CODEWITHSUNDEEP
javaeclipse
TheNoobEternal
TheNoobEternal

Reputation: 27

Exception in thread "main" java.lang.NullPointerException (Please help me..)

I am new in java and trying to learn it.I am getting this error.

Exception in thread "main" java.lang.NullPointerException at ödev.Ogrenci.main(Ogrenci.java:145)

This is my homework. It is about calculating notes of the random students.

PS: I am Turkish and if some words in the code make no sense to you, ignore it. Those words are probably Turkish. :)

package ödev;



public class Ogrenci {



public int no;
public String ad;
public String soyad;

private int vize1;
public int getVize1() {
    return vize1;
}
public void setVize1(int vize1) {
    this.vize1 = vize1;
}
public int getVize2() {
    return vize2;
}
public void setVize2(int vize2) {
    this.vize2 = vize2;
}
public int getFinal() {
    return Final;
}
public void setFinal(int final1) {
    Final = final1;
}
private int vize2;
private int Final;

String[] harf = new String[]{"a", "e", "i", "h", "k", "l", "m", "n"};

public Ogrenci(){



    }

public void randomNot(){

    int i1 = (int)(Math.random()*11);

    int i2 = (int)(Math.random()*101);

    int i3 = (int)(Math.random()*11);


    if(i1<8)
        vize1 = (int)(Math.random()*36+65);
    else
        vize1 = (int)(Math.random()*65);

    if(i2<26)
        vize2 = (int)(Math.random()*51+50);
    else
        vize2 = (int)(Math.random()*50);

    if(i3<8)
        Final = (int)(Math.random()*36+65);
    else
        Final = (int)(Math.random()*65);

}



public String ad(){

    String isim = "";

    int a1 = (int)(Math.random()*6);
    int a2 = a1+3;

    for(int i = 0; i<a2; i++){
        int a3 = (int)(Math.random()*8);

        String a = harf[a3];

        isim += a;
        }

    return isim;

    }

public String soyad(){

        String isim = "";

        int a1 = (int)(Math.random()*6);
        int a2 = a1+3;

        for(int i = 0; i<a2; i++){
            int a3 = (int)(Math.random()*8);

            String a = harf[a3];

            isim += a;
            }

        return isim;

        }

public String harfnotu(){

    double ort = (0.3*vize1)+(0.3*vize2)+(0.4*Final);

    if(ort>=0 && ort<50)
        return "FF";
    else if(ort>=50 && ort<55)
        return "DD";
    else if(ort>=55 && ort<60)
        return "DC";
    else if(ort>=60 && ort<70)
        return "CC";
    else if(ort>=70 && ort<75)
        return "CB";
    else if(ort>=75 && ort<85)
        return "BB";
    else if(ort>=85 && ort<90)
        return "BA";
    else if(ort>=90 && ort<=100)
        return "AA";
    else if(ad.equals("hakan"))
        return "FF";

    return "Harf notu hesaplanamamistir.";
}




public static void main(String[] args){


        Ogrenci[] Ogt =  new Ogrenci[30];

        for(int z=0; z<30; z++){

            Ogt[z].ad();
            Ogt[z].soyad();
            Ogt[z].randomNot();
            Ogt[z].no = 0710000 + z ;

        }





        System.out.println("Numara         Ad   Soyad             1.vize -- 2.vize -- Final -- Harf Notu");

        for(int k = 0; k<30; k++){

            System.out.println(Ogt[k].no+" "+Ogt[k].ad()+" "+Ogt[k].soyad()+" "+Ogt[k].vize1+" "+Ogt[k].vize2+" "+Ogt[k].Final+" "+Ogt[k].harfnotu());

        }








}


}

Upvotes: 0

Views: 439

Answers (5)

Sage
Sage

Reputation: 15418

Ogrenci[] Ogt =  new Ogrenci[30];

With above statement, You have just declared an array which can contain the instance of Ogrenci, where any element ogt[i] is an instance of Ogrenci and initialized to null. You will have to create each element of the array before accessing them using new.

for(int z=0; z < Ogt.length; z++) //<--- don't use numerical value directly(30) 
                                 //while visiting whole array
{
    Ogt[z] = new Ogrenci();
    // other relevant code
}

Upvotes: 1

mttdbrd
mttdbrd

Reputation: 1831

You didn't initialize any of the Ogrenci objects, just allocated an array of them. They're all null until you initialize them. This should fix it:

    Ogrenci[] Ogt =  new Ogrenci[30];

    for(int z=0; z<30; z++){
        Ogt[z] = new Ogrenci();
        Ogt[z].ad();
        Ogt[z].soyad();
        Ogt[z].randomNot();
        Ogt[z].no = 0710000 + z ;

    }

Upvotes: 2

ljgw
ljgw

Reputation: 2771

The problem is here, in your main function:

Ogrenci[] Ogt =  new Ogrenci[30];

    for(int z=0; z<30; z++){

        Ogt[z].ad();
        Ogt[z].soyad();
        Ogt[z].randomNot();
        Ogt[z].no = 0710000 + z ;

    }

You have declared that there are 30 Ogrenci, but you do not initialize any of them.

use:

   Ogrenci[] Ogt =  new Ogrenci[30];

    for(int z=0; z<30; z++){
        Ogt[z] = new Ogrenci();

        Ogt[z].ad();
        Ogt[z].soyad();
        Ogt[z].randomNot();
        Ogt[z].no = 0710000 + z ;

    }

Upvotes: 2

Konstantin
Konstantin

Reputation: 3294

You instantiate Ogt but then before populating it with anything trying to access it's contents Ogt[z].ad(), there is nothing inside hence NPE

Upvotes: 0

rgettman
rgettman

Reputation: 178293

In main, you have this block:

Ogrenci[] Ogt =  new Ogrenci[30];

    for(int z=0; z<30; z++){

        Ogt[z].ad();
        Ogt[z].soyad();
        Ogt[z].randomNot();
        Ogt[z].no = 0710000 + z ;

    }

You declared your array, but the array is initialized to all nulls. Create your Ogrenci objects before you access them:

for(int z=0; z<30; z++){
    Ogt[z] = new Ogrenci();
    // Then call methods on it here.

Incidentally, be careful with numeric literals starting with 0, because that indicates on octal number (unless, of course, that's what you intended).

Upvotes: 2

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