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Generic property

 
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Generic property
m2s87
17 Sep, 2007 - 07:47 AM
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I tried to write class that could be used like:
CODE
    public property<int> something = new property<int>(21);
, but failed. So I started wondering, if Java does have generic property type/class or is it limited with:
CODE
    //begin property <PropertyName>
    public <PropertyType> _<PropertyName>;
    public void set<PropertyName>(<PropertyType> NewValue){
       this._<PropertyName>=NewValue;
    }
    public <PropertyType> get<PropertyName>(){
        return this._<PropertyName>;
    }//end property <PropertyName>
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William_Wilson
RE: Generic Property
17 Sep, 2007 - 08:35 AM
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which compiler are you using. Java 1.5 supports generics, but some IDEs such as jCreator do not as of yet.
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Programmist
RE: Generic Property
17 Sep, 2007 - 10:25 AM
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http://java.sun.com/j2se/1.5.0/docs/guide/...e/generics.html
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m2s87
RE: Generic Property
17 Sep, 2007 - 12:01 PM
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QUOTE(William_Wilson @ 17 Sep, 2007 - 07:35 PM) *

which compiler are you using. Java 1.5 supports generics, but some IDEs such as jCreator do not as of yet.

I'm using Eclipse Java Compiler v_686_R32x, 3.2.2 release, on F/(64bit), with what i can set classfile target level: 1.1 to 1.6
QUOTE(Programmist @ 17 Sep, 2007 - 09:25 PM) *

http://java.sun.com/j2se/1.5.0/docs/guide/...e/generics.html

"If you are familiar with C++'s template mechanism, you might think that generics are similar, but the similarity is superficial. Generics do not generate a new class for each specialization, nor do they permit “template metaprogramming.”
I actually have read the document and the tutorial it has link to before more then one time. I have also seen David J. Eck's tutorial, what i would call - en clair. Ahh, probably i was to sluggish to finish writing the class correctly, because i thought - I could use existing type/class. biggrin.gif


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m2s87
RE: Generic Property
17 Sep, 2007 - 01:25 PM
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Anyhow this was what i came up with:
CODE
class Property<T> {
       private T _value;
       private State _state=State.Normal;

       public enum State { Normal, ReadOnly, WriteOnly }

       public Property( T a ) {  
           _value = a;
       }
       public Property( T a , State NewState) {  
           _state = NewState;
       }
       public T Get() {
           if(_state  != State.WriteOnly){
               if(_value != null){
                   return _value;
               }else{
                   throw new Error("Variable used before being valuated!");;
               }
           }else{
               throw new Error("Variable is writeonly!");;
           }
       }
       public void Set(T NewValue) {
           if(_state  != State.ReadOnly){
               _value=NewValue;
           }else{
               throw new Error("Variable is readonly!");;
           }
       }
    }
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Programmist
RE: Generic Property
17 Sep, 2007 - 01:44 PM
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Looks good. One "caveat" (if you want to call it that) is trying to instantiate a generic array inside a generic class. Imagine that you wanted to have an array of type T in your class. How do you initialize that array? If you look at the procedure for instantiating generic Collections you might guess that this is how you'd do it:

CODE

private T[] myArray;
...
...
myArray = new T[10];


But this is won't work. You'll have to do this:

CODE

myArray = (T[])new Object[10]


Kinda of a pain in the a$$, but that's how you have to do it. I found this out by inspecting Sun's code for ArrayList because I knew it was backed by a generic array (duh).

This post has been edited by Programmist: 17 Sep, 2007 - 01:46 PM
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m2s87
RE: Generic Property
17 Sep, 2007 - 01:48 PM
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QUOTE(Programmist @ 18 Sep, 2007 - 12:44 AM) *


blink.gif Did not know that icon_up.gif

But, i still do not understand why
CODE
public Property<int> something = new Property<int>(32,Property.State.ReadOnly);
//or
public Property<int> something = new Property<int>(21);
gets error " - Syntax error on token "int", Dimensions expected after this token"

And i would probably be using:
ArrayList<Property<int>> myArr = new ArrayList<Property<int>>(); rather then
Property<int>[] myArray=(Property<int>[])new Object[10]

This post has been edited by m2s87: 17 Sep, 2007 - 02:40 PM
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