List 中正确的增删操作

Posted 2020-10-05 AmoryWang_JavaSunny

这个为什么要单独说的原因是，在开发中的对数据库中的增删为最基本的，但是是不是写对了就尤为重要

先来看代码:

 1 public void testLoopInList(){
 2         List<String> a = new ArrayList<String>();
 3         a.add("1");
 4         a.add("2");
 5         a.add("w");
 6         for (String string : a) {
 7             System.out.println(string);
 8         }
 9         
10         for (String temp : a) {
11             if("2".equals(temp)){
12                 a.remove(temp);
13             }
14         }
15         
16         for (String string : a) {
17             System.out.println(string);
18         }

输出：

1
2

java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:859)
    at java.util.ArrayList$Itr.next(ArrayList.java:831)
    at test.Test_ForeachAndIterator.testLoopInList(Test_ForeachAndIterator.java:19)

出现错误了

原因：首先从错误中可以看出，是ArraList中的Iterator的checkForComodification()出现的错误，说明了foreach的实现原理其实就是实现了内部类I特Iterator来进行遍历的，其次为什么会出错呢？

因为在ArrayList和ArrayList的内部类的Iterator中的都会存在remove的方法，而ArrayList和Iterator中都各自有自己的维持长度的变量，前者是modCount，后者是expectModCount，但是源码中ArrayList的remove方法是会改变modCount的值，但却不会直接同步到expectModCount的值的，而Iterator中时刻都插入了checkForComodification()方法来监测modCount是否与expectModCount相等，所以就会很容易出现异常错误，当然下面的代码也是错的

 1     public void testLoopInList(){
 2         List<String> a = new ArrayList<String>();
 3         a.add("1");
 4         a.add("2");
 5         
 6         for (String string : a) {
 7             System.out.println(string);
 8         }
 9         
10         Iterator<String> it = a.iterator();
11         while(it.hasNext()){
12             if("1".equals(it.next())){
13                 a.remove(it.next());
14             }
15         }
16         
17         for (String string : a) {
18             System.out.println(string);
19         }
20     }

错误原因同上

所以在解决问题的关键就是要避免这个异常的出现，也就是时刻让modCount==expectModCount,所以就是使用iterator的remove方法，因为会有使两者相等的代码

即下面：

 1 public void testLoopInList(){
 2         List<String> a = new ArrayList<String>();
 3         a.add("1");
 4         a.add("2");
 5         
 6         for (String string : a) {
 7             System.out.println(string);
 8         }
 9         
10         Iterator<String> it = a.iterator();
11         while(it.hasNext()){
12             if("1".equals(it.next())){
13                 it.remove();
14             }
15         }
16         
17         for (String string : a) {
18             System.out.println(string);
19         }
20     }

 

相关源码提上:

 1 private class Itr implements Iterator<E> {
 2    /**
 3     * Index of element to be returned by subsequent call to next.
 4     */
 5    int cursor = 0;
 6    /**
 7     * Index of element returned by most recent call to next or
 8     * previous.  Reset to -1 if this element is deleted by a call
 9     * to remove.
10     */
11    int lastRet = -1;
12    /**
13     * The modCount value that the iterator believes that the backing
14     * List should have.  If this expectation is violated, the iterator
15     * has detected concurrent modification.
16     */
17    int expectedModCount = modCount;
18    public boolean hasNext() {
19            return cursor != size();
20    }
21    public E next() {
22            checkForComodification();
23        try {
24        E next = get(cursor);
25        lastRet = cursor++;
26        return next;
27        } catch (IndexOutOfBoundsException e) {
28        checkForComodification();
29        throw new NoSuchElementException();
30        }
31    }
32    public void remove() {
33        if (lastRet == -1)
34        throw new IllegalStateException();
35            checkForComodification();
36        try {
37        AbstractList.this.remove(lastRet);
38        if (lastRet < cursor)
39            cursor--;
40        lastRet = -1;
41        expectedModCount = modCount;
42        } catch (IndexOutOfBoundsException e) {
43        throw new ConcurrentModificationException();
44        }
45    }
46    final void checkForComodification() {
47        if (modCount != expectedModCount)
48        throw new ConcurrentModificationException();
49    }

ArrayList中的remove

 1     public boolean remove(Object o) {
 2         if (o == null) {
 3             for (int index = 0; index < size; index++)
 4                 if (elementData[index] == null) {
 5                     fastRemove(index);
 6                     return true;
 7                 }
 8         } else {
 9             for (int index = 0; index < size; index++)
10                 if (o.equals(elementData[index])) {
11                     fastRemove(index);
12                     return true;
13                 }
14         }
15         return false;
16     }