問題描述

有什么方法可以在迭代時修改特定鍵的 HashMap 值?

下面給出一個示例程序:

public static void main(String[] args) {哈希映射<整數，數組列表<字符串>>hm = new HashMap<Integer, ArrayList<String>>();ArrayListar = new ArrayList<String>();for(int i=0;i<50;i++){ar.add(Integer.toString(i));}hm.put(1, ar);for(字符串 s:hm.get(1)){hm.get(1).add("你好");}}

拋出錯誤:

線程main"中的異常 java.util.ConcurrentModificationException在 java.util.ArrayList$Itr.checkForComodification(未知來源)在 java.util.ArrayList$Itr.next(未知來源)在 Excp.main(Excp.java:17)

檢測到對象的并發修改但不允許修改的方法可能會拋出此異常.

以下代碼導致問題.

for(String s:hm.get(1)){hm.get(1).add("你好");}

您正在迭代和修改相同的內容.通過創建 new ArrayList

 ArrayListar1 = new ArrayList();對于(字符串 s:hm.get(1)){ar1.add("你好");}

閱讀這里p>

Is there any way I can modify the HashMap values of a particular key while iterating over it?

A sample program is given below:

public static void main(String[] args) {
    HashMap<Integer,ArrayList<String>> hm = new HashMap<Integer, ArrayList<String>>();      
    ArrayList<String> ar = new ArrayList<String>(); 
    for(int i=0;i<50;i++){              
        ar.add(Integer.toString(i));            
    }

    hm.put(1, ar);      

    for(String s:hm.get(1)){
        hm.get(1).add("hello");
    }

}

Error Thrown:

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
    at java.util.ArrayList$Itr.next(Unknown Source)
    at Excp.main(Excp.java:17)

This exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible.

Below peice of code is causing the problem.

for(String s:hm.get(1)){
        hm.get(1).add("hello");
    }

You are iterating and modifying the same. Avoid this by creating new ArrayList

  ArrayList<String> ar1 = new ArrayList<String>();

for (String s : hm.get(1)) {
            ar1.add("hello");
        }

have a read here

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