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I want to write a class (ListDePersonnes) that handles my array, currently I only have two methods, one to add objects and one to print the array contents to screen.

ListDePersonnes

import java.util.Scanner;

public class ListDePersonnes {
    int count,t;
    personne liste[];
    Scanner s=new Scanner(System.in);

    public void initialize() {                  //
        System.out.println("entrer la taille"); //
        t = s.nextInt();                        // I'm not so sure about this part.
        personne[] liste = new personne[t];     //

    }


    public void addpersonne(){

        System.out.println("what is the name?");
        String nom= s.next();
        System.out.println("what is the age?");
        int age= s.nextInt();
        System.out.println("what is the weight?");
        double poid= s.nextDouble();

        liste[count] = new personne(nom,age,poid);  // weight is poid in frensh 
        count++;
    }



    public void showAll(){
        for (int i=0;i<t;i++ ){
            System.out.println("name: "+ liste[i].getNom() + " / age: "+liste[i].getAge()+" / poid:       "+liste[i].getPoid());
        }
    }
}

personne

public class personne {

    private String nom;
    private int age;
    private double poid;

    public personne(String nom, int age, double poid) {

        this.nom = nom;
        this.age = age;
        this.poid = poid;
    }

}

Main

import java.util.Scanner;
public class LaListe {

    public static void main(String[] args) {
        Scanner s=new Scanner(System.in);
        ListDePersonnes lper= new ListDePersonnes();
        lper.initialize();
        lper.addpersonne();
    }

}

The error thrown is:

Exception in thread "main" java.lang.NullPointerException at ListDePersonnes.addpersonne(ListDePersonnes.java:29) at LaListe.main(LaListe.java:16)

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  • 3
    personne[] liste = new personne[t];. You are shadowing your class field variable. It should be liste = new personne[t]; Commented Nov 28, 2014 at 21:31
  • 1
    take easy on me guys . i'm new to this :) lol SO becoming scary... Just format your code and show some effort, people won't downvote or close Commented Nov 28, 2014 at 21:34
  • 1
    @IslamBouderbala be happy now you have an upvote too :) Commented Nov 28, 2014 at 21:38
  • 1
    You may want to consider having a method to create a larger array to hold more Personne's if the user fills up the array. Or using a different data structure such as an ArrayList, they are very simple to use. Commented Nov 28, 2014 at 21:40
  • 1
    @Islam Bouderbala Do you mean how to delete person from array? Array have static length, so you have to create new array with length of lenght-1 and then "copy" all Objects (except the one you want to remove) to new array. I recommend you to use List. Commented Nov 28, 2014 at 21:40

2 Answers 2

Reset to default
4

You're shadowing the variable liste. Replace

Personne[] liste = new Personne[t];

with

liste = new Personne[t];
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3 Comments

thanks a lot man , any idea on how to delete a personne from the array ?
If you need to delete an Object from the collection then you can use a List, e.g. ArrayList
do i declare it the same as i did with the simple array ?
2

Java includes a class to do such operations more easily, List (docs).

Example usage (from here):

To add an element to the list:

List<Personee> listA = new ArrayList<Peronsee>(); //New ArrayList
listA.add(new Personee("Bob", 29, 165)); //Add element
listA.add(new Personee("Alice", 25, 124)); 
listA.add(0, new Personee("Eve", 34, 136)); //This adds an element to the beginning of the list, index 0.

To remove an element:

listA.remove(personee1); //Remove by object
listA.remove(0); //Remove by index

To access/iterate over the list:

//access via index
Peronsee element0 = listA.get(0);
Personee element1 = listA.get(1);
Personee element3 = listA.get(2);


//access via Iterator
Iterator iterator = listA.iterator();
while(iterator.hasNext(){
  Personee personee = (Personee) iterator.next();
}


//access via new for-loop
for(Personee personee : listA) {
    System.out.println(personee.nom + "," + personee.age + "," + personee.poid);
}
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2 Comments

from what i see . that's an arraylist . what should i do to get objects "personnes" list ?
@IslamBouderbala Updated my answer. The List interface can be made generic, (the List<Personee>) which limits the types that can be added and read from the list. In this case the List object can only handle objects of the Personee type.

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