How do you add integer arrays to an arraylist?

Question

So, I am doing an assignment in a first-year CompSci class, and this array assignment is a little beyond me. I have worked with arrays and arraylists a bit in the past week or two, and for the most part i've been successful, but this assignment is giving me a class (TestArrays) that holds integer arrays, and is asking me to write the code for a class that creates arrays of that TestArrays object. Here is the code I was provided:

public class TestArrays
{
    
    private int[] testArray;
    
    
    public TestArrays()
    {
        testArray = new int[10];
        generateRandomArray();
    }
    
    public void generateRandomArray()
    {
        for(int index = 0; index < testArray.length; index++)
        {
            testArray[index] = (int)(Math.random()*10 +1);
        }
    }
    
    public int[] getArray()
    {
        return testArray;
    }

And here is the code that I have attempted to write:

import java.util.ArrayList;

public class TestArrayLists
{
    private ArrayList<TestArrays> testArrayList;
    /**
     * Complete the contructor for the class.  Instantiate the testArrayList
     * and call the fillArrayList method to add objects to the testArrayList.
     * @param numElements The number of elements in the ArrayList
     */
    public TestArrayLists(int numElements)
    {
        fillArrayList(numElements);
    }

    /**
     * Complete the fillArrayList method.  It should fill the testArrayList with 
     * TestArrays ojects consisting of 10 element int Arrays of random numbers.
     * @param numElements The number of elements in the ArrayList
     */
    private void fillArrayList(int numElements)
    {
        TestArrays testArray;
        for(int i = 0; i < numElements; i++)
        {
            testArray = new TestArrays();
            testArrayList.add(testArray);
        }
    }       

    /**
     * For each TestArrays object in the testArrayList,
     * print out the numbers in the int Array.
     */  
    public void printArrayList()
    {
        for(int i = 0; i < testArrayList.size(); i++)
        {
            System.out.println(testArrayList.get(i));   
        }
    }
}

I get a null pointer exception when I try to run fillArrayList(), which I know means that I didn't instantiate the testArray properly, but the testArray constructor is supposed to do that (i think) and nothing that I have tried has worked.

I would prefer if you didn't just give me the code that I need to do this right, because although i'm sure its an easy fix that I am just not far enough to know about, I would prefer to know how to do this for the future as well. If you would be willing to explain to me what you're doing, I would really appreciate it. Thank you for time.

The field `private ArrayList testArrayList` is `null` when you try to add to this `ArrayList`. Initialize it immediately or in the constructor. — Nowhere Man, Mar 26 '21 at 16:40
Additional hint: your `printArrayList()` method doesn't print the numbers. The first for-loop is correct but then you also need a second for-loop or use Stream-API to iterate over the int[] that you will receive by calling `testArrayList.get(i).getArray()`. — falknis, Mar 26 '21 at 17:00

score 0 · Answer 1 · answered Mar 26 '21 at 16:40

0

You forgot to instantiate testArrayList in your TestArrayLists constructor. Try adding

testArrayList = new ArrayList<>();

to your constructor.

answered Mar 26 '21 at 16:40

Alex Grounds

256
2
12

How do you add integer arrays to an arraylist?

1 Answers1