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java class reference problem - why?

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moutanna

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#11

Oct 16th, 2009

please what you mean by this declaration:
public class ValidRecords<Student>

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bentlogic

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#12

Oct 16th, 2009

This is a standard way of declaring a java class that implements 'generics' ( sun has a pdf you can download on generics... )
There is a load of stuff on the web about generics in java; we are expected to make use of generics in our programming course when generics make sense... In this case I am making the type ArrayList<Student> an arraylist that is only able to have Student class elements stored in it... type safety...

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#13

Oct 16th, 2009

It's the .class files that need to be in the Assignments dir; the source .java files' locations are irrelevant to this.
The classpath must point to the folder that contains the Assignments folder, NOT to the folder where the files are.

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bentlogic

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#14

Oct 16th, 2009

yep, that's the case. I realize this but alas there's no change to my situation... the clocks ticking and I don't know which wire to cut, red or blue???

This really has to be simple surely, but what have I missed? Maybe I'll try another computer in case it's environmental (or just mental).

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#15

Oct 16th, 2009

Are you sure?

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Originally Posted by bentlogic

1. i put both classes into a directory called Assignment
2. i changed the package declarations (in both files) to be "package Assignment;"
3. i changed the classpath to the absolute pathname where the source files are

1. If the second compile always fails you cannot have a second .class file to put into the directory.
2. Thay already had that. What did you change?
3. This is wrong - the classpath must point to the folder that contains the "Assignment" folder that contains the class files. It's irrelevant where you out the source files.

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bentlogic

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#16

Oct 16th, 2009

yes, I can always get Student.java to compile producing a .class file. I have never been able to get ValidRecords.java to compile.

The classpath is correct and is C:\ST152-Warwick\Assignment currently; this is where the files reside (the 2 source files and the Student.class file)...

I have just tried on another computer with EXACTLY the same result.
public class ValidRecords<Student> {

I am wondering about the generic declaration for ValidRecords class but not sure about any of this... reason:
compiler throws-
ValidRecords.java:74: unexpected type
found : type parameter Student
required : class
Student sCopy = new Student( s1 );
^

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bentlogic

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#17

Oct 16th, 2009

the whitespace in front of the ^ (caret) has been blown away by the formatter thingy on this site; oops!

so the caret in above post should go under the 'S' in Student after the new keyword. So the constructor bit NOT the first Student in front of the sCopy

Hope that's clear

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#18

Oct 16th, 2009

Remove "<Student>" from ValidRecords class opening, problem solved...

 Help with Code Tags 
 java Syntax (Toggle Plain Text) 
package Assignment;
 
import java.util.*;
/**
 *
 * @author Nigel Novak
 */
public class ValidRecords {
 
    // Class Fields and Constants Follow
    private ArrayList<Student> list;
    private int sortOrder;
package Assignment;

import java.util.*;
/**
 *
 * @author Nigel Novak
 */
public class ValidRecords {

    // Class Fields and Constants Follow
    private ArrayList<Student> list;
    private int sortOrder;

Last edited by peter_budo; Oct 16th, 2009 at 7:14 pm.

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LJC - London Java Community, Graduate & Undergraduate Software Development Community, JAVAWUG (Java Web User Group), The London Android Group

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bentlogic

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#19

Oct 16th, 2009

[QUOTE=peter_budo;1017052]Remove "<Student>" from ValidRecords class opening, problem solved...

Peter that's Sensational!

was under the impression that i needed that for the generic stuff to work, i.e. ArrayList<Student> ...

Obviously I don't understand generics too well (what newbie does

)?

Is ArrayList<ClassName> or any valid var name placeholder a special case of generics? I understand the type safety side but why don't I need to make the class declaration with this generic placeholder present for compiler??

Anyway, thanks so much for your help. Really appreciated!

Also, interested in the LJC Undergrad website although down-under in Perth, Western Australia.

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#20

Oct 17th, 2009

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Originally Posted by bentlogic

Peter that's Sensational!

was under the impression that i needed that for the generic stuff to work, i.e. ArrayList<Student> ...

Obviously I don't understand generics too well (what newbie does )?

Is ArrayList<ClassName> or any valid var name placeholder a special case of generics? I understand the type safety side but why don't I need to make the class declaration with this generic placeholder present for compiler??

Anyway, thanks so much for your help. Really appreciated!

Also, interested in the LJC Undergrad website although down-under in Perth, Western Australia.

Nothing special, already another user questioned your use of class declaration placeholder.
I guess your reference to Sun tutorial on generics in PDF format is to this article Generics in the Java Programming Language. It is my opinion that this is not well presented. Try to read SCJP Sun Certified Programmer for Java 6 Study Guide from chapter 7 and see if that makes more sense for you (definitely easier to read then my endless rumbling).

As for anything similar to Java user/meetup groups I'm member in London I did not found anything in your location through meetup site, however your best chance would be get in touch with Australia Java Champion Mike Cannon-Brookes and see if he is aware of any Java group in your area. Good luck

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