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Thread: can someone help with this error?

  1. #1
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    Default can someone help with this error?

    Hi I'm a beginner in java. I have some problem with my code.
    can someone help me?

    import java.util.Scanner;
     
     
    public class HW11of2 { 
     
    	public static void main(String[] args) {
    		// TODO Auto-generated method stub
    		Scanner console = new Scanner(System.in); 
    		student[] students=new student[4];
    		for(int i=0;i<students.length;i++){
     
    			System.out.println("Please enter the name: ");  
    	        students[i].name = console.nextLine();  
    			System.out.println("Please enter a number for the id: ");  
    	        students[i].id = console.nextInt();  
    	        int sum=0;
    	        for(int j=0;j<5;j++){
    	        	System.out.println("Please enter the grade: ");  
    		        students[i].grades[j] = console.nextInt();
    		        sum=sum+ students[i].grades[j];
    	        }
    	        students[i].avg=sum/5;
    		}
    		int avgtotal=0;
    		 for(int i=0;i<students.length;i++){
    			 avgtotal=avgtotal + students[i].grades[2];
    		 }
    		 System.out.println(avgtotal);
    		 int min=min(students[0].grades);
    		 for(int i=1;i<students.length;i++){
    			 int temp=min(students[i].grades);
    			 if(temp<min){
    				 min=temp;
    			 }
    		 }
    		 int max=max(students[0].grades);
    		 for(int i=1;i<students.length;i++){
    			 int temp=max(students[i].grades);
    			 if(temp>max){
    				 max=temp;
    			 }
    		 }
            console.close();
    	}
     
    	public static int min(int[] x){
    		int min=x[0];
    		for (int i=0; i<x.length;i++)
    		{
    			if (x[i]<min)
    			{
    				min=x[i];
    			}
    		}
    		return min;
    	}
    	public static int max(int[] x){
    		int max=x[0];
    		for (int i=0; i<x.length;i++)
    		{
    			if (x[i]>max)
    			{
    				max=x[i];
    			}
    		}
    		return max;
    	}
    	public class student
    	{
    		int id;
    		String name;
    		int[] grades=new int[5];
    		int avg;
    	}
    }

    and the error I get is:
    Exception in thread "main" java.lang.NullPointerException
    	at HW11of2.main(HW11of2.java:13)

    Thanks!


  2. #2
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    Default Re: can someone help with this error?

    Welcome to the forum! Thanks for taking the time to learn to post your code correctly and ask a complete question.

    In Java, class names begin with capital letters, even inner classes.

    This line:

    student[] students = new student[4];

    creates an array that is large enough to hold 4 Student objects, but it doesn't add create or add any student objects to the array.

    So in this and similar subsequent lines:

    students[i].name = console.nextLine();

    the reference to students[i] is null. Each element of students[] must first be filled with a Student instance, as in:

    students[i] = new Student();

    Hope this helps.

  3. #3
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    Default Re: can someone help with this error?

    Thanks!
    that helped and also I've forgot to put static where the class of students.

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