在Java使用迭代器时出现 ConcurrentModificationException

ConcurrentModificationException已被检测到对象的并发修改的方法抛出，而此类修改是不允许的。如果线程在使用快速失败迭代器迭代集合时直接修改集合，则迭代器将抛出此ConcurrentModificationException 。在这里，我们将通过一个例子来理解这个异常，它为什么会发生，以及如何同时进行更改，这是此异常的根本原因。在后面的部分，我们将了解如何修复它。

示例 1： ConcurrentModificationException

Java

// Java Program to ConcurrentModificationException while
// using Iterator
  
// Importing ArrayList and Iterator classes from java.util
// package
import java.util.ArrayList;
import java.util.Iterator;
  
// Main class
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
        // Creating an object of ArrayList class
        // Declaring object of Integer type
        ArrayList list = new ArrayList<>();
  
        // Adding custom integer elements to the object
        list.add(1);
        list.add(2);
        list.add(3);
        list.add(4);
        list.add(5);
  
        // Iterating over object elements using iterator
        Iterator iterator = list.iterator();
  
        // Condition check
        // It holds true till there is single element
        // remainin in the List
        while (iterator.hasNext()) {
  
            // Rolling over to next element using next()
            // method
            Integer value = iterator.next();
  
            // Print the element value
            System.out.println("value: " + value);
  
            // If element equals certain value
            if (value.equals(2)) {
  
                // Display command for better readability
                System.out.println(
                    "========================");
  
                // Removing entered value in object
                System.out.println("removing value: "
                                   + value);
  
                // Making changes simultaneously
                System.out.println(
                    "========================");
                list.remove(value);
            }
        }
    }
}

输出：

输出说明：

ConcurrentModificationException 在调用next()方法时抛出，因为迭代器正在迭代 List，并且我们同时在其中进行修改。现在为了避免这种异常，让我们讨论一种直接使用迭代器的方法，如下所示：

示例 2：解决 ConcurrentModificationException

Java

// Java Program to Avoid ConcurrentModificationException by
// directly using Iterator
  
// Importing ArrayList and Iterator classes
// from java.util package
import java.util.ArrayList;
import java.util.Iterator;
  
// Main class
public class Main {
  
    // Mai driver method
    public static void main(String[] args)
    {
        // Creating an ArrayList object of integer type
        ArrayList list = new ArrayList<>();
  
        // Custom integer elements are added
        list.add(1);
        list.add(2);
        list.add(3);
        list.add(4);
        list.add(5);
  
        // Iterating directly over elements of object
        Iterator iterator = list.iterator();
  
        // Condition check
        // It holds true till there is single element
        // remaining in the List using hasNext() method
        while (iterator.hasNext()) {
  
            // Rolling over elements using next() method
            Integer value = iterator.next();
  
            // print the values
            System.out.println("value: " + value);
  
            // If value equals certain integer element
            // entered Say it be 2
            if (value.equals(2)) {
  
                // Display command only
                System.out.println(
                    "========================");
  
                // Removing the entered value 
                System.out.println("removing value: "
                                   + value);
                 
                // Display command only
                System.out.println(
                    "========================");
                
                // Removing current value in Collection
                // using remove() method 
                iterator.remove();
            }
        }
    }
}

输出：

输出说明：

不会抛出 ConcurrentModificationException，因为 remove() 方法不会导致 ConcurrentModificationException。