如何从ArrayList中删除元素？

Question

I have added data into ArrayList and now want to update that list be deleting some element from it.

I have element something like 1,2,3,4 in ArrayList of type CartEntry .

Code :

ArrayList<CartEntry> items = new ArrayList<CartEntry>();

public void remove(int pId)
{
    System.out.println(items.size());

    for(CartEntry ce : items)
    {
        if(ce.getpId() == pId)
        {
            items.remove(ce);
            //System.out.println(items.get(1));             
        }
    }   
    items.add(new CartEntry(pId));
}

CartEntry Code :

public long getpId() {
    return pId;
}

Constructor :

public CartEntry(long pId) {
    super();
    this.pId = pId;     
}

when I am trying this code it gives me an error:

java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
    at java.util.ArrayList$Itr.next(Unknown Source)

Here pId is the argument that specify that item should be deleted from items. Suppose I want to delete item that have 2 data then what will I have to do ?

Answer 1

You are facing ConcurrentModificationException because you are doing two operations on the same list at a time. ie looping and removing same time.

Inorder to avoid this situation use Iterator,which guarantees you to remove the element from list safely .

A simple example looks like

Iterator<CartEntry> it = list.iterator();
    while (it.hasNext()) {
        if (it.next().getpId() == pId) {
            it.remove();
            break;
        }
    }

Answer 2

There are at least two problems with your code:

1. you call remove on the collection you iterate over, that will cause a ConcurrentModificationException if you continue iterating after the remove .

   There are two ways to fix this:

   • stop iterating after you found the object you want to remove (just add a break or a return ) or
   • switch from the enhanced for-loop to using an Iterator and its remove method .

2. you add an element in your remove method, that's probably not what you want.

So I'd use this code (this assumes that there is only ever one CartEntry with a given id in the list):

public void remove(int pId)
{
    for(CartEntry ce : items)
    {
        if(ce.getpId() == pId)
        {
            items.remove(ce);
            return;
        }
    }   
}

If the assumption with the unique id is not correct, then you'll need to use the Iterator approach:

public void remove(int pId)
{
    Iterator<CartEntry> it = items.iterator();
    while(it.hasNext())
    {
        CartEntry ce = it.next();
        if(ce.getpId() == pId)
        {
            it.remove();
        }
    }   
}

Answer 3

you have created an Arraylist of type carEntry . So you need to create an Iterator of type CarEntry

Iterator<CarEntry> it =  items.iterator();    
while(it.hasNext())
{
   if(it.next().getPId == PId)
   it.remove();
}

Answer 4

在CartEntry中实现.equals然后使用ArrayList.remove（CartEntry）或循环遍历数组列表，找到具有某些条件的项，标记索引，并调用ArrayList.remove（index） - 循环之后

Answer 5

Try,

public void remove(int pId){

Iterator<CartEntry> it = items.iterator();

 while(it.hasNext()) {
    CartEntry entry = it.next();
    if (entry.getpId() == pId) {
        it.remove();
    }
  }
}

Answer 6

The enhanced-for (or for each) loop for iterating over an Expression which is a subtype of Iterable<E> or raw Iterable , basically equivalent to the following form:

  for (I #i = Expression.iterator(); #i.hasNext(); ) {   
      VariableIdentifiers_opt TargetType Identifier = (TargetType) #i.next();
         Statement
   }

This is clearly stated in jls 14.14.2. The enhanced for statement section.

For your context the Expression is an ArrayList . the iterators returned by ArrayList 's iterator method is fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException .

use an Iterator instead and use it's own remove() method:

  Iterator<E>iterator = list.iterator();
  while(iterator.hasNext())
    if(iterator.next().equals(E))
      iterator.remove();