ClassCastException：ArrayList不能转换为String [英] ClassCastException: ArrayList cannot be cast to String

问题描述

我收到此错误：

java.lang.ClassCastException：java.util.ArrayList无法强制转换为
java .lang.String

java.lang.ClassCastException: java.util.ArrayList cannot be cast to java.lang.String

这是代码：

public static List<String>produit_cart(ArrayList l)
{   
    List<String>re=new ArrayList();
    List<String>l4=new ArrayList();

    re=(List<String>) l.get(0);

    for(int i=1;i<l.size();i++)
    {
        l4=new ArrayList();

        for(int j=0;j<re.size();j++)
        {
            for(int k=0;k<((ArrayList)l.get(i)).size();k++)
            {
                l4.add(re.get(j) +
                        " " +
                        (String)((ArrayList)l.get(i)).get(k));

            }   
        }

        re=new ArrayList();
        for(int m=0;m<l4.size();m++)
        {
            re.add(l4.get(m));
        }   
    }

    return re;
}

问题出在以下几行：

l4.add(re.get(j)+" "+(String)((ArrayList)l.get(i)).get(k));

我试图将其重写为：

l4.add(re.get(j)+" "+((ArrayList)l.get(i)).get(k));

但它不起作用。我该怎么办？

but it didn't work. What should I do ?

推荐答案

你的括号有问题。

您想在上调用 get（i）和 get（k） ArrayList ，然后将其强制转换为字符串。

You want to call get(i) and get(k) on your ArrayList, then cast that to a String.

您的代码转换为 l.get（i）到字符串，然后尝试调用 get（k） 就可以了。

Your code casts the result of l.get(i) to a String, then tries to call get(k) on it.

......实际上，我几乎无法确定你的代码是做什么的，因为它在当前状态下难以阅读。

... Actually, I can barely determine what your code does, because it is so difficult to read in its current state.

以下内容应该可以使用并使代码更容易理解：

The following should work and also make your code much easier to follow:

ArrayList tempList = (ArrayList) l.get(i);
String tail = (String) tempList.get(k);
l4.add(re.get(j) + " " + tail;

一真正帮助的事情是确保列表 l 是一个字符串列表列表

One thing that would really help would be to ensure that list l is a list of string lists. Then you wouldn't have to cast at all. You could achive that by using the following declaration (if at all possible):

List<ArrayList<String>> listL = new ArrayList<ArrayList<String>>();

---

以下是一些建议，说明如何重新编写上述内容并使其更容易理解：

---

Here are some suggestions of how you could re-write the above and make it much easier to follow:

public static List<String> produitCart(List<ArrayList<String>> listOfLists)
{   
    List<String> returnList = listOfLists.get(0);

    for (int i = 1; i < listOfLists.size(); i++) {

        List<String> listFour = new ArrayList<String>();

        for (int j = 0; j < returnList.size(); j++) {
            ArrayList<String> tempList = listOfLists.get(i);
            for (int k = 0; k < tempList.size(); k++) {
                listFour.add(returnList.get(j) + " " + tempList.get(k));
            }   
        }

        returnList = new ArrayList();

        for (int m = 0; m < listFour.size(); m++) {
            returnList.add(listFour.get(m));
        }   
    }

    return returnList;
}

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