Java中的ConcurrentModificationException是什麼?
例子
讓我們來看一個例子來說明這個異常是如何發生的。import java.util.List;
import java.util.Iterator;
import java.util.ArrayList;
//Test driver for ConcurrentModificationException
public class ConcurrentModificationExceptionTestDriver1 {
public static void main(String[] args) {
List<String> businessDays = new ArrayList<>();
businessDays.add("Monday");
businessDays.add("Tuesday");
businessDays.add("Wednesday");
businessDays.add("Thursday");
businessDays.add("Friday");
businessDays.add("Saturday");
businessDays.add("Sunday");
Iterator<String> iterator = businessDays.iterator();
while (iterator.hasNext()) {
String day = iterator.next();
if (day.equals("Sunday")) {
// removing a list item while iterating
// leading to java.util.ConcurrentModificationException
businessDays.remove(day);
}
}
}
}
輸出
線程「main」中的異常java.util.ConcurrentModificationException 在java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1042) 在java.base/java.util.ArrayList$Itr.next(ArrayList. java :996)在milestone.thirty.seven.ConcurrentModificationExceptionTestDriver1.main(ConcurrentModificationExceptionTestDriver1.java:22)
Java中如何避免ConcurrentModificationException?
ConcurrentModificationException有不同的解決方法。這裡列出了一些。您可以根據您的需求選擇任何人。- 避免迭代時修改。
- 使用傳統循環進行同時迭代和修改。
- 建立要在使用迭代器時修改的值的副本。迭代結束後更新值。
例子
以下是如何避免ConcurrentModificationException的範例。import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
//Test driver for ConcurrentModificationException
public class ConcurrentModificationExceptionTestDriver2 {
public static void main(String[] args) {
List<String> businessDays = new ArrayList<>();
businessDays.add("Monday");
businessDays.add("Tuesday");
businessDays.add("Wednesday");
businessDays.add("Thursday");
businessDays.add("Friday");
businessDays.add("Saturday");
businessDays.add("Sunday");
System.out.println("Initial Business Days: " + businessDays);
// Method II - Using traditional for loop
for (int i = 0; i < businessDays.size(); i++) {
if (businessDays.get(i).equals("Sunday")) {
businessDays.remove(i);
}
}
Iterator<String> iterator = businessDays.iterator();
List<String> businessDaysToBeRemoved = new ArrayList<>();
// Method III - Creating a copy of elements
while (iterator.hasNext()) {
String day = iterator.next();
if (day.equals("Saturday")) {
// Add values to be removed in the list
businessDaysToBeRemoved.add(day);
}
}
businessDays.removeAll(businessDaysToBeRemoved);
System.out.println("Final Business Days: " + businessDays);
}
}
輸出
初始工作日:[週一、週二、週三、週四、週五、週六、週日] 最終工作日:[週一、週二、週三、週四、週五]
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