How to iterate through ArrayList of objects?

Question

I have a class called SparseMatrix. It contains an ArrayList of Nodes (also class). I am wondering of how to iterate through the Array and access a value in Node. I have tried the following:

 //Assume that the member variables in SparseMatrix and Node are fully defined.
 class SparseMatrix {
     ArrayList filled_data_ = new ArrayList();
     //Constructor, setter (both work)

     // The problem is that I seem to not be allowed to use the operator[] on
     // this type of array.
     int get (int row, int column) {
         for (int i = 0; i < filled_data_.size(); i++){
             if (row * max_row + column == filled_data[i].getLocation()) {
                 return filled_data[i].getSize();
             }
         }
         return defualt_value_;
     }
 }

I will probably switch to static arrays (and remake it every time I add an object). If anyone has a solution, I would very much appreciate you sharing it with me. Also, thank you in advance for helping me.

Feel free to ask questions if you don't understand anything here.

You should be using generics, and you can't use [i] to get an element out of an ArrayList, you have to use .get(i). — Louis Wasserman, Oct 08 '16 at 04:54

score 3 · Answer 1 · answered Oct 08 '16 at 05:18

3

Assuming filled_data_ is a list that contains list of objects of a class named Node.

List<Nodes> filled_data_ = new ArrayList<>();
for (Node data : filled_data_) {
    data.getVariable1();
    data.getVariable2();
}

More info http://crunchify.com/how-to-iterate-through-java-list-4-way-to-iterate-through-loop/

answered Oct 08 '16 at 05:18

HARDI

394
5
12

1

I didn't mention this, but I am not going to parse the entire ArrayList. Still, this is good to know. Thank you. – sudomeacat Oct 08 '16 at 18:07

score 2 · Accepted Answer · edited May 23 '17 at 12:33

2

First of all, you should not use raw types. See this link for more info: What is a raw type and why shouldn't we use it?

The fix is to declare the type of object held by your array list. Change the declaration to:

 ArrayList<Node> filled_data_ = new ArrayList<>();

Then you can access each element in the array list using filled_data_.get(i) (as opposed to filled_data_[i], which would work for a regular array).

`filled_data_.get(i)`

The above will return the element at index i. Documentation here: https://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#get(int)

edited May 23 '17 at 12:33

Community

1
1

answered Oct 08 '16 at 05:02

nhouser9

6,730
3
21
42

This helps a lot, thank you. (I have experience with C++, so Java concepts is rather new to me.) I found a way to do this with static arrays (inefficient), so I will take this into account. Again, thank you for your help. – sudomeacat Oct 08 '16 at 18:04
@diuqSehTnettiK no problem =] – nhouser9 Oct 08 '16 at 19:01

Faizal · Answer 3 · 2016-10-09T10:13:37.597

If you didn't use generic, then you need to cast the object

//Assume that the member variables in SparseMatrix and Node are fully defined.
 class SparseMatrix {
 ArrayList filled_data_ = new ArrayList();
 //Constructor, setter (both work)

 // The problem is that I seem to not be allowed to use the operator[] on
 // this type of array.
 int get (int row, int column) {
     for (int i = 0; i < filled_data_.size(); i++){
         Node node = (Node)filled_data.get(i);
         if (row * max_row + column == node.getLocation()) {
             return node.getSize();
         }
     }
     return defualt_value_;
 }

}

That sounds interesting, please provide an example of what you mean. — sudomeacat, Oct 08 '16 at 18:05

score 0 · Answer 4 · answered Oct 08 '16 at 05:07

0

If array list contains Nodes which defines getLocation() you could use :

((Nodes)filled_data_.get(i)).getLocation()

You could also define

ArrayList<Nodes> filled_data_ = new ArrayList<Nodes>();

answered Oct 08 '16 at 05:07

c0der

18,467
6
33
65

score 0 · Answer 5 · answered Oct 08 '16 at 09:35

When you create the ArrayList object, you should specify the type of the contained elements with <> brackets. It is also good to keep the reference to the List interface - not ArrayList class. To iterate through such a collection, use foreach loop:

Here is an example of the Node class:

public class Node {
    private int value;

    public Node(int value) {
        this.value = value;
    }

    public void setValue(int value) {
        this.value = value;
    }

    public int getValue() {
        return value;
    }
}

Here is an example of the Main class:

public class Main {

    public static void main(String[] args) {

        List<Node> filledData = new ArrayList<Node>();
        filledData.add(new Node(1));
        filledData.add(new Node(2));
        filledData.add(new Node(3));

        for (Node n : filledData) {
            System.out.println(n.getValue());
        }
    }
}

How to iterate through ArrayList of objects?

5 Answers5