英文:
java.util.ConcurrentModificationException while mutating an object
问题
我正在对 CustomObject 的 List 进行迭代，而在此迭代过程中，我通过向此自定义对象的标签列表添加标签来改变此对象。我没有将任何 CustomObject 添加或移除到 customObjects（List<CustomObject>）中。但我仍然遇到 java.util.ConcurrentModificationException。
以下是代码部分：
public class CustomObject {
private List<Tag> tags;
// Tag 的 Getter 方法
// Tag 的 Setter 方法
}
public class DummyClass {
List<CustomObject> addMissingTag(List<CustomObject> customObjects) {
for (CustomObject object : customObjects) { // 第 5 行
// 下面的方法将一个标签添加到 CustomObject 的标签列表中
mutateObjectByAddingField(object); // 第 7 行
// 一些代码
}
return customObjects;
}
void mutateObjectByAddingField(CustomObject customObject) { // 第 13 行
Boolean found = false;
for (Tag tag : customObject.getTags()) { // 第 15 行
if ("DummyKey".equalsIgnoreCase(tag.getKey())) {
found = true;
break;
}
}
if (!found) {
Tag tag = new Tag("DummyKey", "false");
List<Tag> tags = customObject.getTags();
tags.add(tag);
customObject.setTags(tags);
}
}
}
以下是堆栈跟踪：
java.util.ConcurrentModificationException: null
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901) ~[?:1.8.0_131]
at java.util.ArrayList$Itr.next(ArrayList.java:851) ~[?:1.8.0_131]
at DummyClass.mutateObjectByAddingField(DummyClass.java:15)
at DummyClass.addMissingTag(DummyClass.java:7)
这是否意味着即使我们只尝试修改对象而不是从列表/集合中删除或添加，我们仍然可能会遇到 ConcurrentModificationException？
英文:
I am iterating over a List of CustomObject and while doing this iteration, I am mutating this object by adding a tag to tags list of this custom object. I am not adding or removing any CustomObject to or from customObjects (List<CustomeObject>). I am still getting java.util.ConcurrentModifcationException.
public class CustomObject {
private List<Tag> tags;
// Getter for Tag
// Setter for tag
}
public class DummyClass {
List<CustomObject> addMissingTag(List<CustomObject> customObjects) {
for (CustomObject object:customObjects) { // line 5
// following method adds a tag to tags list of CustomObject
mutateObjectByAddingField(object); // line 7
// Some Code
}
return customObjects;
}
void mutateObjectByAddingField(CustomObject customObject) {//line 13
Boolean found = false;
for (Tag tag:customObject.getTags()) { // line 15
if ("DummyKey".equalsIgnoreCase(tag.getKey())) {
found = true;
break;
}
}
if (!found) {
Tag tag = new Tag("DummyKey", "false");
List<Tag> tags = customObject.getTags();
tags.add(tag);
customObject.setTags(tags);
}
}
}
Here is the stack trace:
java.util.ConcurrentModificationException: null
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901) ~[?:1.8.0_131]
at java.util.ArrayList$Itr.next(ArrayList.java:851) ~[?:1.8.0_131]
at DummyClass.mutateObjectByAddingField(DummyClass.java:15)
at DummyClass.addMissingTag(DummyClass.java:7)
Does this mean we can get ConcurrentModifcationException even if we just try to modify the object and not remove or add from/to the list/collection?
答案1
得分: 1
首先，在for循环中您正在使用List类型来迭代元素，因此增强型的for语句相当于使用迭代器的for循环，正如此处所提到的，因为List实现了Iterator接口。从堆栈跟踪中也可以明显看出。
当使用Iterator时，您不能在正在迭代的列表上进行修改，正如GeeksForGeeks中所述，否则将会抛出ConcurrentModificationException。
因此，为了解决这个问题，您可以例如显式地使用整数来实现for循环，如下所示：
for (int i = 0; i < customObjects.length(); i++) {
CustomObject object = customObjects.get(i);
// 下面的方法将向CustomObject的标签列表添加一个标签
mutateObjectByAddingField(object);
// 一些代码
}
英文:
First, you are using a List type in the for loop to iterate the elements, so the enhanced for statement is equivalent to a for using an iterator, as mentioned here, because List implements Iterator. Also, it's obvious from the stack trace.
When using Iterator, you can't make modifications on the list you are iterating, as mentioned GeeksForGeeks, you will get ConcurrentModificationException.
So, to solve this issue, you can for example implement explicitly the for loop using integers like below:
for (int i=0;i < customObjects.length(); i++) {
CustomObject object = customObjects.get(i);
// following method adds a tag to tags list of CustomObject
mutateObjectByAddingField(object);
// Some Code
}
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