add method from Arraylist - question

**Adomini** · 10-21-2010, 04:11 AM

Good evening everyone. I was doing a homework assignment, and the assignment is to use the "add method" from the ArrayList class. I have to basically come up with my own code to make the "add" method work (to add an element into an ArrayList and return the list with the added element)

here is my code, and the professors comments. Any advice on how to do what he asks me to do? this should be simple.....

Java Code:

public void add(Object o)
 {
	Object [ ] old = a;
    a = (Object []) new Object[ a.length * 2 + 1 ];

    /****************************************
    You do not need to cast a.
    */
    for( int i = 0; i < size( ); i++ )
        a[ i ] = old[ i ];

        /********************************
        You have not added a.
        */
 }

Any advice (not looking for handouts here...)

**Fubarable** · 10-21-2010, 04:45 AM

Where do you use ArrayList in the method or return an ArrayList from the method? What is your actual assignment?

**Adomini** · 10-21-2010, 04:57 AM

Assignment says....

(Implementing ArrayList) ArrayList is implemented in the Java API. Implement ArrayList and the methods defined using your own program.

His notes on my code says that I am not adding 'a' to it. Should I return
a[ i ] = old[ i + a ]; as the last line of the code then?

Does that make more sense?

**Zack** · 10-21-2010, 06:52 AM

Notice what you are doing. Remember that size() is the CURRENT size of the array. Let's say theoretically that your list is {5,4,3,2} (even though it's an object list--just pretend ;)). Now, you want to add the number 8. So let's walk through your loop:

size() == 4, so our loop goes from i = 0 to i = 3:
i = 0: a[0] = 5
i = 1: a[1] = 4
i = 2: a[2] = 3
i = 3: a[3] = 2

After this is done, you don't add 8 anywhere. You have to do something like, a[size()] = o, or something similar (just throwing code around to get the ideas going).

EDIT: The above is actually incorrect relative to your assignment. See JosAH's post below as to why.

Additionally I don't know why you multiplied a's length by 2 first. You're just adding an element. You only need a[old.length + 1].

And casting... yeah, that's self explanatory. (Object[]) is not necessary.