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Thread: Array of a Class

  1. #1
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    Default Array of a Class

    Hi, I'm triying to fill an array of class "Months"

    All I'm triying to fill is something like this: "array[position].atribute"
    Every Month must have name and days;

    I get java.lang.NullPointerException , I don't understand why, I tryed using getter and setter but I don't fix it, I don't understand what's wrong, I don't get any compilation errors.

    Thank you.
    import java.util.Scanner;
     
    public class Arrays {
    	public static void main(String[] args) {
    		Scanner keyboard = new Scanner(System.in);
    		Months [] array = new Months[12];
     
    		for(int i=0; i<array.length; i++){
    			System.out.println("Introduzca el nombre del mes:\t"+(i+1));
    			array[1].name = keyboard.nextLine();
    			System.out.println("Introduzca los días que tiene el mes:\t");
    			array[i].days = keyboard.nextInt();
    		}
    		keyboard.close();
    		for(int i=0; i<array.length; i++){
    			System.out.println(array[0].name);
    			System.out.println(array[0].days);
    		}
    	}
    }
    class Months {
    	String name;
    	int days;
    }


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Array of a Class

    I get java.lang.NullPointerException
    Please copy the full text of the error message and post it here. It has information about where the error happened.

    Where does the code put any Months objects into the array? The array has been defined but no Months objects have been put into it so all its slots have null values.
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: Array of a Class

    Exception in thread "main" java.lang.NullPointerException
    at Ejercicio03.main(Ejercicio03.java:26)

    The name of the class is really Ejercicio03 and there is more lines but I simplified because if i fix to add a name i will know to add more attributes tt

  4. #4
    Super Moderator Norm's Avatar
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    Default Re: Array of a Class

    Our posts might have crossed.

    Where does the code put any Months objects into the array? The array has been defined but no Months objects have been put into it so all its slots have null values.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Array of a Class

    Sorry for my noob question, but, are you telling me that I need to instance 12 objects and full my array with these 12 objects? like this:

     
    Months m = new Months();
    Months[] array = new Months[12];
     
    for(int i=0;i<array.length;i++)
    	array[i]=m;

    But how could I loop for do 12 objects with different attributes each??

  6. #6
    Super Moderator Norm's Avatar
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    Default Re: Array of a Class

    Inside the loop create a new instance with the different attributes before adding it to the array.

    Your loop adds a reference to the same (one) instance of the object to all the slots of the array.
    If you don't understand my answer, don't ignore it, ask a question.

  7. The Following User Says Thank You to Norm For This Useful Post:

    Rexshine (January 20th, 2013)

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    Default Re: Array of a Class

    Thank you very much, now I understand it better, I did this:
    Meses [] array = new Meses[2];
     
    for(int i=0;i<array.length;i++)
    	array[i]= new Meses();
    I still had problems, but the problem is now solved, I'm just blind I was filling the months fine, but all positions of array show me the First position.
    If you check the code I always showing array[0] (12 times -.-)

    Now works great.

    Thank you again.

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