Getters, Setters and Constructors in Java - Making a car and stocking it

问题

I'm trying to create a class in Java using BlueJ. My class is named Automobile. My goal is to be able to use my Constructor method to create cars with the variables: year, color, brand, number of doors, number of kilometers, if it's automatic (boolean), if it's sold (boolean), a description, and an identification number. All the variables have a set default value, a minimum and a maximum accepted value.

I have to use getVariablename and setVariablename for my methods. My color and brand variables are int, and I made methods to retrieve their String counterparts in a table in my class.

My issue is I don't understand the principle of setting my variable in one method and getting it in another (while making sure it's an accepted value). Also, once I have my Setter and Getter method, what do I have to write down in the creation of my Constructor method?

Up to now, I have this :

public class Automobile {

    private static final String[] COLORS = { "Other", "Noir", "Blanc", "Bleu Nuit", "Bleu Clair", "Vert Pomme", "Vert Bouteille", "Taupe", "Argent", "Sable"};

    private static final String[] BRANDS = { "Autre", "Mazda", "Toyota", "Ford", "GM", "Hyunday", "BMW", "SAAB", "Honda"};    


    public static final int COLOR_DEF = 8;
    public static final int COLOR_MIN = 0;
    public static final int COLOR_MAX = COULEURS.length - 1;

    public static final int BRAND_DEF = 4;
    public static final int BRAND_MIN = 0;
    public static final int BRAND_MAX = MARQUES.length - 1;

    public static final double KILO_DEFAULT = 55000;
    public static final double KILO_MIN = 15000;
    public static final double KILO_MAX = 140000;

    public static final int TWO_DOORS = 2;
    public static final int FOUR_DOORS = 4;
    public static final int DOORS_DEFAULT = FOUR_DOORS;

    public static final boolean AUTO_DEF = true;
    public static final int YEAR_MIN = 1997;
    public static final int YEAR_MAX = 2016;
    public static final int YEAR_DEFAUT = 2007;

    public static final String COMM_DEFAUT = "";


     public static String color (int cou) {

         String chainecolor = "";

         if (cou >= COLOR_MIN && cou <= COLOR_MAX) {
             chainecolor = COLORS[cou];
         }

         return chainecolor;
      } //This method is to return the String value of a color from its int value using the COLORS table. If invalid it returns an empty chain.


     public static String brand (int br) {

        String chainebrand = "";

        if (ma >= BRAND_MIN && ma <= BRAND_MAX) {
            chainebrand = BRANDS[br];
        }
        return chainebrand;
      } //same thing for the brand

    public Automobile (int brand, int year, int color, boolean automatic, double kilometers,int nbrDoors, String description, boolean sold){

        //To be completed          
    }

    //here i'm supposed to create getters that return int values for everything but automatic, sold and description

    public void setYear ( int year ) {
        if (year >= YEAR_MIN && YEAR <= YEAR_MAX) {
        year = year;
        }
    } // supposed to be the setter for my year, as long as it's within the accepted values

    public void setMarque (int brand){
       if (brand >= BRAND_MIN && brand <= BRAND_MAX) {
           brand = brand;
       }
    } //same, for the brand

    public void setColor (int color) {

      if (color >= COLOR_MIN && color <= COLOR_MAX){
          color = color;
      }
    }// same for the color

    public void setNbrDoors (int p) {

        if (p == TWO_DOORS || p == FOUR_DOORS){
            p = p;
        }
    } // same for the door. I am forced to use (int p) as the variable for this method, which confuses me as to how I will refer to it from nbrDoors up in the Automobile constructor method

} // Automobile

So my difficulties lie in:

Are the examples of setters that I made valid for this purpose? I do not understand the need for p = p, or color = color...
How do I create a getter method that will be able to go pick up the Variable p from setNbrDoors, return its value and have it be used for nbrDoors in the Automobile constructor?
What am I supposed to write in the Constructor method, such as it will be able to get its values from the getters?

This is all because the second part is I will have to create a little code to ask the user to input all the values for variables, then create a table to stock the Cars the user creates.

P.S.: the work is originally in french, so I translated the variable and method names best I could for your better understanding. Also, the variable names, methods, etc are all imposed, I am FORCED to make the class this way exactly.

EDIT: As such, the use of static for brand and color conversion are also imposed. Those 2 methods are solely for returning a String of character from an int value. they are not used in the Constructor. Finally, the exceptions will be handled in the second part of the work using a separate validation loop. The Automobile class is really used solely to handle the creation of the "car" object.

回答1:

There are few issues with your code:

(1) You did not have any proper instance variables (like year, brand, etc..) for Automobile

(2) You did not use this. to set the instance variables (because you did not create them)enter code here. Just note that, this always refers to the current object, refer here i.e., when you say this.year= year, you are actually assigning the right hand side year value to the current object's year variable (left hand side).

You can refer the below code with comments:

public class Automobile {

        private int year;
        private int color;
        private int brand;

        //add other fields

        public Automobile (int brand, int year, int color, boolean automatic, double kilometers,int nbrDoors, String description, boolean sold) {

            if (year >= YEAR_MIN && year <= YEAR_MAX)  {
                this.year = year;
            } else {
                new IllegalArgumentException("Invalid Year Passed to construct Automobile");
            }

            //Similarly add other validations for brand, color, etc..
        }

        public void setYear ( int year ) {
            if (year >= YEAR_MIN && YEAR <= YEAR_MAX) {
                //USE 'this.' as shown below' to set the given year to 'this' object's year
                this.year = year;
            }
        } 

       public int getYear() {
           return year;
       }

        //Similarly add setters and getters for year, color, brand, etc...
}