[Java] Java 如何绕过迭代器遍历时的数据修改异常

List<String> list = new ArrayList<>();
Collections.addAll(list, "Hello", "World", "C++", "Java");
// 我想输出 Hello,World,Java,迭代时发现多一个 C++，所以直接删除掉。
Iterator iterator = list.iterator();
System.out.println(iterator.next());
System.out.println(iterator.next());
list.remove("C++");
System.out.println(iterator.next());

Hello
World
 
java.util.ConcurrentModificationException
  at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:907)
  at java.util.ArrayList$Itr.next(ArrayList.java:857)
  at com.wdbyte.lab.jdk.ModCountDemo.updateCollections(ModCountDemo.java:26)

final void checkForComodification() {
  if (modCount != expectedModCount)
    throw new ConcurrentModificationException();
}

/**
 * The number of times this list has been <i>structurally modified</i>.
 * Structural modifications are those that change the size of the
 * list, or otherwise perturb it in such a fashion that iterations in
 * progress may yield incorrect results.
 *
 * <p>This field is used by the iterator and list iterator implementation
 * returned by the {@code iterator} and {@code listIterator} methods.
 * If the value of this field changes unexpectedly, the iterator (or list
 * iterator) will throw a {@code ConcurrentModificationException} in
 * response to the {@code next}, {@code remove}, {@code previous},
 * {@code set} or {@code add} operations.  This provides
 * <i>fail-fast</i> behavior, rather than non-deterministic behavior in
 * the face of concurrent modification during iteration.
 *
 * <p><b>Use of this field by subclasses is optional.</b> If a subclass
 * wishes to provide fail-fast iterators (and list iterators), then it
 * merely has to increment this field in its {@code add(int, E)} and
 * {@code remove(int)} methods (and any other methods that it overrides
 * that result in structural modifications to the list).  A single call to
 * {@code add(int, E)} or {@code remove(int)} must add no more than
 * one to this field, or the iterators (and list iterators) will throw
 * bogus {@code ConcurrentModificationExceptions}.  If an implementation
 * does not wish to provide fail-fast iterators, this field may be
 * ignored.
 */
protected transient int modCount = 0;

public void trimToSize() {
  modCount++;
  if (size < elementData.length) {
    elementData = (size == 0)
      ? EMPTY_ELEMENTDATA
      : Arrays.copyOf(elementData, size);
  }
}

List<String> list = new ArrayList<>();
Collections.addAll(list, "Hello", "World", "C++", "Java");
 
list.listIterator();
Iterator iterator = list.iterator();
System.out.println(iterator.next());
System.out.println(iterator.next());
list.remove("C++");
// 40 多亿次遍历，溢出到负数，继续溢出到原值
for (int n = Integer.MIN_VALUE; n < Integer.MAX_VALUE; n++) ((ArrayList) list).trimToSize();
System.out.println(iterator.next());

List<String> list = new ArrayList<>();
Collections.addAll(list, "Hello", "World", "C++", "Java");
 
list.listIterator();
Iterator iterator = list.iterator();
System.out.println(iterator.next());
System.out.println(iterator.next());
 
// 开始操作
list.set(2, "Java");
Phaser phaser = new Phaser(2);
Thread main = Thread.currentThread();
new Thread(() -> {
  synchronized (System.out) {
    phaser.arriveAndDeregister();
    while (main.getState() != State.BLOCKED) {
      try {
        Thread.sleep(100);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
    list.remove(3);
  }
}).start();
phaser.arriveAndAwaitAdvance();
 
System.out.println(iterator.next());
 
// 输出集合
System.out.println(list);
 
/**
 * 得到输出
 * 
 * Hello
 * World
 * Java
 * [Hello, World, Java]
 */

List<String> list = new ArrayList<>();
Collections.addAll(list, "Hello", "World", "C++", "Java");
 
list.listIterator();
Iterator iterator = list.iterator();
System.out.println(iterator.next());
System.out.println(iterator.next());
 
// 开始操作
((List)list).set(2, new Object() {
  public String toString() {
    String s = list.get(3);
    list.remove(this);
    return s;
  }
});
 
System.out.println(iterator.next());