ArrayList 的删除姿势你都知道了吗

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.ConcurrentModificationException
        deleteByArrayList(getList(), predicate);
        // 不能删除 报错java.util.ConcurrentModificationException
        deleteByForeach(getList(), predicate);
        //不能删除 报错 java.util.ConcurrentModificationException
        deleteByEnhancedForLoop(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。
```java
 public 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`
```java
  public 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();
            }
        }