String problem java.lang.NullPointerException

Question

Hii all :)
How are u?? :)
I have a little a problem with String object in this class (You can say its a homework ^__^ ).
public class Personne {
     private String nom;
     private String prenom;
     private int age;
     public Personne(){
          this(null, null, 0);
     public Personne(String nom, String prenom, int age){
          setNom(nom);
          setPrenom(prenom);
          setAge(age);
     public void setNom(String nom){
          this.nom = new String(nom);
     public void setPrenom(String prenom){
          this.prenom = new String(prenom);
     public void setAge(int age){
          this.age = age;
     public String getNom(){
          return this.nom;
     public String getPrenom(){
          return this.prenom;
     public int getAge(){
          return this.age;
     public String toString(){
          System.out.print("Nom : " + this.getNom() + ", Penom : " + this.getPrenom() + ", Age : " + this.getAge() + " ");
          return null;
}When i call the class personne with the Personne() i get these errors in compiling-time :
Exception in thread "main" java.lang.NullPointerException
     at java.lang.String.<init>(Unknown Source)
     at Personne.setNom(Personne.java:18)
     at Personne.<init>(Personne.java:12)
     at Personne.<init>(Personne.java:8)
     at Main.main(Main.java:4) // The line wich i inisialize my object in my main method.
can someone help me??
Thank you alll ^__^
Edited by: La VloZ on 18 mai 2013 22:50
Edited by: La VloZ on 18 mai 2013 22:51
Edited by: La VloZ on 18 mai 2013 22:51
Edited by: La VloZ on 18 mai 2013 22:53
Edited by: La VloZ on 18 mai 2013 22:55

Answer 1

Why the error? Because you invoke the constructor String(String s), with s null. The constructor would produce a new String with the same character sequence as s, but if s is null, it cannot do this, so it throws NullPointerException. What else could it do? Without throwing an exception, a constructor has to create a new object, but there is no raw material, so to speak, for the creation. Think what would/could you do if you had to write such a 'copy constructor' for your own class and you would be passed null.
Now, the String copy constructor is probably useless, as the javadoc hints. When you have a String already, you don't need to create an identical one, as the original will:
- Not be garbage collected, if you keep a reference to it
- Never be modified, as strings are immutable
You are right that the code you show would 'copy the reference of nom to this.nom' and this is ok. It's exactly what you need, for the reasons above.