How can you make an array grow

[Victoria Preston]

I need to have grow method in my program. I know I would not need this if I used an arrayList but I cannot use that. I have been instructed to use arrays. So can anyone tell me how to make my array bigger.

[Keith Lynn]

Actually you cannot change the size of an array once it's created. You can however create another array that is larger than your original and use System.arraycopy to copy the contents from the first into the second. Then assign the reference to your first array to point to the second.

[Garrett Rowe]

The size of an array is fixed. Your only option is to copy the values into a larger array using System.arraycopy and then assign the original reference to the larger array. Like this

double[] values = new double[10];
 
//fill the array
for (int i = 0; i < values.length; i++)
{
   values[i] = i * i;
}
 
//oops now we need a larger array
double[] temp = new double[values.length * 2]
 
//Copy old array into temp array using System.arraycopy
System.arraycopy(values, 0, temp, 0, values.length);
 
//Assign old reference to new array
values = temp;
}

[ February 04, 2006: Message edited by: Garrett Rowe ]

[Victoria Preston]

Are you positive becuase my teacher said something about a method he said it started like private void grow()

[Keith Lynn]

An array is an object but has no methods associated with it.

[Garrett Rowe]

Maybe he meant that the grow() method was one you needed to implement using other methods in the API.

[Victoria Preston]

What does API mean....i'm sorry but I am new at this

[Keith Lynn]

API is the Application Programming Interface.

[Garrett Rowe]

Originally posted by Garrett Rowe:
Maybe he meant that the grow() method was one you needed to implement using other methods in the API.

Sorry, didn't mean to confuse you. By API I meant the classes and methods in the standard java library.

[Victoria Preston]

okay i know it starts out like

private void grow()
       {
    

What would I put in the middle what time of method would I call

[Keith Lynn]

What class definition is the method in?

[Victoria Preston]

again huh?

[Keith Lynn]

I mean what class definition contains that method?

[Victoria Preston]

are you asking what toher methods are avaliable...
I know that the name of my class is EmployeeList....I have another one named Employee

I did not import anything to build these

[Keith Lynn]

What I'm confused about the method grow. Is that a method you are supposed to write or are you looking for a class that has that method?

[Victoria Preston]

i'm sorry

grow is a method I need to write

[Keith Lynn]

The code that Garrett showed should give you a start.

[Victoria Preston]

okay thanks I will play around with it....I appreciate the help

[Tony Morris]

Your only option is to copy the values into a larger array using System.arraycopy and then assign the original reference to the larger array.

Not quite true - if you digress to requirements, you'll find that arrays contain a number of abstraction leaks (exceeding requirements), and the need to copy is one consequence of this. If you use a more appropriate structure that does not have such a leak, then this restriction disappears. I've already ranted about it elsewhere, so I won't do it again.

An array is an object but has no methods associated with it.

An array object has all the methods of java.lang.Object and the clone() method has been overridden with a lesser access scope (public).

[Victoria Preston]

so Tony what would you do.....

I have had no luck at all i'm stressed i'm going to cry

[Keith Lynn]

Originally posted by Tony Morris:

An array object has all the methods of java.lang.Object and the clone() method has been overridden with a lesser access scope (public).

Sorry for the mistake. I was intending to say that there are no methods like the one she mentioned, (private void grow)

[Rusty Shackleford]

Since you have to use an array, just mess around with an earlier suggestion.

This is a great assignment to learn from. It touches on what an ArrayList does, and that is far more valuable then just learning how to use ArrayList.

[luc comeau]

do you know how much the array has to grow by? Should it be specified in a parameter to the grow method...eg: public void grow(int growByThisMany)
or maybe...public void grow(int newSize), where newSize must be >=the old array size.

Also do you just have to initialize the rest of the new "grown" array elements to anything?Like did the question specify? I am just curous about these things.

Also if your not allowed to use the arrayCopy() method, you can just loop though each element in the old array, and add each element of the old one into a new one with the new specified size.

Let me know if your confused

[Martin Mathis]

Originally posted by Victoria Preston:
so Tony what would you do.....

I have had no luck at all i'm stressed i'm going to cry

All you need to do is...

1. Create a new array (larger)
2. Copy the contents of the old array into the new one (System.arraycopy(..), or a loop)
3. Make the reference to the original array point to the new one

All you have to do is convert those 3 lines to code and you're finished.

[Tony Morris]

Originally posted by Victoria Preston:
so Tony what would you do.....

I have had no luck at all i'm stressed i'm going to cry

Easy, I'd write a structure that has much less abstraction leak (though still not optimal as per Java), and use that. In fact, I'd call it a sequence in a project called ContractualJ, then I'd publish it for anyone else who chooses to learn why arrays are horribly broken. Data copying (as you no doubt have been advised to use via System.arraycopy) is a symptom of the leak - not a solution. That's what I'd do

http://contractualj.com/api/net/tmorris/adt/sequence/Sequence.html
[ February 07, 2006: Message edited by: Tony Morris ]

[Ernest Friedman-Hill]

Tony, please. Victoria is in a beginning Java programming class, and they want her to learn the how-to-implement-a-vector thing; feeding her this "abstraction leak" stuff sure isn't going to help her succeed in her immediate goals. You have to walk before you can run.

And I would imagine that a Sequence is implemented using a Java array, anyway, right?

Victoria, Martins's three steps

1. Create a new array (larger)
2. Copy the contents of the old array into the new one (System.arraycopy(..), or a loop)
3. Make the reference to the original array point to the new one

Are exactly what you need to do. It's the right way, it's the standard way, and it's what you need to do.

[Tony Morris]

Originally posted by Ernest Friedman-Hill:
[QB]Tony, please. Victoria is in a beginning Java programming class, and they want her to learn the how-to-implement-a-vector thing; feeding her this "abstraction leak" stuff sure isn't going to help her succeed in her immediate goals. You have to walk before you can run.

Point taken. However, I have observed that the vulnerable mind is much more prone to accepting reality than the trained/misled mind. That is, I can explain to my university students a concept that claimed "experienced" people still fail "to get". My conclusions are based on the kinds of questions that are asked. In any case, I concede.

And I would imagine that a Sequence is implemented using a Java array, anyway, right?

Not at all. A Sequence has nothing to do with arrays - why would I put a coating of strawberry cream on a big poo? A Sequence can indeed "grow" and "shrink" despite being an immutable (for some definition of immutable) structure without the need for data copying as a result of a more appropraiate abstraction (i.e. less leakage).