I can't find the error

[Alex Garci]

Hi,

I have the following code:

public class Anadeobjetosalalistamultimedia 
{
    
    
    public static void main(String args[])
    {
        EstanteriaMultimedia[] e=new EstanteriaMultimedia[20];  
        boolean icorrecta=false; //para comprobar si la inserción del objeto es correcta
        
        Pelicula pl1=new Pelicula("No habra paz para los malvados","Enrique Urbizu",120,"Jose Coronado");
        Pelicula pl2=new Pelicula("Chico y Rita","Fernando Trueba",130,"Chico");
        Pelicula pl3=new Pelicula("La piel que habito","Pedro Almodovar",150,"Antonio Banderas");
        Pelicula pl4=new Pelicula("Los otros","Alejandro Amenabar",150,"Nicole Kidman");
        Pelicula pl5=new Pelicula("Juana la Loca","Vicente Aranda",140,"Aitana Sanchez Gijon");
        
        //icorrecta=e[0].add(new Pelicula("No habra paz para los malvados","Enrique Urbizu",120,"Jose Coronado"));
        
        icorrecta=e[0].add(pl1);
        if (!icorrecta) System.out.println("No es correcto");
        icorrecta=e[1].add(pl2);
        if (!icorrecta) System.out.println("No es correcto");
        icorrecta=e[2].add(pl3);
        if (!icorrecta) System.out.println("No es correcto");
        icorrecta=e[3].add(pl4);
        if (!icorrecta) System.out.println("No es correcto");
        icorrecta=e[4].add(pl5);
        if (!icorrecta) System.out.println("No es correcto");
        

The rest of classes is here:

public class EstanteriaMultimedia 
 {
    Multimedia[] listamultimedia;
    int contador;
    int tmx; //guardo el tamaño maximo del array
    
    public EstanteriaMultimedia(int tmaximo)
     {
         //EstanteriaMultimedia e=new EstanteriaMultimedia(tmaximo);
         listamultimedia=new Multimedia[tmaximo];
         tmx=tmaximo;
         contador=0;
     }     
    
    int size()
     {
         return listamultimedia.length;
     }
    
    boolean add(Multimedia m)
     {
         if (listamultimedia.length==tmx) return false;
         else
          {
              listamultimedia[contador]=m;
              contador++;
          }
        
        return true;
     }
    
    Multimedia get(int posicion)
     {
         return listamultimedia[posicion];
     }    
    
    int indexOf(Multimedia m)
       {
              
        for(int i=0;i<tmx;i++)
         {
             if(m.equals(listamultimedia[i]))
              {
                  return i;
              }  
         }
        
        return -1;
       }        
 
    //@Override
    public String toString(int p) 
    {
        String pm=null; 
        
        pm="Título"+listamultimedia[p].titulo+"\nAutor"+listamultimedia[p].autor+"\nDuracion"+listamultimedia[p].duracion;
                
        return pm;
    }       

 }

public class Multimedia 
 {
    String titulo;
    String autor;
    int duracion;
    
    Multimedia(String titulo, String autor, int duracion)
    {
        this.titulo=titulo;
        this.autor=autor;
        this.duracion=duracion;
    }
    
    @Override
    public String toString()
      
     {
       return this.titulo + " " + this.autor + " " + this.duracion;
     }
    
    @Override
    public boolean equals(Object M)
     {
        Multimedia objeto = (Multimedia) M;
        if(this.titulo.equals(objeto.titulo)) 
         {
            return true;
         }
	
        return false;
     }       
 }

public class Pelicula extends Multimedia 
{
    String actorprincipal;
    
    Pelicula(String titulo,String autor,int duracion,String actorprincipal)
     {
        super(titulo,autor,duracion);
        this.actorprincipal=actorprincipal;
     }        

    @Override
    public String toString()
    {
        super.toString();
        return "Actor principal "+ actorprincipal;
    }       
}

When I run the code I display the following error:

Exception in thread "main" java.lang.NullPointerException
at Anadeobjetosalalistamultimedia.main(Anadeobjetosalalistamultimedia.java:18)
Java Result: 1

After reading some information about this error in Internet I only know the error is due to a null reference in the code. But I don't understand the meaning of this.

How can I fix it?

Thanks.

[Jesper de Jong]

In line 7 of Anadeobjetosalalistamultimedia you are creating a new array with elements of type EstanteriaMultimedia, with 20 elements.

But you are not initializing the content of the array anywhere. So the array will contain 20 null references. This is a mistake that many beginners make with arrays.

In line 18 you're trying to call a method on e[0], but e[0] is null so you get a NullPointerException.

You need to fill the array with EstanteriaMultimedia objects somewhere.

[Sunil Sunny]

initialize you array as e[0]=new EstanteriaMultimedia(1) or whatever parameter you want to give

[Alex Garci]

Hi,

I initialized array EstanteriaMultimedia but I displayed the same error. But I not initialized EstanteriaMultimedia array, I also did the same in listamultimedia array. But the NullPointer exception error didn't disappear.

The code I modified:

Class Anadeobjetosalalistamultimedia

public class Añadeobjetosalalistamultimedia 
{
        
    public static void main(String args[])
    {
        int tm=20;
        EstanteriaMultimedia[] e=new EstanteriaMultimedia[tm];  
        boolean icorrecta=false; 
        
        //I initialize Array EstanteriaMultimedia
        for(int i=0;i<tm;i++)
        {
           e[i].contador=0;
           e[i].tmx=0;
          
        }
        
                
        Pelicula pl1=new Pelicula("No habra paz para los malvados","Enrique Urbizu",120,"Jose Coronado");
        Pelicula pl2=new Pelicula("Chico y Rita","Fernando Trueba",130,"Chico");
        Pelicula pl3=new Pelicula("La piel que habito","Pedro Almodovar",150,"Antonio Banderas");
        Pelicula pl4=new Pelicula("Los otros","Alejandro Amenabar",150,"Nicole Kidman");
        Pelicula pl5=new Pelicula("Juana la Loca","Vicente Aranda",140,"Aitana Sanchez Gijón");
        
}

Class EstanteriaMultimedia

public class EstanteriaMultimedia 
 {
    Multimedia[] listamultimedia;
    int contador;
    int tmx; 
    
    public EstanteriaMultimedia(int tmaximo)
     {
         
         listamultimedia=new Multimedia[tmaximo];
         for(int i=0;i<tmaximo;i++) //I initialize the array listamultimedia
         {
             listamultimedia[i].autor="\n";
             listamultimedia[i].titulo="\n";
             listamultimedia[i].duracion=0;
         }    
         tmx=tmaximo;
         contador=0;
     }     
    
    int size()
     {
         return listamultimedia.length;
     }
    
    boolean add(Multimedia m)
     {
         if (listamultimedia.length==tmx) return false;
         else
          {
              listamultimedia[contador]=m;
              contador++;
          }
        
        return true;
     }
    
    Multimedia get(int posicion)
     {
         return listamultimedia[posicion];
     }    
    
    int indexOf(Multimedia m)
       {
              
        for(int i=0;i<tmx;i++)
         {
             if(m.equals(listamultimedia[i]))
              {
                  return i;
              }  
         }
        
        return -1;
       }        
 
    //@Override
    public String toString(int p) 
    {
        String pm=null; //para guardar la cadena a mostrar
        
        pm="Título"+listamultimedia[p].titulo+"\nAutor"+listamultimedia[p].autor+"\nDuración"+listamultimedia[p].duracion;
                
        return pm;
    }       

 }

What's wrong?

Thanks.

[Jesper de Jong]

You haven't solved the problem yet. Look at your code:

public class Añadeobjetosalalistamultimedia 
{
        
    public static void main(String args[])
    {
        int tm=20;
        EstanteriaMultimedia[] e=new EstanteriaMultimedia[tm];  
        boolean icorrecta=false; 
        
        //I initialize Array EstanteriaMultimedia
        for(int i=0;i<tm;i++)
        {
           e[i].contador=0;
           e[i].tmx=0;
          
        }

        // ...
}

Note that in line 13 and 14, e[i] is null. You haven't created a new EstanteriaMultimedia object yet and assigned it to e[i].

[Alex Garci]

Hi,

I have initialized array e[i] typing the following:

e[i]=new EstanteriaMultimedia(20);

But the NullPointerexception error was displayed in class EstanteriaMultimedia. I typed something similar as on the class Anadeobjetosalalistamultimedia:

listamultimedia[i]=new Multimedia[20];

When I thought the error was fixed, the java compiler throws the following error:

incompatible types

required Multimedia
found Multimedia[]

Here the new code:

class EstanteriaMultimedia

public class EstanteriaMultimedia 
 {
    Multimedia[] listamultimedia=new Multimedia[0];
    int contador;
    int tmx; //guardo el tamaño maximo del array
    
    public EstanteriaMultimedia(int tmaximo)
     {
         //EstanteriaMultimedia e=new EstanteriaMultimedia(tmaximo);
                    
         for(int i=0;i<tmaximo;i++)
         {
             listamultimedia[i]=new Multimedia[0]; //incompatible types error
         }    
         for(int i=0;i<tmaximo;i++)
         {
             listamultimedia[i].autor="\n";
             listamultimedia[i].titulo="\n";
             listamultimedia[i].duracion=0;
         }    
         tmx=tmaximo;
         contador=0;
     }     
    
    int size()
     {
         return listamultimedia.length;
     }
    
    boolean add(Multimedia m)
     {
         if (listamultimedia.length==tmx) return false;
         else
          {
              listamultimedia[contador]=m;
              contador++;
          }
        
        return true;
     }
    
    Multimedia get(int posicion)
     {
         return listamultimedia[posicion];
     }    
    
    int indexOf(Multimedia m)
       {
              
        for(int i=0;i<tmx;i++)
         {
             if(m.equals(listamultimedia[i]))
              {
                  return i;
              }  
         }
        
        return -1;
       }        
 
    //@Override
    public String toString(int p) 
    {
        String pm=null; //para guardar la cadena a mostrar
        
        pm="Título"+listamultimedia[p].titulo+"\nAutor"+listamultimedia[p].autor+"\nDuración"+listamultimedia[p].duracion;
                
        return pm;
    }       

 }

class Multimedia

public class Multimedia 
 {
    String titulo;
    String autor;
    int duracion;
    
    Multimedia(String titulo, String autor, int duracion)
    {
        this.titulo=titulo;
        this.autor=autor;
        this.duracion=duracion;
    }
    
    @Override
    public String toString()
      
     {
       return this.titulo + " " + this.autor + " " + this.duracion;
     }
    
    @Override
    public boolean equals(Object M)
     {
        Multimedia objeto = (Multimedia) M;
        if(this.titulo.equals(objeto.titulo)) 
         {
            return true;
         }
	
        return false;
     }       
 }

class Anadeobjetosalalistamultimedia

public class Añadeobjetosalalistamultimedia 
{
        
    public static void main(String args[])
    {
        int tm=20;
        EstanteriaMultimedia[] e=new EstanteriaMultimedia[tm];  
        boolean icorrecta=false; //para comprobar si la inserción del objeto es correcta
        
        
        for(int i=0;i<tm;i++)
            e[i]=new EstanteriaMultimedia(20); //no errors compiling
        
        
        for(int i=0;i<tm;i++)
        {
           e[i].contador=0;
           e[i].tmx=0;
          
        }
        
        Pelicula pl1=new Pelicula("No habrá paz para los malvados","Enrique Urbizu",120,"José Coronado");
        Pelicula pl2=new Pelicula("Chico y Rita","Fernando Trueba",130,"Chico");
        Pelicula pl3=new Pelicula("La piel que habito","Pedro Almodovar",150,"Antonio Banderas");
        Pelicula pl4=new Pelicula("Los otros","Alejandro Amenabar",150,"Nicole Kidman");
        Pelicula pl5=new Pelicula("Juana la Loca","Vicente Aranda",140,"Aitana Sanchez Gijon");
        
        

How can I fix it?

Thanks.

[Henry Wong]

Alex Garci wrote:
I have initialized array e[i] typing the following:

e[i]=new EstanteriaMultimedia(20);

But the NullPointerexception error was displayed in class EstanteriaMultimedia.

Basically, you fixed the problem, only to reveal a different problem in a different location. As for how to fix it, it is basically the same issue, and the fix is similar to the earlier one -- only for different types.

Alex Garci wrote:
I typed something similar as on the class Anadeobjetosalalistamultimedia:

listamultimedia[i]=new Multimedia[20];

When I thought the error was fixed, the java compiler throws the following error:

incompatible types

required Multimedia
found Multimedia[]

Don't know why you decided to do a different fix -- but obviously, you can assign an array to an non-array, and the compiler is telling you that.

Henry

[Alex Garci]

Thanks for replying. The issue is solved now.