Issue with array of an object

Question

I am learning java and was trying some stuff. This is what I am trying to do.

I am trying to create an table of an array of an object. e.g. I am creating an object called animal by which i can add any number of animal and its breed on user choice.

package tt;

import java.util.Scanner;

public class animal {

    String aname;
    String abreed;

    public animal() {

        mainprog aa = new mainprog();

        System.out.printf("eneter name of your %s..\n", aa.Animalcat);
        Scanner name = new Scanner(System.in);
        aname = name.nextLine();

        System.out.printf("eneter breed of your %s..\n", aa.Animalcat);
        Scanner breed = new Scanner(System.in);
        abreed = breed.nextLine();

    }

    public String getbreed() {
        return abreed;

    }

    public String getname() {
        return aname;
    }
}

So the main program ask for how many animals I want to add.

package tt;

import java.util.Scanner;

public class mainprog {

    public static String Animalcat;

    public static void main(String[] args) {

        System.out.println("How many animals you want to add..");

        Scanner an = new Scanner(System.in);

        int animalNumbers = an.nextInt();

        animal[][] addAnimal = new animal[animalNumbers][1];
        animaltype[] at = new animaltype[animalNumbers];

        for (int i = 0; i < animalNumbers; i++) {

            at[i] = new animaltype();
            Animalcat = at[i].getAnimalType();

            for (int j = 0; j < 1; j++) {

                addAnimal[i][j] = new animal();
            }
        }

        Display(addAnimal, at);

    }

    public static void Display(animal x[][], animaltype y[]) {
        System.out.println("Your animals are..");
        for (int m = 0; m < x.length; m++) {
            System.out.printf("Following are the name and the breed of %s ",
                    y[m].getAnimalType());
            System.out.println();
            for (int n = 0; n < x[m].length; n++) {

                System.out.printf(" %s", x[m][n].aname);
                System.out.printf(" %s", x[m][n].abreed);
                System.out.println();

            }
        }
    }

}

---

package tt;

import java.util.Scanner;

public class animaltype {
    String animalType;

    public animaltype() {
        System.out.println("What kind of animal you want to add..");
        Scanner at = new Scanner(System.in);
        animalType = at.nextLine();
    }

    public String getAnimalType(){
        return animalType;
    }

}

The issue I have here is I can ask for how many types of animals I want to add but I can't control how many animals of that type I want to add. While adding animal I can only add 1 number of animal manually by declaring it as [1] and can't by user input.

addAnimal = new animal[animalNumbers][1];

The problem in if I can do that by declaring animal[][] addAnimal = null; and then later initialize it with like:

animal[][] addAnimal = new animal[animalNumbers][animaltypenumbers];

But I get NullPointerException all time. Is there anyway I can have this done?

Answer 1

When you create an array, it will be filled by the default value of the element it contains. Since Animal is an Object, then it will be filled by null values, and you cannot use any variable which has null value. Since you're just filling animal[i][0] in your current code, you won't get any problem. But it will appear when you try to access to animal[i][1]. This happens in Display method:

public static void Display(animal x[][], animaltype y[]) {
    System.out.println("Your animals are..");
    for (int m = 0; m < x.length; m++) {
        System.out.printf("Following are the name and the breed of %s ",
                y[m].getAnimalType());
        System.out.println();
        for (int n = 0; n < x[m].length; n++) {
            //you only filled elements in x[m][0]
            //x[m][n] when n > 0 is null
            //so you will get NullPointerException
            System.out.printf(" %s", x[m][n].aname);
            System.out.printf(" %s", x[m][n].abreed);
            System.out.println();

        }
    }
}

A better option:

Use Animal[] addAnimal instead, you don't need it to have it as an array of arrays:

public static void main(String[] args) {
    System.out.println("How many animals you want to add..");
    Scanner an = new Scanner(System.in);
    int animalNumbers = an.nextInt();
    animal[] addAnimal = new animal[animalNumbers];
    animaltype[] at = new animaltype[animalNumbers];
    for (int i = 0; i < animalNumbers; i++) {
        //at[i] = new animaltype();
        //Animalcat = at[i].getAnimalType();
        Animalcat = new animaltype();
        //for (int j = 0; j < 1; j++) {
        //    addAnimal[i][j] = new animal();
        //}
        addAnimal[i] = Animalcat;
    }
    Display(addAnimal, at);
}
//modify Display method accordingly

A better option:

Use List<Animal> backed by ArrayList<Animal> instead of Animal[]. List let's you handle a list of elements that grows dynamically.

public static void main(String[] args) {
    System.out.println("How many animals you want to add..");
    Scanner an = new Scanner(System.in);
    int animalNumbers = an.nextInt();
    List<Animal> animals = new ArrayList<Animal>();
    for (int i = 0; i < animalNumbers; i++) {
        Animalcat = new animaltype();
        animals.add(Animalcat);
    }
    Display(animals);
}
//modify Display method accordingly