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Index › Programming Questions & Help › Syntax Questions › . operator with ArrayList.get()
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. operator with ArrayList.get()? (Read 512 times)
rebirth
. operator with ArrayList.get()?
Dec 12th, 2008, 10:10am  
why doesn't this work? i thought get() returns an object. so why can't i use . to access it's methods?

Code:

ArrayList foo;
Bar blah;

void setup() {
  foo = new ArrayList();
  blah = new Bar();
  foo.add(blah);
  foo.get(0).cha();
}

class Bar {
 public void cha(){
   println("Cha cha cha cha");
 } 
}
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eskimoblood
Re: . operator with ArrayList.get()?
Reply #1 - Dec 12th, 2008, 11:04am  
Cause Java didn't know what type your object is. You have to cast it before calling the function:

Code:

((Bar)foo.get(0)).cha(); 
or
Bar b = (Bar)foo.get(0);
b.cha();


You could also type your array list by using <Type> so you don't need to cast.

Code:

ArrayList<Bar> foo;
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Andreas Köberle {eskimoblood}
222550793  
koogy
Re: . operator with ArrayList.get()?
Reply #2 - Dec 12th, 2008, 11:04am  
because it doesn't know what type of object that it has just got.

if you cast the return value from foo.get(0) then it'll work

Bar bar = foo.get(0);
bar.cha();

or, if you want to be obtuse:

((Bar)foo.get(0)).cha();  // untested 8)

java 5 lets you define the type of things in your arraylist using generics:

ArrayList<Bar> foo;
foo = new ArrayList<Bar>();

so it knows that anything retrieved from this will be a Bar (it'll also complain if you try and put non-Bar items into it)

(edit: so close... 8)
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PhiLho 
Re: . operator with ArrayList.get()?
Reply #3 - Dec 12th, 2008, 11:08am  
In Java 1.4 (Processing still uses this syntax), when you put an object in a Collection, it becomes anonymous, it is only seen as an Object.
That's what foo.get(0) returns.
You have to cast the value returned to the original type to be able to use it:
((Bar)foo.get(0)).cha();

Since Java 1.5, we can declare ArrayList as holding only objects of a given type:
ArrayList<Bar> foo = new ArrayList<Bar>();
Then you can use directly your code, because Java knows it is actually a Bar (I think behind the scene, it just silently adds the cast...).

[EDIT] Wow, you guys are too fast! Happy
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WWW  
rebirth
Re: . operator with ArrayList.get()?
Reply #4 - Dec 12th, 2008, 6:56pm  
Hey thanks everyone!

I managed to get it to work with the cast and with assigning the function to a Bar object.  I couldn't get
Code:
ArrayList<Bar> foo;
to work though. It gives an "unexpected token: void" on setup().
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PhiLho 
Re: . operator with ArrayList.get()?
Reply #5 - Dec 12th, 2008, 9:25pm  
eskimoblood was mistaken, and I wasn't precise enough: by "Processing still uses this syntax", I meant it doesn't support, yet, the <T> syntax. See Bug 598 "Update ANTLR grammars to support Java 1.5 syntax "
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