Learning about classes

Question

Hi,
I'm working my way through "The Java Programming Language, Fourth Editition" by Sun Microsystems. There are exercises through the book, but they don't give the solutions (which is helpful!!!).
They show a simple class, then ask you to produce an identical one to keep track of vehicles. Based on their sample code this should be the code:
class Vehicle {
     public int speed;
     public String direction;
     public String owner;
     public long idNum;
     public static long nextID = 0;
  }Then then ask you to write a class called LinkedList giving no coding examples 'that has a field type Object and a reference to the next LinkedList element in the list. Can someone help me to decipher what this means and the appropriate code?
They then show how to create objects linked to the vehicle class, which based on their sample code should be
Vehicle car = new Vehicle();
car.idNum = Vehicle.nextID++;
car.speed = 120
car.direction = "North"
car.owner = "Metalhead":
This is the bit that next confuses me. They ask you to write a main method for your Vehicle class that creates a few vehicles and prints their field values.
The next exercise is to write a main method for LinkedList class that creates a few objects of type vehicle and places them into successive nodes in the list.
Is'nt it correct that the above code starting 'Vehicle car' would be the code used to create the object which would go in the LinkerList calling on the Vehicle method which has already been defined? For example, successive entries could be for bus, bike etc in the LinkerList. What does it mean though to place these into successive 'nodes' on the list? And why would you create vehicles in the main method of the Vehicle class. Shouldn't they just be created in the LinkerList?
I'm only learning about classes and I'm confused already?
Any help (and code) would be great!!!!
Thanks for any help,
Tim
p.s. I know what I have written sounds vague, bu the book doesn't really give much extra helpful info.

Answer 1

First of all, the variables of the "Vehicle" class should all be private.
You should not be able to directly retrieve each variable from this class
Example:
Vehicle car = new Vehicle();
car.idNum = Vehicle.nextID++;
car.speed = 120
car.direction = "North"
car.owner = "Metalhead"You should have methods that can change this data as well as retrieve the data.
Example:
class Vehicle {
     private int speed;
     private String direction;
     private String owner;
     private static long nextID = 0;
        public Vehicle() {
            speed = 0;
            direction = "";
            owner = "";
            ++nextID;
        // These methods retrieve the attributes
     public int getSpeed() { return speed; }
        public String getDirection()  { return new String(direction);}
        public String getOwner() { return new String(owner);}
        public long getidNum() { return idNum; }
        // These methods change the attributes
     public int setSpeed(int i) { speed = i; }
        public String setDirection(String s)  { direction = new String(s); }
        public String setOwner(String s) { owner = new String(s); }
}Now, they want you to create another CLASS that shows what your Vehicle class can do.
Example:
public class SomeRandomExampleClass {
    public static void main(String[] args) {
        Vehicle v1 = new Vehicle();
        // Now let's fix up our vehicle
        v1.setSpeed(100);
        v1.setOwner("Zeus");
        v1.setDirection("North");
        // Do note that since idNum is a static variable, it will be increased everytime you create
        // an instance of a Vehicle, and works with the class rather than the object.
        // For instance, if you create Vehicle's v1, v2, and v3, then v1's idNum = 1, v2's idNum = 2;
        // and v3's idNum = 3.
        Vehicle v2 = new Vehicle() ;
        v2.setSpeed(500);
        v2.setOwner("Ricky Bobby");
        v2.setDirection("NorthEast");
        System.out.println(v1.getSpeed()); // prints v1's speed.
        System.out.println(v1.getOwner()); // prints Owner
        System.out.println(v1.getDirection); // Prints Direction
        System.out.println(v1.getidNum());  // prints idNum
        System.out.println(v2.getSpeed()); // prints v1's speed.
        System.out.println(v2.getOwner()); // prints Owner
        System.out.println(v2.getDirection); // Prints Direction
        System.out.println(v2.getidNum());  // prints idNum
}A linked list is what it sounds like. It can hold a list of (let's say vehicles) and you can print their
attributes from there.
For instance, let's say we have a list called list1.
list1.add(vehicle1);
list1.add(vehicle2);
Then you can iterate(move) through your list and print the attributes of each vehicle, instead of having to
call v1.doThis() v1.doThis(), v2.doThis().
I find it amusing they want you to create a LinkedList when you are first using/learning classes.
To understand how to create a linked list, you can visit:
http://leepoint.net/notes-java/data/collections/lists/simple-linked-list.html
Or you can use:
http://java.sun.com/j2se/1.4.2/docs/api/java/util/LinkedList.html
*Note: I wrote the code from nothing so some of it could have syntax errors.