public static void deleteByReverseOrder(List<String> list, Predicate<String> predicate) {for (int i = list.size() - 1; i >= 0; i--) {if (predicate.test(list.get(i))) {list.remove(list.get(i));}}System.out.println(Thread.currentThread().getStackTrace()[1].getMethodName()+":"+list.toString());}/*** 普通for循环正序删除*可以删除  结果不正确* @param list* @param predicate*/public static void deleteByOrder(List<String> list, Predicate<String> predicate) {for (int i = 0; i < list.size(); i++) {if (predicate.test(list.get(i))) {list.remove(list.get(i));}}System.out.println(Thread.currentThread().getStackTrace()[1].getMethodName()+":"+list.toString());}/*** 迭代器循环，使用ArrayList的remove()方法删除* 可以删除  结果不正确* @param list* @param predicate*/public static void deleteByArrayList(List<String> list, Predicate<String> predicate) {Iterator<String> iterator = list.iterator();while (iterator.hasNext()) {if (predicate.test(iterator.next())) {list.remove(iterator.next());}}System.out.println(Thread.currentThread().getStackTrace()[1].getMethodName()+":"+list.toString());}/*** 迭代器循环，使用迭代器的remove()方法删除* 可以正常删除结果正确* @param list* @param predicate*/public static void deleteByIterator(List<String> list, Predicate<String> predicate) {Iterator<String> iterator = list.iterator();while (iterator.hasNext()) {if (predicate.test(iterator.next())) {iterator.remove();}}System.out.println(Thread.currentThread().getStackTrace()[1].getMethodName()+":"+list.toString());}/*** java8 forEach方法删除*不能删除 报错 java.util.ConcurrentModificationException* @param list* @param predicate*/public static void deleteByForeach(List<String> list, Predicate<String> predicate) {list.forEach(p -> {if (predicate.test(p)) {list.remove(p);}});System.out.println(Thread.currentThread().getStackTrace()[1].getMethodName()+":"+list.toString());}/*** 增强版for循环删除*不能删除 报错 java.util.ConcurrentModificationException* @param list* @param predicate*/public static void deleteByEnhancedForLoop(List<String> list, Predicate<String> predicate) {for (String string : list) {if (predicate.test(string)) {list.remove(string);}}}}/*** 调用批量删除方法* @param list* @param predicate*/public static void deleteAll(List<String> list, Predicate<String> predicate) {List<String> removeList = new ArrayList<>();for (String string : list) {if (predicate.test(string)) {removeList.add(string);}}list.removeAll(removeList);System.out.println(Thread.currentThread().getStackTrace()[1].getMethodName()+":"+list.toString());}

@SuppressWarnings("unchecked")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();}

private void fastRemove(int index) {modCount++; //modCount`都会进行加1操作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}```
所以上面两个方法都会抛出`ConcurrentModificationException`异常。
#### java8 forEach方法删除(抛出异常)
-  **java8 forEach方法删除**（`deleteByForeach`）为什么也会抛**`ConcurrentModificationException`异常呢？答案还是在源码里面。
同上面一样删除一个元素后`modCount` 进行了加`1`而`expectedModCount` 没有变化。
```javapublic void forEach(Consumer<? super E> action) {Objects.requireNonNull(action);final int expectedModCount = modCount;@SuppressWarnings("unchecked")final E[] elementData = (E[]) this.elementData;final int size = this.size;for (int i=0; modCount == expectedModCount && i < size; i++) {action.accept(elementData[i]);}if (modCount != expectedModCount) { // 是不是又是这个判断throw new ConcurrentModificationException();}

for (int i = 0; i < list.size(); i++) {if (predicate.test(list.get(i))) {list.remove(list.get(i));// 新增这个修正i的值i--;}}```
是不是又get了一个骚操作。
![在这里插入图片描述](https://img-blog.csdnimg.cn/20200610234309264.png)
#### 使用迭代器的remove()方法删除（推荐做法）
**迭代器循环，使用迭代器的remove()方法删除**（`deleteByIterator`）这个比较简单我们直接看迭代器的删除
关键代码就一行 `expectedModCount = modCount`
```javapublic void remove() {if (lastRet < 0)throw new IllegalStateException();checkForComodification();try {ArrayList.this.remove(lastRet);cursor = lastRet;lastRet = -1;expectedModCount = modCount; //调用ArrayList的删除方法后多了这么一句话。} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}