根据arraylist元素之间的特定比较，删除自定义对象类型的arraylist的内容

Question

I have an ArrayList of type RemoveTest, where RemoveTest is a user defined class. RemoveTest has two properties of String type mId and rmId. I need to find in the ArrayList, elements(Objects of type RemoveTest) satisfying the below criteria: such that the value of obj1.rmId() is same as obj2.getmId() and then remove both of these elements(obj1 and obj2).

I tried this by writing the below code:

import java.util.ArrayList;
import java.util.Iterator;

public class RemoveItr {

    public static void main(String[] args){
        ArrayList<RemoveTest> eleList = new ArrayList<RemoveTest>();
        RemoveTest obj1 = new RemoveTest();
        obj1.setmId("m1");
        obj1.setRmId("");
        RemoveTest obj2 = new RemoveTest();
        obj2.setmId("m2");
        obj2.setRmId("m1");
        RemoveTest obj3 = new RemoveTest();
        obj3.setmId("m3");
        obj3.setRmId("");
        RemoveTest obj4 = new RemoveTest();
        obj4.setmId("m4");
        obj4.setRmId("m3");
        RemoveTest obj5 = new RemoveTest();
        obj5.setmId("m5");
        obj5.setRmId("");
        eleList.add(obj1);
        eleList.add(obj2);
        eleList.add(obj3);
        eleList.add(obj4);
        eleList.add(obj5);
        Iterator<RemoveTest> i = eleList.iterator();
        while(i.hasNext()){
            RemoveTest fwdM =(RemoveTest)i.next();
            String fwdId = fwdM.getmId();
            Iterator<RemoveTest> ni = eleList.iterator();

            while(ni.hasNext()){
                RemoveTest revM =(RemoveTest)ni.next();
                String revId = revM.getRmId();
                if(fwdId.equals(revId)){
                    System.out.println("fwdId "+fwdId+"- revId "+revId);
                     i.remove();
                     ni.remove();

                }

            }
        }
    }
}

public class RemoveTest {

    String mId;
    String rmId;

    public String getmId() {
        return mId;
    }
    public void setmId(String mId) {
        this.mId = mId;
    }
    public String getRmId() {
        return rmId;
    }
    public void setRmId(String rmId) {
        this.rmId = rmId;
    }

    }

Note: both classes are Public as they were not in the same source file.

But, I got ConcurrentModificationException and I believe it is because, that as I was already in the middle of iterating through the arraylist, and then other loop(iterator) steps in and tries to operate on the same ArrayList.

Is this understanding correct? and If so, how can I resolve this problem and achieve the solution.

Answer 1

You can't modify a collection while iterating over it, except by using Iterator.remove() .

This rule implies that you can't use two iterators simultaneously and expect remove() to work, because calling remove() on one iterator will violate the rule for the other iterator.

The solution is to collect all the entries that need deleting in a separate Set , then after your logic has completed, call List.removeAll(set) .

Given this, you don't need iterators at all - just use the foreach syntax:

For a simplistic example:

List<RemoveTest> list = new ArrayList<RemoveTest>();
// populate list
Set<RemoveTest> removals = new HashSet<RemoveTest>();
for (RemoveTest i : list)
    for (RemoveTest j : list)
        if (...)
            removals.add(i); // j will get added in another iteration
list.removeAll(removals);

Answer 2

使用CopyOnWriteArrayList而不是ArrayList，它应该可以解决您的问题。