Passing a class by reference

Programming Questions

uncleage

in Programming Questions • 1 year ago

First off, a big hello to everyone! (as this is my first post)

I am trying to teach myself Processing and OOP in general. I programmed a few things years ago in Assembly and a few other languages, Processing seems to be a neat implementation so I am easing into it. I ran across a program called Glitch Sequencer by Bret Truchan. In addition to making the program free he also makes the code available (which is awesome Bret!). His code, the reference here and purchasing the "Learning Processing" has made this learning process pretty simple so far.

The small snippet below is from his code (I apologize in advance is this is a violation of some sort).

void send_pending_note_offs()

{

Note note;

for(int index = this.open_notes.size() - 1; index >= 0; index--)

{

note = (Note) this.open_notes.get(index);

if (note.duration <= 1)

---------------------------------------------------------------------------------------------------------

class Note

{

int pitch, velocity, duration;

Note(int pitch, int velocity, int duration)

{

this.pitch = pitch;

this.velocity = velocity;

this.duration = duration;

}

In the above code I do not understand what is happening in the highlighted section. My guess is that "note = (Note)" is equivalent to "note = new Note()". If this is the case then I guess it's a matter of personal style/preference coding and I get what's happening. But that would also lead to me to ask, when is one syntax preferable over the other? Or, is there a difference in the impact of each on the program?

Thanks in advance for any knowledge you can share.

Derrick

Replies(5)

byhring

Re: Passing a class by reference

1 year ago

This is a way of forcing the program to interpret the datatype of whatever is returned from:

this.open_notes.get(index);

As an object of type "Note".

It is similar to doing:

float aFloat = 1.2345;
int anInt = (int)aFloat;
//Should print "1"
println(anInt);

If the object that is returned from this.open_notes.get(index) is of the wrong type, you will get a runtime error.

dimiro

Re: Passing a class by reference

1 year ago

Most probably he has made an arraylist (or an another container) that holds Note objects. Most probably there is a declaration like this somewhere:
List note=new ArrayList()

an arraylist does not know what it contains, so you have to cast it to get the elements out of the container, that is why hehas added the (Note) before the get(index)

if you write it like this
List <Note> note=new ArrayList<Note>();
you do not have to add the (Note) before you get it....

(i am relatively new to this, but i think that this is the case :)

PhiLho

Re: Passing a class by reference

1 year ago

I suppose that open_notes is an array list. In the old way of using them, we put any objects in them, the only common point between them being that they inherit from the Object fundamental class. So, when you did a get() from an array list (or any other old style collection), you only know it is an Object. Some undifferentiated blob of data...
There is a way to tell what kind of object it is (as long as you know the types of objects you put in the list), but in practice, most of the time, all the objects you put in a list are of same type.
So, you have to put a cast on the obtained object to tell it is really a Note (and the VM will throw an error at runtime if that's not the case) to put this object into a Note case (reference).

In modern Java (since v.1.5), we use generics to indicate upfront what type of objects we plan to put in a collection. This allows the compiler to check that we won't put any other kind of object in it: strong typing is good for coding!
The notation is:
ArrayList<Note> open_notes = new ArrayList<Note>();
and then you can get objects in a simpler way:
Note note = open_notes.get(i);
Note there is no need to prefix all fields of the class by this, this is an habit some programmers have, but I find this verbose and redundant. This is a bit subjective, so I won't argue on it...
Note also that Java 5 offers also a nicer notation to loop on such typed array lists:
for (Note note : open_notes)
{
if (note.duration <= 1) ...
}
Even less verbose... But if you really need to iterate on the reverse order of the array list, this won't work.

jas0501

Re: Passing a class by reference

1 year ago

new = (Note) ...

Is called casting, and is assigning the type to the object of unknown or different type.

The same thing as

float floatValue;

answer = question( (int)floatValue);

where question is a function that takes an int and you have a float. Without casting the float-->int the compiler would complain abount not having function question(float).

uncleage

Re: Passing a class by reference

1 year ago

Wow, it's really nice to read all the great responses. I'm clear on it now.

Thanks again

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