问题
I am learning about HashMap class and wrote this simple program.
this code works good for adding elements to the hashmap and while removing elements from the hashmap , I am encountering java.util.ConcurrentModificationException
for example here is a copy of my terminal,
[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'
code:
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();
}
}
UPDATE: working code
thanks to both (rgettman,Nathan Hughes) of you.
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();
}
}
回答1:
Your for-loop gets the map.entrySet and uses the iterator on it to work through the map entries (this version of the for-loop requires an Iterable, it gets the iterator from the Iterable). When you are using an iterator on a map, but remove things from the map without using that iterator, you get the ConcurrentModificationException. That is the map telling the iterator that it's out of date.
You can write a for loop using the iterator explicitly, like this:
for (Iterator<Map.Entry<String, Integer> it = map.entrySet().iterator();
it.hasNext();) {
and use the iterator's remove method when you need to delete an entry.
回答2:
You are calling remove while you're iterating over the Map. This line, the enhanced for loop, runs an Iterator implicitly:
for(Map.Entry<String,Integer> entry : map.entrySet() )
When an Iterator detects that its collection is modified, it throws a ConcurrentModificationException. However, you can call remove() on the Iterator itself without that exception being thrown. Use an Iterator explicitly:
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.
}
}
回答3:
You cannot remove an element from a map which you are currently iterating over. You could define an iterator, or you could make a few simple modifications to your code within the block for case 3.
case 3:
System.out.print("Enter Value to remove : ");
value = input.nextInt();
if(map.containsValue(new Integer(value)))
{
Map.Entry<String,Integer> foo = null;
for(Map.Entry<String,Integer> entry : map.entrySet() )
if(entry.getValue().intValue() == value)
foo = entry;
System.out.println("Key : "+foo.getKey()+" Removed.");
map.remove(foo.getKey());
}
break;
回答4:
Just Use Iterator class to remove the Iterating Element as
for (Iterator<Map.Entry<String, Integer> it = map.entrySet().iterator();
while(it.hasNext()) {
Map.Entry<String, Integer> e= itr.next();
key = e.getKey();
Value = e.getValue();
//Your Other Code Here
it.remove(); //It removes the current Itertaion from Map
}
来源：https://stackoverflow.com/questions/26494197/java-util-concurrentmodificationexception-when-removing-elements-from-a-hashmap