print toString() doesn't print ArrayList Class member objects

Question

Why does print toString() doesn't print Array Class member objects? It says "cannot return a void result". What type of result does it want when my ArrayList private objects contain values.

Eg: In Person class-

public class Person { 

    private ArrayList <String> name;

    private ArrayList <Integer> number;

    public Person(ArrayList <String> thatName, ArrayList <Integer> thatNumber) 
    {
        name = new ArrayList <String>(thatName);
        number = new ArrayList <Integer>(thatNumber);
    }

    public ArrayList<String> getName() 
    {
        return name;
    }

    public ArrayList<Integer> getNumber() 
    {
        return number;
    }

    public String toString() 
    {
        return System.out.println(name + "" + number); //Gives error "cannot return a void result"    
    }   

Also, can I make String.format(,) work in this case? I tried, but it doesn't work as the ArrayList object types are not primitive data types.

EDIT:

Here is how I am calling Person class but toString() not printing anything.

import java.util.ArrayList;
public class PersonTest {

public static void main(String[] args) {

    ArrayList <String> nam = new ArrayList <String>();
    ArrayList <Integer> numb = new ArrayList <Integer>();

    nam.add("Zeus");
    numb.add(99);

    Person pr = new Person(nam, numb);

    pr.toString();

}

}

Answer 1

System.out.println is a void, therefore can't return anything. What you should do is :

public String toString()
{
  return name + " " + number;
}

and in the caller, use person instance to print :

System.out.println(person.toString());

or below, since printing an object will automatically invoke the toString() method.

System.out.println(person);