ArrayList help

[ilovejava]

I had to make a arrayList and i do not know why is it not working

public class ArrayList<E> {

	private E[] theArray;
	private int size;
	private int capacity;


	public ArrayList(int initialCapacity) {
		this.size = 10;
		this.capacity = initialCapacity;
		theArray = (E[]) new Object[initialCapacity];
		
		
	}

	public ArrayList() {
		this(1024);
	}

	public E get(int index) throws Exception {
		
		if(index > size) {
			throw new Exception("the index is out of bounds");
		}else{
			return theArray[index];
		}
	}

	public void set(int index, E e) {
		E old = theArray[index];
		theArray[index] = e;
		
	}

	public void add(int index, E e) {
		if (size != theArray.length){
			E[] newArray = (E[])(new Object[size *2]);
			System.arraycopy(theArray, 0, newArray, 0, size);
			theArray = newArray;
		}
		for ( int i = size - 1; i <= index ; i--){
			theArray[i +1] = theArray[i];
			theArray[index] = e;
			size++;
		}
	}

	public void remove(int index) {
		for( int i = index; i < size - 1; i++){
				theArray[i] = theArray[i+1];
		}
		theArray[index] = null;
		size--;
	}

	public int getSize() {
		return this.size;
	}

	public boolean isEmpty() {
		return this.size == 0;
	}

	@Override
	public String toString() {
		StringBuilder sb = new StringBuilder("[");
		for (int i = 0; i <size; i++) {
			sb.append(theArray[i]);
			if (i < size - 1)
				sb.append(", ");
		}
		return sb.toString() + "]";

	}

}

and this is the tester

public class MyTest3 {

	public static void main(String[] sa) {
		try {
			
			new MyTest3().tests();
		} catch (Exception ex) {
			System.err.println("*** Failure: " + ex.getMessage() + " ***");
		} finally {
			
		}
	}

	private void tests() throws Exception {
		

		
		ArrayList<String> als = new ArrayList<String>();
		als.add(0, "hello");
		
		System.out.println(als.get(0));

		
	}

}

I do not know what is wrong with this code please help me!

[Dean_Grobler]

Are you getting an error message? We can look at that then take it from there...

[stultuske]

why do you have a size ánd a capacity? normally, this is one and the same value.

since you start of with the values:
size = 10
length = 1024

what exactly do you think this will do:

if (size != theArray.length){
			E[] newArray = (E[])(new Object[size *2]);
			System.arraycopy(theArray, 0, newArray, 0, size);
			theArray = newArray;
		}

?

[ilovejava]

that is making the array bigger if there are more elements than there are supposed too, like the ensureCapcity mehtod

[stultuske]

yes, but what is the logic behind it?
just work in your mind, or write it down, every step your actual running application is taking, and maybe you'll see what I mean.

but normally: the length(which is the size) and the capacity should always be the same, since the size defines the capacity of an array.

[JamesCherrill]

In a classic array list (eg java.util.ArrayList) you have a size which is the number of objects actually stored in the list at any time (initial value 0), and a capacity, which is the length of the private array that's used to hold the list (initial value is an arbitrary "big enough"). Capacity only increases when the array overflows (size > capacity) and you have to allocate a new bigger one.

[stultuske]

true, but here:

theArray = (E[]) new Object[initialCapacity];

when 1024 is passed as initialCapacity, both the actual size and capacity is 1024, or am I missing something here?

[JamesCherrill]

This is the OP's code, so he can do what he wants, but the "normal" implementation is that "size" refers to the current contents (number of added objects - number of removed objects). So initially the capacity is 1024 (length of the backing array), but the size is 0 (the array list contains zero objects). Add an object and the size becomes 1 and the capacity remains 1024. Add another 1023 objects and the size becomes 1024. Add yet one more object and the size becomes 1025, but internally the array will have been resized and the capacity will be some larger value, eg 2048.

[stultuske]

hmmm ...
I used to create a Generic array, and with each add test whether or not there were still elements at the end that are null. if so, I used the first null, else, if all elements taken, resizing the array with a value that could be set in the ArrayList class and copy over the values of the initial array to the resized one.
for each remove, I would just "trim" the array, to make sure that space was no longer occupied.

[JamesCherrill]

Your algorithm for add seems equivalent to just maintaining a size, unless the definition of your implementation allows null values to be added (like java.util.ArrayList does).
The algorithm for remove is valid, but could be really expensive if you have to create a new array and copy all the contents every time. Much slicker to null out the extra element ( or size-- )

[stultuske]

mm ... the app I was working on needed an implementation which didn't take null values. but I see your point, if it should, testing whether the last element(s) is(are) null wouldn't tell me whether they were assigned or not.

at remove, I wasn't creating a new Array. I passed either the object or the index to be removed, (if object, searched the index) and the
for ( int i = index; i < length-1; i++)
array = array[i+1];
and set the last element to null.
true, could be better when we're talking huge lists, but .. :)

[JamesCherrill]

OK, sorry, misunderstood what you initially said about remove. Nulling the last element after shifting down makes perfect sense if you don't allow null values.

[stultuske]

yeah well, a part of our assignment had to handle Objects in a List, and in the normal flow, a null wasn't possible. since I got tired of trying to explain my lab-partner what a NullPointerException was and how to handle it properly, I decided on a bit more of an idiot-proof (or Jan-proof, for that matter) implementation of the List, and hence, null marks the end :)

[ilovejava]

public void add(int index, E e) {
		if (size > capacity){
			E[] newArray = (E[])(new Object[size *2]);
			System.arraycopy(theArray, 0, newArray, 0, size);
			theArray = newArray;
		}
		for ( int i = size; i <= index ; i++){
			theArray[i +1] = theArray[i];
			theArray[index] = e;
			size++;
		}
	}

it now works there was a problem with my add method