ArrayList 的 ArrayList 添加和检索元素

我有一个整数数组列表（mainList），里面有整数数组（subList），我试图将整数数组元素添加到 mainList 并稍后显示它们。将 subList 添加到 mainList 并显示 subList 中的所有元素。

2 subLists = {1,2,4},{3,2,1}
mainList =[{1,2,4},{3,2,1}]
Display : 1,2,4,3,2,1

1. 如何轻松地从 mainList 中检索元素
2. 如何一次添加子列表而不循环

以下是我尝试将子列表添加到主列表的方法

//Adding elements
 ArrayList<ArrayList<Integer>> mainList = new ArrayList<ArrayList<Integer>>();
ArrayList<Integer> subList = new ArrayList<Integer>();
for(int i=0;i<10;i++) {
    for(int j=i+1;j<10;j++){
        //Do something and add elements to subList
        subList.add(element[j]) }
        mainList.add(subList);

        // When I clear subList mainList is also getting cleared. I want to add the elements of subList to mainList. I was able to do it with loops but how to do this way
        subList.clear();
    }

    //Printing, How do I get the elements from mainList which will have subLists
    for(Integer i:mainList){ 
        System.out.println(i);
    }
}

你的代码

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

无法编译，请参阅我在下面的代码中的编辑：

import com.sun.deploy.util.StringUtils;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Main {

    public static void main(String[] args) {
        ArrayList<ArrayList<Integer>> mainList = new ArrayList<ArrayList<Integer>>();

        Integer[] sub1 = {1,2,4};
        Integer[] sub2 = {3,2,1};
        ArrayList<Integer> subList1 = new ArrayList<>(Arrays.asList(sub1));
        ArrayList<Integer> subList2 = new ArrayList<>(Arrays.asList(sub2));

        mainList.add(subList1);
        mainList.add(subList2); //[[1, 2, 4], [3, 2, 1]]


        ArrayList<Integer> intValues = new ArrayList<>();
        for(ArrayList<Integer> inner: mainList){
            intValues.addAll(inner); // add inner list elements to Integer list
        }

        List<String> strings =  intValues.stream().map(Object::toString)
                .collect(Collectors.toList());  // converts to string list

        System.out.println(StringUtils.join(strings, ","));  // prints comma separated string
    }
}

Output:

1,2,4,3,2,1

我的解释在代码注释中。