Java中ArrayList的ArrayList

我们已经讨论过 ArrayList 的数组是不可能没有警告的。一个更好的主意是使用 ArrayList 的 ArrayList。

// Java code to demonstrate the concept of
// array of ArrayList
  
import java.util.*;
public class Arraylist {
    public static void main(String[] args)
    {
        int n = 3;
  
        // Here aList is an ArrayList of ArrayLists
        ArrayList > aList = 
                  new ArrayList >(n);
  
        // Create n lists one by one and append to the 
        // master list (ArrayList of ArrayList)
        ArrayList a1 = new ArrayList();
        a1.add(1);
        a1.add(2);
        aList.add(a1);
  
        ArrayList a2 = new ArrayList();
        a2.add(5);
        aList.add(a2);
  
        ArrayList a3 = new ArrayList();
        a3.add(10);
        a3.add(20);
        a3.add(30);
        aList.add(a3);
  
        for (int i = 0; i < aList.size(); i++) {
            for (int j = 0; j < aList.get(i).size(); j++) {
                System.out.print(aList.get(i).get(j) + " ");
            }
            System.out.println();
        }
    }
}

输出：

1 2 
5 
10 20 30