Get an item within an arraylist of an arraylist

Question

import java.util.ArrayList;

public class ArrayListPractice {
   public static void main(String[] args){
        ArrayList arr = new ArrayList();
        arr.add(10);
        arr.add(20);
        // arr is now [10,20]
        ArrayList arr2 = new ArrayList();
        arr2.add(new ArrayList(arr));
        arr2.add(30);
        // arr2 is now [[10,20],30]
        System.out.println(arr2.get(0)); // Prints out [10,20]
   }
}

I can print out the first element of arr2. But how can I print out the first element of the first element? (I want to print out 10 from arr2.)

If you're using Java 5 or later, please consider using ArrayList<Integer> arr etc. so that you can make use of the type information. The concept is "Generics". — C-Otto
– C-Otto, Commented Apr 11, 2017 at 12:34

Tim · Accepted Answer · 2017-04-11 11:52:56Z

1

You need to cast arr2.get(0) to ArrayList so you can call the get method on it (your ArrayList ArrayList arr = new ArrayList(); is not typed - so it holds instances of Object).

Like this:

package test;
import java.util.ArrayList;

public class ArrayListPractice {
   public static void main(String[] args){
        ArrayList arr = new ArrayList();
        arr.add(10);
        arr.add(20);
        // arr is now [10,20]
        ArrayList arr2 = new ArrayList();
        arr2.add(new ArrayList(arr));
        arr2.add(30);
        // arr2 is now [[10,20],30]
        System.out.println(((ArrayList)arr2.get(0)).get(0)); // Prints out [10]
   }
}

edited Apr 11, 2017 at 11:52

answered Apr 11, 2017 at 11:46

Tim

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1 Comment

Jens Over a year ago

You should use the generic way.

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