Healty structure in java

I'm trying to set up a simple program to handle a couple of objects. Up to this point l just used brute force while writing java. I did a file handling the objects like this.

Code:

---

public class SimObjects
{
  public static void main(String[] args)
  {
      1. if (no objects){//Create object}
      2. else {//start iterate to let the objects act. }
  }

---

I quite new to java and I like to get some advice on how to plane this better. I tried to put the creation of objects in a constructor. I take it java only use constructors once for every class, or? Then iterate in a main method. Any pros or cons? I'd be thrilled if someone showed me a proper way of doing this.

You generally don't want to do anything in the main method other than get your program running. I suggest a structure like:

Code:

---

public class Driver{
    public static void main(String[] args){
        new Driver();
    }
   
    ArrayList<SimObjects> list;

    public Driver(){
        if(list == null){
            list = new ArrayList<SimObject>();
            for(int i=0; i<1000; i++){
                list.add(new SimObject());
            }
        } else{
            for(SimObject so : list){
                //do something with so
            }
        }
    }
}

---

Quote:

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I take it java only use constructors once for every class

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Correct, hence the name 'Constructor' - only used in construction.

Good Luck! :D

P.S. Considering the behavior you want though, the above code is a little useless since its in the constructor. Please see my modified code which makes more sense:

Code:

---

public class Driver{
    public static void main(String[] args){
        new Driver();
    }
   
    public Driver(){
        //FYI, an empty constructor like this is not required. When you don't supply a
        //constructor, the java compiler creates one automatically
    }

    ArrayList<SimObjects> list;
   
    //First time print is called, it will create the objects. Subsequent calls will simply
    //print the existing list.
    public void print(){
        if(list == null){
            list = new ArrayList<SimObject>();
            for(int i=0; i<1000; i++){
                list.add(new SimObject());
            }
        } else{
            for(SimObject so : list){
                System.out.println(so.toString());
            }
        }
    }
}

---

Thanks!

Things are getting slightly, clearer.

I'm sorry, I made a typo in my code above.
list = new ArrayList<SimObject>;
should be
list = new ArrayList<SimObject>();
I forgot the parens! I'll update my previous post to reflect this.

It's all right. I really liked the ArrayList you showed and what you were able to do with it. Could you describe the difference between Array and and ArrayList. Or maybe post a link? Pretty please!

Yeah, no problem.
An Array is a language feature and is used in this way:

Code:

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int[] arrayOfInts = new int[10];

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You can make an array of any type simply by declaring a variable with its type suffixed by []'s. The 10 is the size of the array, and it is fixed, it cannot be changed.

ArrayList on the other hand is actually a class written with java. Its what is known as a dynamic array or dynamic vector. This means it can start small and grow as large as you want. Its based on arrays behind the scenes, but as it runs low on space, it automatically creates a larger array and copies all data into it, replacing the original.

Arrays are great to use when you have a fixed, known number of elements. They are light weight, and very simple to work with. ArrayLists are great when the number of elements is unknown or frequently changing. ArrayList also has many convenience methods built in - insertion, deletion, contains, iteration, etc... You can also use the java Collections class to do things like sorting: Collections.sort(someArrayList);

Does that help?

Quote:

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Originally Posted by quad64bit

Does that help?

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More than you'll ever know. Thanks a million!

Sure any time! Please feel free to ask more questions like that - they are good questions and beneficial to many.

Hi again!

I did an exercise some days ago, dealing with arrays. Now I'd like to try using ArrayList. I have a class called Turtle and the program is supposed to get these turtles to mess about in a 2D world.

When I did the exercise I used placed my Objects in an over sized array. But now knowing what you said about ArrayList I like to try a more dynamic approach. I start out like this to create an ArrayList containing Turtles.

Code:

---

    Random randGenerator = new Random();
   
   
    //Create world
    int worldSize = randGenerator.nextInt(200) + 600;
    World w = new World(worldSize,worldSize);
   
//Create ArrayList containing Turtle(s)
 ArrayList<Turtle> turtles = new ArrayList<Turtle>();
    turtles.add(new Turtle(40, 60, w)); //looped in the program
    turtles.add(new Turtle(50, 50, w));
    turtles.add(new Turtle(60, 40, w));

    System.out.println(turtles);

---

Your way seems a bit out of reach at the moment. Said by a newbie. This seems to get the work done. Well, obviously I populate the ArrayList in a loop.

I'll let you know if I mess up.

Your code looks ok to me! You can get the turtles back out with turtles.get(3) where 3 is the 4th turtle in the list. You can also do faster iteration using a for each loop:

Code:

---

for(Turtle turtle: turtles){
    //do something with turtle
    System.out.println(turtle);
}

---

Remember! turtle in the above example is an OBJECT. This means if you just print it like that, you only get it's memory address. To print something other than the memory address, you need to add a toString method to your Turtle class:

Code:

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public class Turtle{
    //...
   
    public String toString(){
        return "X: " + x +"\nY: "+ y +"\nName: "+name;
    }
}

---

In my example, I am printing the x, y and name fields of the turtle (or whatever fields you want!). If Turtle has this method (it must be called toString() and have a String return type) then doing System.out.println(someTurtle); will print whatever you put in the toString() method.