java - 无法弄清楚 ConcurrentModificationException 的原因

Question

以下方法抛出一个ConcurrentModificationException:

方法一(有异常(exception)):

public List getData(int n)
{
    List l = new ArrayList(statement.values());
    Iterator i = l.iterator();


    Calculation calculation = null;

    while (i.hasNext())
    {
        calculation = (Calculation)i.next();
        if (!calculation.total.hasScores())
            i.remove();
    }
    Collections.sort(l);
    int size = l.size();

    return l.subList(0,size > n? n : size);
}

当我将上述方法更改为:

方法二(无一异常(exception)):

public List getData(int n)
{
    List l = new ArrayList(statement.values());
    Iterator i = l.iterator();
    Collections.sort(l);

    Calculation calculation = null;

    while (i.hasNext())
    {
        calculation = (Calculation)i.next();
        if (!calculation.total.hasScores())
            i.remove();
    }

    int size = l.size();

    return l.subList(0,size > n? n : size);
}

那就没有异常(exception)了!想不通为什么？

异常跟踪:

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

完成类(class):

public class MyData 
{
    private Map <Integer, Calculation> statements = new HashMap<Integer, Calculation>();

    public List getListData()
    {
        List listData = this.getData(5);

        return listData;
    }

    public List getData(int n)
    {
        List l = new ArrayList(statement.values());
        Iterator i = l.iterator();

        Calculation calculation = null;

        while (i.hasNext())
        {
            calculation = (Calculation)i.next();
            if (!calculation.total.hasScores())
                i.remove();
        }

        Collections.sort(l);
        int size = l.size();

        return l.subList(0,size > n? n : size);
    }

}

现在调用 getListData 时会发生异常。

Answer 1

由于 sort() 方法会打乱整个集合，您必须先调用 sort() 方法。之后调用 iterator()。