如何从二维数组列表中获取价值 [英] how to get value from 2d arraylist

问题描述

我有一个名为sub和main的数组列表，

i have arraylists named sub and main,

ArrayList main = new ArrayList();

ArrayList sub=new ArrayList();

我向sub添加值，然后向main添加sub.

i add value to sub and then add sub to main.

示例；

 sub.add(1);
 sub.add(2);
 main.add(sub);

现在我想在sub内获取所有值

now i want to get all values inside sub

 so i used following one but .get(j) gives me the error get >> canot find symbol

for (int i=0;i<main.size();i++) {
       System.out.println();

       for (int j=0;j<sub().size();j++) {
            System.out.print(main.get(i).get(j));//error line
       }
}

如何获取主arraylist的子数组中的所有值

how can i get all values inside subarray of main arraylist

推荐答案

将变量声明为

ArrayList main;

此列表包含Object个.这意味着即使添加ArrayList，main.get(i)也只会返回Object.这就是为什么会出现编译器错误的原因:Object没有名为get()的方法.

This list holds Objects. This means that main.get(i) will only return an Object, even if you add ArrayLists. That's why you get a compiler error: Object doesn't have a method named get().

要解决此问题，您需要使用泛型:

To fix the problem, you need to use generics:

ArrayList<List<Integer>> main = new ArrayList<>();

ArrayList<Integer> sub=new ArrayList<>();

现在get()将返回具有get()方法的List<Integer>，因此编译器错误将消失.

Now get() will return a List<Integer> which has a get() method, so the compiler error will disappear.

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