How does an Arraylist of Arraylist work? How to resort to an Arraylist of Arraylist?

Question

ArrayList<ArrayList<String>> ArrayList2;

How does an Arraylist work? How to resort to an Arraylist of Arraylist?

Answer 1

The working doesn’t change. It’s just an object storing another object of the same type (roughly).

To go through the lists and sublists just use two (or one of each):

• for
• while
• forEach
• do..while
• Iterator
• Stream etc..

Below I used with a for and the forEach (lambda expression of Java 8).

import java.util.ArrayList;

class HelloWorld
{   
    public static void main(String[] args){

        /* Lista Final */
        ArrayList<ArrayList<String>> List = new ArrayList<ArrayList<String>>() {{

            /* Cria uma List de frutas */
            add( new ArrayList<String>() {{
                add("Abacaxi");
                add("Banana");
                add("Cajá");
                add("Caqui");
            }} );

            /* Cria uma List de armas */
            add( new ArrayList<String>() {{
                add("Parafal");
                add("M4");
                add("PT938");
                add("AR-15");
            }} );

            /* Cria uma List de bandas */
            add( new ArrayList<String>() {{
                add("Evanescence");
                add("Linkin Park");
                add("Epica");
                add("Xandria");
            }} );
        }};

        /* Percorre a primeira lista */
        for (int i = 0; i < List.size(); i++) {

            /**
             * Percorre as sublistas utilizando expressão Lambda (Java 8)
             * Caso não utilize Java 8, substitua por um `for`
             */
            List.get(i).forEach( x -> {
                System.out.println( x );
            });
        }
    }
}

^{Demonstration: https://ideone.com/HSLcbp}