从hashmap中删除元素时的java.util.ConcurrentModificationException

Question

我正在学习HashMap课程,并写了这个简单的程序.这段代码适用于向hashmap添加元素,同时从hashmap中删除元素,我遇到java.util.ConcurrentModificationException 的例如这里是我的终端的副本,

[ravi@doom test]$ java TestHashMap 
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :1

 Key : A

 Value : 1
Key/Value : (A,1) added to storage.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :1

 Key : B

 Value : 2
Key/Value : (B,2) added to storage.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :1

 Key : C

 Value : 3
Key/Value : (C,3) added to storage.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :1

 Key : D

 Value : 4
Key/Value : (D,4) added to storage.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :4
( D , 4 );
( A , 1 );
( B , 2 );
( C , 3 );
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :2
Key to REMOVE : 
D
Pair (D,4) Removed.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :4
( A , 1 );
( B , 2 );
( C , 3 );
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :3
Enter Value to remove : 2
Key : B Removed.
Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextEntry(HashMap.java:922)
    at java.util.HashMap$EntryIterator.next(HashMap.java:962)
    at java.util.HashMap$EntryIterator.next(HashMap.java:960)
    at TestHashMap.start(TestHashMap.java:60)
    at TestHashMap.main(TestHashMap.java:87)
ABRT problem creation: 'success'

码:

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

class TestHashMap
{
    private Map<String,Integer> map;
    public TestHashMap()
    {
        map = new HashMap<String,Integer>();
    }
    public void displayMenu()
    {
        System.out.println(".....MENU.....");
        System.out.println("1. Add");
        System.out.println("2. remove key");
        System.out.println("3. remove value");
        System.out.println("4. display");
        System.out.println("7. Exit");
        System.out.print("Your choice :");
    }
    public void start()
    {
        Scanner input = new Scanner(System.in);
        int menuChoice,value;
        String key;
        while(true)
        {
            displayMenu();
            menuChoice = input.nextInt();
            switch(menuChoice)
            {
                case 1:
                    System.out.print("\n Key : ");
                    input.nextLine();
                    key = input.nextLine();
                    System.out.print("\n Value : ");
                    value = input.nextInt();
                    map.put(key,new Integer(value));
                    System.out.println("Key/Value : ("+key+","+value+") added to storage.");
                break;
                case 2:
                    System.out.println("Key to REMOVE : ");
                    input.nextLine();
                    key = input.nextLine();
                    Integer v = map.get(key);
                    if(v == null)
                        System.out.println("No value exists for key "+key);
                    else
                    {
                        map.remove(key);
                        System.out.println("Pair ("+key+","+v.intValue()+") Removed.");
                    }
                    break;
                case 3:
                    System.out.print("Enter Value to remove : ");
                    value = input.nextInt();
                    if(map.containsValue(new Integer(value)))
                    {
                        for(Map.Entry<String,Integer> entry : map.entrySet() )
                        {
                            if(entry.getValue().intValue() == value)
                            {
                                System.out.println("Key : "+entry.getKey()+" Removed.");
                                map.remove(entry.getKey());
                            }
                        }
                    }
                break;
                case 4:
                    for(Map.Entry<String,Integer> entry : map.entrySet() )
                    {
                        System.out.println("( "+entry.getKey()+" , "+entry.getValue()+" );");
                    }
                break;
                case 7:
                    input.close();
                    System.exit(0);
                default:
                    System.out.println("Invalid Choice !");
            }
        }
    }
    public static void main(String args[])
    {
        TestHashMap thm = new TestHashMap();
        thm.start();
    }
}

更新:工作代码

感谢你们(rgettman,Nathan Hughes).

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
import java.util.Iterator;

class TestHashMap
{
    private Map<String,Integer> map;
    public TestHashMap()
    {
        map = new HashMap<String,Integer>();
    }
    public void displayMenu()
    {
        System.out.println(".....MENU.....");
        System.out.println("1. Add");
        System.out.println("2. remove key");
        System.out.println("3. remove value");
        System.out.println("4. display");
        System.out.println("7. Exit");
        System.out.print("Your choice :");
    }
    public void start()
    {
        Scanner input = new Scanner(System.in);
        int menuChoice,value;
        String key;
        while(true)
        {
            displayMenu();
            menuChoice = input.nextInt();
            switch(menuChoice)
            {
                case 1:
                    System.out.print("\n Key : ");
                    input.nextLine();
                    key = input.nextLine();
                    System.out.print("\n Value : ");
                    value = input.nextInt();
                    map.put(key,new Integer(value));
                    System.out.println("Key/Value : ("+key+","+value+") added to storage.");
                break;
                case 2:
                    System.out.println("Key to REMOVE : ");
                    input.nextLine();
                    key = input.nextLine();
                    Integer v = map.get(key);
                    if(v == null)
                        System.out.println("No value exists for key "+key);
                    else
                    {
                        map.remove(key);
                        System.out.println("Pair ("+key+","+v.intValue()+") Removed.");
                    }
                    break;
                case 3:
                    System.out.print("Enter Value to remove : ");
                    value = input.nextInt();
                    if(map.containsValue(new Integer(value)))
                    {
                        for (Iterator<Map.Entry<String, Integer>> it = map.entrySet().iterator();it.hasNext();) 
                        {
                            Map.Entry<String,Integer> x = it.next();
                            if(x.getValue().intValue() == value)
                            {
                                key = x.getKey();
                                it.remove();
                                System.out.println("Key : "+key+" Removed.");
                            }
                        }
                    }
                break;
                case 4:
                    for(Map.Entry<String,Integer> entry : map.entrySet() )
                    {
                        System.out.println("( "+entry.getKey()+" , "+entry.getValue()+" );");
                    }
                break;
                case 7:
                    input.close();
                    System.exit(0);
                default:
                    System.out.println("Invalid Choice !");
            }
        }
    }
    public static void main(String args[])
    {
        TestHashMap thm = new TestHashMap();
        thm.start();
    }
}

Answer 1

你的for循环获取map.entrySet并使用它上面的迭代器来处理映射条目(这个版本的for循环需要一个Iterable,它从Iterable获取迭代器).在地图上使用迭代器时,如果不使用该迭代器从地图中删除内容,则会得到ConcurrentModificationException.那是告诉迭代者它已经过时的地图.

您可以使用iterator显式编写for循环,如下所示:

for (Iterator<Map.Entry<String, Integer> it = map.entrySet().iterator();
it.hasNext();) {

并在需要删除条目时使用iterator的remove方法.

Answer 2

remove你在迭代时正在打电话Map.这一行,增强的for循环,Iterator隐含地运行:

for(Map.Entry<String,Integer> entry : map.entrySet() )

当Iterator检测到其集合被修改时,它会抛出一个ConcurrentModificationException.但是,您可以remove()在Iterator不抛出异常的情况下调用自身.Iterator明确使用:

Iterator<Map.Entry<String, Integer>> itr = map.entrySet().iterator();
while(itr.hasNext())
{
   Map.Entry<String, Integer> entry = itr.next();
   if(entry.getValue().intValue() == 2)
   {
      System.out.println("Key : "+entry.getKey()+" Removed.");
      itr.remove();  // Call Iterator's remove method.
   }
}