Implementing my own ArrayList using arrays

Question

I have a project I have been assigned in java and I have to basically create my own class that mimics basic features of the ArrayList class.

I have to do it in the format shown below using these variables but I am stuck on one part. When the array fills up I want to make a new one with 10 extra spaces, then copy all the old array elements to the new one and use that from now on. How do I make my program automatically use this array from now on? Because it will continue to use the first array.

public MyArrayList()
{
    array = new String[10];
    arraySize = 10;
    arrayElements = 0;
}

public void add (String string)
{
    if (arrayElements < arraySize)
    {
        array[arrayElements] = string;
        arrayElements++;
    } else
    {
        arraySize += 10;
        arrayElements++;
        String[] array2 = new String[arraySize];

        for (int i = 0; i < arrElements;i++)
        {
            array2[i] = array[i];
        }
        //missing code here I think?
        array2[arrayElements] = string;
    }
}

I think arrayElements++; should come after you copy #arrayElements over to a new array. — zapl
– zapl, Commented Oct 16, 2015 at 23:11

Elliott Frisch · Accepted Answer · 2015-10-16 23:07:06Z

4

Assign array2 to the array reference. Something like,

//missing code here I think?
array2[arrayElements] = string;
array = array2;

Or, use Arrays.copyOf(int[], int) and something like

arraySize += 10;
arrayElements++;
array = Arrays.copyOf(array, arraySize); // <-- will pad with zeros.

answered Oct 16, 2015 at 23:07

Elliott Frisch

202k20 gold badges166 silver badges265 bronze badges

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

4 Comments

toastedDeli Over a year ago

I'm confused, how can I assign array the value of array2? array2 will be double the size? I want the code to be able to continue, so I can keep adding items and it will keep updating the size of the array. My problem is that when it gets to the 11th element, it will try to add the element to array still, instead of array2. I need to make array2 the main array for the object essentially.

Elliott Frisch Over a year ago

You'd assign the reference back to array. And it would be the same array that array2 refered to (the doubled array).

Andreas Over a year ago

@toastedDeli The array field is a reference to the array, not the array itself. array2 is a reference to another array. array = array2 updates the reference stored in array to refer to the other array. The first array is then unreferenced, and will be deleted / garbage collected by the JVM.

toastedDeli Over a year ago

I updated my code to include array = array2 where I said missing code should be but I still get an exception? I think I understand what you mean about the reference changes but I can't get it to work :(

Scott Hunter · Accepted Answer · 2015-10-16 23:06:55Z

1

MyArrayList uses array to hold its contents; in add you have created a new version of this in array2; just assign that to array.

answered Oct 16, 2015 at 23:06

Scott Hunter

50k12 gold badges64 silver badges107 bronze badges

3 Comments

toastedDeli Over a year ago

I cant assign array2 to array though because its double the size? :(

Scott Hunter Over a year ago

Have you tried? I mean just array = array2, not trying to copy individual elements.

toastedDeli Over a year ago

It's okay I got it :-) Thank you so much for the help!

Andreas · Accepted Answer · 2015-10-16 23:13:55Z

0

You have multiple problems:

You create a new temporary array (array2) and copy existing values, but you don't update the array field to point to the new array.
You increment arrayElements too early.

Your code should be (pseudo-code):

add(newVal) {
    if (array is full) {
        newarray = new [array.size + extra]
        copy values from array to newarray
        array = newarray
    }
    add newVal to array and increment value count
}

See. You should only add new value in one place.
That is the DRY principle (Don't Repeat Yourself).

answered Oct 16, 2015 at 23:13

Andreas

160k13 gold badges164 silver badges262 bronze badges

Comments

Petter Friberg · Accepted Answer · 2015-10-17 17:00:51Z

0

This can be a solution to increase the size of your array

String[] array2 = new String[array.length + 10];
System.arraycopy(array , 0, array2 , 0, array.length);
array =  array2 ;

edited Oct 17, 2015 at 17:00

answered Oct 16, 2015 at 23:06

Petter Friberg

21.8k10 gold badges67 silver badges116 bronze badges

1 Comment

President James K. Polk Over a year ago

maybe you could add a few words to make this a better answer, maybe set arraySize to array.length + 10, maybe use newArray instead of array2...

Collectives™ on Stack Overflow

Implementing my own ArrayList using arrays

4 Answers 4

4 Comments

3 Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

4 Comments

3 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related