如何在主 class 双列表中导入另一个 class？

Question

I'm beginner programmer. I made a double list in an external class, and I want to print it out in the main class.

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

public class A {

    private List<List<Object>> A=new ArrayList<>();

    public List<List<Object>> getA() {
        return A;
    }

    public void setA(List<List<Object>> A) {
        this.A.add(new ArrayList(Arrays.asList("1+1")));
        this.A.add(new ArrayList(Arrays.asList("dog")));
    }
}

In this class, I made double list and I will put this double list to main class

public class Main {
    public static void main(String[] args) {    
        A a=new A();    
        System.out.println(a.getA());       //this result []
    }
}

I want to print[[1+1],[dog]] , but it does show [] .

What is problem??

I want to print the desired result.

Answer 1

You never actually call your setA method from your main class.

public class Main {
public static void main(String[] args) {    
    A a=new A();   
    a.setA(a); // <-- populate array list here 
    System.out.println(a.getA());       //this result []
}