[英]Java - Parsing ArrayList with 2 iterators and the dreaded ConcurrentModificationException
I am searching through a ArrayList and doing a compare with 2 iterators. 我正在搜索ArrayList并与2个迭代器进行比较。 I am writing out values to a String buffer that will eventually be a XML output. 我正在将值写到String缓冲区,该缓冲区最终将是XML输出。 As I parse through the values I am checking for matching itemIds. 当我解析这些值时,我正在检查匹配的itemIds。 The matches are normally parts and drawings. 火柴通常是零件和图纸。 There can be many drawings to a part. 一个零件可以有很多图纸。 For my XML I have to know the type and name of all the matches and append the values together. 对于我的XML,我必须知道所有匹配项的类型和名称,并将值附加在一起。
Using this ArrayList: 使用此ArrayList:
itemId type name itemId类型名称
1000 part hammer 1000零件锤
1001 part nail 1001部分指甲
1000 dwg semantic 1000 dwg语义
1002 part ruler 1002尺子
My sample XML output would roughly look like : 我的样本XML输出大致如下所示:
<Master itemId=1000 type=part name=hammer>
<Extra type=dwg name=semantic>
</Master>
<Master itemId=1001 type=part name=nail>
</Master>
<Master itemId=1002 type=part name=ruler>
</Master>
This is my first loop 这是我的第一个循环
while (theBaseInterator.hasNext()){
ImportedTableObjects next = theBaseInterator.next();
currentEntry = next.identiferId;
currentType = next.typeId;
currentDatasetName = next.nameId;
compareInterator = tArray.listIterator(theBaseInterator.nextIndex());
compareEntriesofArray(currentEntry, currentType, currentDatasetName, compareInterator); <======= calling method for 2nd loop and compare check
}
I written a method for the second loop and compare 我为第二个循环编写了一个方法并进行比较
private void compareEntriesofArray(Object currentEntry, Object currentType, Object currentDatasetName, ListIterator<ImportedTableObjects> compareInterator)
object nextEntry;
while (compareInterator.hasNext()) {
ImportedTableObjects next = compareInterator.next();
nextEntry = next.identiferId;
if(nextEntry.equals(currentEntry)) {
compareInterator.remove();
}
When it finds a match I am trying to remove the matching entry from the list. 当找到匹配项时,我正尝试从列表中删除匹配项。 There is no need to re-check entries that has been matched. 无需重新检查已匹配的条目。 So when the 1st loop continues down the list - it does not have to check that entry again. 因此,当第一个循环继续在列表中向下移动时-不必再次检查该条目。
But of course I am getting the ConcurrentModificationException. 但是当然我得到了ConcurrentModificationException。 I fully understand why. 我完全理解为什么。 Is there a way that instead of trying to remove the entry, I can some how mark it with a boolean or something, so when the first loop gets to that entry in the list it knows to skip it and go to the next? 有没有一种方法可以代替尝试删除该条目,而是可以用布尔值或其他方式标记它,因此当第一个循环到达列表中的该条目时,它知道将其跳过并转到下一个?
Add all elements you want to remove into a new List. 将要删除的所有元素添加到新列表中。
After iterating, call: 迭代后,调用:
coll1.removeAll (coll2);
Not with iterators, and their hasNext/next, but with Lists, you can iterate with a for-loop from top to bottom. 不是使用迭代器及其hasNext / next,而是使用列表,您可以从上到下使用for循环进行迭代。 removing element (7) bevore visiting element (6) and so on has never been a problem for me, but I haven't seen it being recommended. 删除元素(7)之前访问元素(6)等等对我来说从来都不是问题,但我还没有看到有人推荐它。
Here complete code 这里完整的代码
import java.util.*;
public class GuessGame
{
public static void main ( String [] args )
{
char [] ca = "This is a test!".toCharArray ();
List <Character> ls = new ArrayList <Character> ();
for (char c: ca)
ls.add (c);
show (ls);
// first method: remove from top/end and step backwise:
for (int i = ls.size () - 1; i >= 0; --i)
{
char c = ls.get (i);
if (c == 'i' || c == 'a' || c == 'e')
ls.remove (i);
}
show (ls);
// second approach: collect elements to remove ...
ls = new ArrayList <Character> ();
for (char c: ca)
ls.add (c);
show (ls);
// ... in a separate list and
List <Character> toRemove = new ArrayList <Character> ();
for (char c: ls)
{
if (c == 'i' || c == 'a' || c == 'e')
toRemove.add (c);
}
// ... remove them all in one go:
ls.removeAll (toRemove);
show (ls);
}
private static void show (List <Character> ls)
{
for (char c: ls)
System.out.print (c + " ");
System.out.println ();
}
}
Output: 输出:
T h i s i s a t e s t !
T h s s t s t !
T h i s i s a t e s t !
T h s s t s t !
最简单的方法可能是创建另一个列表,在该列表中放置“匹配的”条目,然后对照该列表进行检查。
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