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Index › Programming Questions & Help › Syntax Questions › What does it mean to "cast"
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What does it mean to "cast"? (Read 682 times)
Phattee
What does it mean to "cast"?
Jul 2nd, 2009, 12:22pm  
Hello,

I was going through the Processing reference for subset()

http://processing.org/reference/subset_.html

and I came across this line:

Quote:
"When using an array of objects, the data returned from the function must be cast to the object array's data type. For example: SomeClass[] items = (SomeClass[]) subset(originalArray, 0, 4)."


I have no idea what the part in bold means. What does it mean to "cast" to the object array's data type?
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PhiLho 
Re: What does it mean to "cast"?
Reply #1 - Jul 2nd, 2009, 12:37pm  
Nothing to do with spells nor Harry Potter, I think...
Probably more related to molds...

To cast to a type is to force its types to something related/compatible, in order to get more functionalities or to force it to have some characteristics.

For example, you can cast a float to an int to get rid of the decimal part:
float x = 5.236;
int i = (int) x;

In the case of collections, like ArrayList, you store only generic objects, so when you get them back, you have to cast them to their original type.

I don't know why it is necessary for subset, but it in the same spirit.
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Jack Perkins
Re: What does it mean to "cast"?
Reply #2 - Jul 2nd, 2009, 7:29pm  
I came up against this when I started using ArrayLists instead of regular arrays to hold my objects. When you make a regular array, you make it of the same type as the object it holds. Arraylists don't have a specific type (as far as i understand this) and when operating on an object in that ArrayList, it becomes necessary to put it into a placeholder object, using casting.
Code:

Arraylist stuff;

stuff.add(new ball());
stuff.add(new ball());
stuff.add(new ball());

for(int i=0;i<stuff.size();i++){
ball frank = (ball) stuff.get(i);

frank.doStuff();
frank.bounce();
}
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