Collection in Java Question

[cyrus]

Hello everyone! I am rather new to the concept of Collections in Java (I have used extensively collections in VB).

I am trying to find how collections in java work, so I used a CASE tool that automatically generates code. I am trying to model the simplest case of a maritime company that owns some ships.

However the code that is generated by this tool can not be compiled in JBuilder. I have two classes one called “Company” and another called “Ship”. The code is the following one (the errors that I receive are in the code):

//Company.java

public class Company
{

    
    public final Set getShips()
    {

        if (ships == null)
        {
            return java.util.Collections.EMPTY_SET;
        }
        return java.util.Collections.unmodifiableSet(ships);
    }
    

// I GET AN ERROR IN THE FOLLOWING LINE SAYING “"Company.java": cannot resolve symbol: class Ship in class shipcompany.Company at line 31, column 32”

    public final void addShips(Ship arg)
    {

        if (arg != null)
        {
            if (ships == null)
            {
                ships = new LinkedHashSet();
            }
            if (ships.add(arg))
            {
                arg.setCompany(this);
            }
        }

    }
    
// I GET AN ERROR FOLLOWING LINE  “Company.java": cannot resolve symbol: class Ship in class shipcompany.Company at line 48, column 35”

    public final void removeShips(Ship arg)
    {

        if (ships != null && arg != null)
        {
            if (ships.remove(arg))
            {
                arg.setCompany(null);
            }
        }

    }
    
    public Company()
    {

    }
    
    public Set ships;
    
    protected String CName;

}


//Ship.java

public class Ship
{

    
    public final Company getCompany()
    {

        return company;
    }
    
    public final void setCompany(Company arg)
    {

        if (company != arg)
        {
            Company temp = company;
            company = null;//to avoid infinite recursions
            if (temp != null)
            {
                temp.removeShips(this);
            }
            if (arg != null)
            {
                company = arg;
                arg.addShips(this);
            }
        }

    }
    
    public Ship()
    {

    }
    
    public Company company;
    
    protected String SName;
    
    protected int NumOfCrew;
    
    protected String TypeOfShip;

}

I have read the Java tutorial on collections, but I was wondering if there are any good code samples on Java Collections. I am after a code sample that implements a relationship 1 to many (like one Company has many ships).

Thank you very much for your responses. I am sorry for the long post!

[cjard]

Collections is a utility class for working on objects that implement the Collection interface. After implmenting the interface, an object is guaranteed to have a number of methods that make it manipulable as a collection, and suitable for processing by the Collections utility class..

so, per se.. there are no "Collections" in java.. there are, as you have discovered, a number of classes that represent collected data, all of which implement the Collection inteferace.

personally, i'd have started with a simpler one:

public class test101{
  public static void main(String[] argv){
    java.util.ArrayList al = new java.util.ArrayList(50);
    
    //populate the arraylist
    for(int i=0; i<50; i++){
      al.add(new Double(Math.random()*1000));
    }

    //print out the list to show the random order
    java.util.ListIterator li = al.listIterator();
    Double d;
    while(li.hasNext()){
      d = (Double) li.next();
      System.out.println(d.toString());
    }

    //use the collections object to sort the list:
    Collections.sort(al);
    
    //print out the list to show the ordered list
    li = al.listIterator();
    while(li.hasNext()){
      d = (Double) li.next();
      System.out.println(d.toString());
    }

    //use the collections object to shuffle the list:
    Collections.shuffle(al);
    
    //print out the list to show the newly randomized list
    li = al.listIterator();
    while(li.hasNext()){
      d = (Double) li.next();
      System.out.println(d.toString());
    }
  }
}

that fragment demonstrates Collections... you can work through it, and use it as a base to move to a 1-M relationship based thing like a LinkedHashSet..

i dont know which way round the LHS works, actually, if it is a linked list of hashtables or a hashtable of linked lists.. i would presume the latter.. hash collisions cause the data to e appended to the list, thus, calling get, on the header (the one company) produces a linked list of many ships..

[cjard]

also, if you could see your way to removing those long comments about errors, then the forum will return to normal..

youre getting an arror about no Ship class either because:

you havent compiled Ship.java

you havent set the classpath to contain the current dirrectory ( . ) at the end...

[cyrus]

Thank you very much for your reply cjard

The error was after all caused by JBuilder.

I compiled by source code from the command prompt and it works perfectly.

However your comments and your piece of code are really valuable to a beginer like me!

Thank you man!