NullPointerException help

[laichunkitjacky]

I am working on a project on Student Score management system.
When I am running the program to start the bubblesort after entering the students information, NullPointerException occur.
I know this is due to the null value of array.
But I don't know how to fix it because I should have entered the students' records and the "list" array should be present and contain value. Why it still appears as null ??

This is the error:
Exception in thread "main" java.lang.NullPointerException
at Students.bubbleSort(Students.java:121)
at Students.main(Students.java:108)

import java.util.*;
public class Students 
{

	private String id,fn,ln;
	private int age;
	private char gender;
	private double feg;
	static Students list[];
	static int select,number;
	
		
	public Students()
	{
	}
	
	public Students(String a,String b, String c, int d,char e, double f)
	{
		this.id=a;
		this.fn=b;
		this.ln=c;
		this.age=d;
		this.gender=e;
		this.feg=f;
	}

	public String getID()
	{
		return this.id;
	}
	
	public double getFeg()
	{
		return this.feg;
	}
	
	public String getName()
	{
		return this.fn+" "+this.ln;
	}
	
		
	public static void main(String []args)
	{
		Scanner keyboard = new Scanner(System.in);
		System.out.println("Press 1 to input student information");
		System.out.println("Press 2 to list all students according to final grades");
		System.out.println("Press 3 to search for a specific student's information");
		select = keyboard.nextInt();
		if(select==1)
		{
			System.out.println("How many students' information you want to enter?");
			number=keyboard.nextInt();
			
			Students  list [] =new Students[number];
			
			for(int i=0;i<list.length;i++)
			{
				String ida,fna,lna;
				int agea;
				char gendera;
				double fega;
								
				
				System.out.println("Input id");
				ida=keyboard.next();
				System.out.println("Input First Name");
				fna=keyboard.next();
				System.out.println("Input Last Name");
				lna=keyboard.next();
				System.out.println("Input Age");
				agea=keyboard.nextInt();
				System.out.println("Input Gender");
				gendera=keyboard.next().charAt(0);
				System.out.println("Input Score");
				fega=keyboard.nextInt();
				list[i]= new Students(ida,fna,lna,agea,gendera,fega);
			}
					
			
			for(int j=0;j<list.length;j++)
			{
				System.out.println(list[j]+list[j].getID());
			}
		}
		
		System.out.println("Succeed to input "+number+" records.");
		System.out.println(" ");
		System.out.println("Press 2 to list all students according to final grades");
		System.out.println("Press 3 to search for a specific student's information");
		
		
				
		select = keyboard.nextInt();
		if(select ==2)
		{					
			bubbleSort(list);

			for (int i=0; i < list.length; i++) 
				{
				System.out.println( list[i].getName() + " " +
					list[i].getFeg() );
				}
		}
		
	}
	
	public static void bubbleSort(Students [] array)
	{
		for (int pass=0; pass < array.length-1; pass++) 
			for (int k=0; k < array.length-pass-1; k++) 
				if (array[k].getFeg() > array[k+1].getFeg())
					swap(array, k, k+1);
	}

	public static void swap(Students [] array, int first, int second)
	{
		Students	temp;

		temp = array[first];
		array[first] = array[second];
		array[second] = temp;
	}

	
	
}

[laichunkitjacky]

The line generating error should be 97 and 110

[JosAH]

You have a static array 'list' in line #9, but you also have a local one (which hides the other one) in line #55; don't create a local one in line #55 but initialize the static one there.

kind regards,

Jos

[laichunkitjacky]

Thank you for your reply!! But I want to let the user to create the array with their own size. Is there any way that I can do this?

[JosAH]

Thank you for your reply!! But I want to let the user to create the array with their own size. Is there any way that I can do this?

Sure you can, just don't introduce a local array at line #55, i.e. change it to:

list [] =new Students[number];

kind regards,

Jos