Removing items from a list

Question

When navigating through a list, I would like to remove the list item depending on the condition. See code below.

This gives me a ConcurrentModification exception.

 for (Object a : list) { if (a.getXXX().equalsIgnoreCase("AAA")) { logger.info("this is AAA........should be removed from the list "); list.remove(a); } } 

How can I do that?

Answer 1

You need to use Iterator and call remove() on Iterator instead of using a for loop.

Answer 2

 for (Iterator<String> iter = list.listIterator(); iter.hasNext(); ) { String a = iter.next(); if (...) { iter.remove(); } } 

Making an additional assumption that the list has strings. As already mentioned, list.iterator() is required. listIterator can do a bit of navigation.

Answer 3

You cannot do this because you are already looping on it.

To avoid this situation, use Iterator, which ensures that you remove the item from the list safely ...

 List<Object> objs; Iterator<Object> i = objs.iterator(); while (i.hasNext()) { Object o = i.next(); //some condition i.remove(); } 

Answer 4

You cannot and should not modify the list while iterating over it. You can solve this problem by temporarily saving objects for deletion:

 List<Object> toRemove = new ArrayList<Object>(); for(Object a: list){ if(a.getXXX().equalsIgnoreCase("AAA")){ toRemove.add(a); } } list.removeAll(toRemove); 

Answer 5

Besides the excellent solutions offered here, I would like to propose another solution.

I'm not sure you can add dependencies, but if you can, you can add https://code.google.com/p/guava-libraries/ as a dependency. This library adds support for many basic functional operations in Java and can make it easier to work and work with collections.

In the code, I replaced the List type with T, since I don’t know what your list is printed on.

This problem can be solved with guava as follows:

 List<T> filteredList = new Arraylist<>(filter(list, not(XXX_EQUAL_TO_AAA))); 

And somewhere else you define XXX_EQUAL_TO_AAA as:

 public static final Predicate<T> XXX_EQUAL_TO_AAA = new Predicate<T>() { @Override public boolean apply(T input) { return input.getXXX().equalsIgnoreCase("AAA"); } } 

However, this is probably too large in your situation. This is what becomes more powerful the more you work with collections.

Ohw, you also need these static imports:

 import static com.google.common.base.Predicates.not; import static com.google.common.collect.Collections2.filter; 

Answer 6

 //first find out the removed ones List removedList = new ArrayList(); for(Object a: list){ if(a.getXXX().equalsIgnoreCase("AAA")){ logger.info("this is AAA........should be removed from the list "); removedList.add(a); } } list.removeAll(removedList);