Java ArrayList remains empty when I add an object containing Integers

Question

Hello I'm having an issue storing an ArrayList of Integers into an ArrayList of ArrayList of Integers. Here is the full code:

public class SetZeroMatrix {

  public static void main(String [] args){
    ArrayList<ArrayList<Integer>> zeroMatrix = new ArrayList<ArrayList<Integer>>();
    ArrayList<Integer> insertionList = new ArrayList<Integer>();

    try {
        FileReader in = new FileReader("matrixInput.txt");
        BufferedReader br = new BufferedReader(in);
        Scanner matrixScanner = new Scanner(br);

         while(matrixScanner.hasNextLine()){
            Scanner rowReader = new Scanner(matrixScanner.nextLine());
            while(rowReader.hasNextInt()){
                insertionList.add(rowReader.nextInt());
            }
            //testing to see if insertionList is empty
            System.out.println("Arraylist contains: " + insertionList.toString()); 
            zeroMatrix.add(insertionList);
            insertionList.clear();
         }
         matrixScanner.close();
      }
      catch(FileNotFoundException ex) {
            System.out.print("File not found" + ex);
      }
      //testing to see if zeroMatrix is empty
      ArrayList<Integer> testList = new ArrayList<Integer>();
      testList = zeroMatrix.get(1);
      System.out.println("ArrayList contains: " + testList.toString()); 
  }

}

This program is reading from a text file "matrixInput.txt" that contains:

34
20

The problem is after I added insertionList into zeroMatrix, zeroMatrix prints an empty ArrayList (during the last line of the code). I suspect its because I'm not inserting insertionList correctly into zeroMatrix? Or maybe I'm printing it incorrectly?

You delete everything inside of insertionList after adding it to zeroMatrix. What did you expect it to print out after doing that? — takendarkk
– takendarkk, Commented Sep 5, 2014 at 3:31
with in while loop, you should not clear insertionList instead you should create a new instance of insertionList — Prasad Khode
– Prasad Khode, Commented Sep 5, 2014 at 3:31

Elliott Frisch · Accepted Answer · 2014-09-05 03:31:51Z

2

You aren't adding a copy of the List, just a reference. So when you do,

zeroMatrix.add(insertionList);
insertionList.clear(); // <-- this

You clear the List you added to the zeroMatrix. You can copy the List,

zeroMatrix.add(new ArrayList<>(insertionList)); // <-diamond operator, see below.
insertionList.clear(); // <-- now you can clear the insertionList.

Or you can move the insertionList declaration into your loop body -

while(matrixScanner.hasNextLine()){
  ArrayList<Integer> insertionList = new ArrayList<Integer>();

In Java 7 or above you can use the diamond operator -

  ArrayList<Integer> insertionList = new ArrayList<>();

For Java 6 and earlier you had to specify the type on both sides like

  ArrayList<Integer> insertionList = new ArrayList<Integer>();

answered Sep 5, 2014 at 3:31

Elliott Frisch

202k20 gold badges166 silver badges265 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

anon9928175234 Over a year ago

Thank you Elliott! I incorrectly thought insertionList would create distinct copies of itself. You are correct, I should be creating new instances insertionList.

Avinash Kumar Pandey · Accepted Answer · 2014-09-05 03:47:06Z

The issue in your code is in how you scope your variables. Now, insertionList is a variable which should have been inside the scope of your outer while loop, so instead of declaring it beforehand and clearing it afterwards or reassigning it later, just declare and instantiate it inside the inner while loop.

public class SetZeroMatrix{

  public static void main(String [] args){
    ArrayList<ArrayList<Integer>> zeroMatrix = new ArrayList<ArrayList<Integer>>();

    try{
        FileReader in = new FileReader("matrixInput.txt");
        BufferedReader br = new BufferedReader(in);
        Scanner matrixScanner = new Scanner(br);

        while(matrixScanner.hasNextLine()){
            Scanner rowReader = new Scanner(matrixScanner.nextLine());
            ArrayList<Integer> insertionList = new ArrayList<Integer>();    
            while(rowReader.hasNextInt()){
                insertionList.add(rowReader.nextInt());
            }
            //testing to see if insertionList is empty
            System.out.println("Arraylist contains: " + insertionList.toString()); 
            zeroMatrix.add(insertionList);
            insertionList.clear();
        }

        matrixScanner.close();
    }
    catch(FileNotFoundException ex){
        System.out.print("File not found" + ex);
    }
    //testing to see if zeroMatrix is empty
    ArrayList<Integer> testList = new ArrayList<Integer>();
    testList = zeroMatrix.get(1);
    System.out.println("ArrayList contains: " + testList.toString()); 

}

Collectives™ on Stack Overflow

Java ArrayList remains empty when I add an object containing Integers

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related