concurrency error

[Ed Grace]

import java.io.*;
import java.util.Arrays;









class grades
{
	public static void main(String[]args)
		throws java.io.IOException
		{
			//variable
			int i;
			String name;
			double x = 0, y, z;
			
			//input
			InputStreamReader isr = new InputStreamReader(System.in);
		    BufferedReader br = new BufferedReader(isr);
			//declare array
			String names[];
			//allocate array
			names = new String[4];
			//master grades array
			String gradesmaster[];
			//allocate master array
			gradesmaster= new String[4];
			//declair 2 array
			double grade1[];
			//allocate 2 array
			grade1 = new double[4];
			//declair 4 array
			double grade2[];
			//allocate 4 array
			grade2 = new double[4];
			//declair 3 array
			double grade3[];
			//allocate 3 array
			grade3 = new double[4];
			
			//getting students names
			for (i = 1; i< names.length; i++)
			{	
			System.out.println("What is your student's name?");
		    names[i] = br.readLine();
		    }
		    //sorting  names
		   
		    Arrays.sort(names);
		    //getting 1st students grades
			for (i = 1; i< grade1.length; i++)
			{	
			System.out.println("What is " +names[1]+ "'s grades?");
		    gradesmaster[i] = br.readLine();
		    grade1[i]= Double.parseDouble(gradesmaster[i]);
		    }
		    //getting 2nd students grades
			for (i = 1; i< grade1.length; i++)
			{	
			System.out.println("What is " +names[2]+ "'s grades?");
		    gradesmaster[i] = br.readLine();
		    grade2[i]= Double.parseDouble(gradesmaster[i]);
		    }
		    //getting 3rd students grades
			for (i = 1; i< grade1.length; i++)
			{	
			System.out.println("What is " +names[3]+ "'s grades?");
		    gradesmaster[i] = br.readLine();
		    grade3[i]= Double.parseDouble(gradesmaster[i]);
		    
		    }
		    
		    
		    System.out.println("\t This is a report card for the 2009 Spring session");
		    	// display names
		    	System.out.println( "Student name:"+ names[1] +" ," + names[2]+" ," +names[3]+","  );
		    
		    	System.out.println("___________________________________________________________________________________________");
		    	
		    	System.out.println("Student name" +"\tGrade 1"+"\tGrace 2"+"\tGrade 3"+"\tAverage");
		    	//display the  1st student
		    	System.out.print(names[1]+"\t\t");
		    		for (i = 1; i< grade1.length; i++)
		    		{
		    			System.out.print("\t "+ grade1[i]);
		    			
		    			
		    		}
		    		    x= grade1[1]+grade1[2]+grade1[3];
		    			x= x/3;
		    			x= Math.round(x);
		    	
		    	System.out.print("\t"+x);
		    		//display second student
		    		System.out.print("\n"+names[2]+"\t\t");
		    		for (i = 1; i< grade2.length; i++)
		    		{
		    			System.out.print("\t "+ grade2[i]);
		    			
		    			
		    		}
		    		x= grade2[1]+grade2[2]+grade2[3];
		    			x= x/3;
		    			x= Math.round(x);
		    	
		    	
		    	System.out.print("\t"+x);
		    		//display third  student
		    		System.out.print("\n"+names[3]+"\t\t");
		    		for (i = 1; i< grade3.length; i++)
		    		{
		    			System.out.print("\t "+ grade3[i]);
		    			
		    			
		    		}
		    	 x= grade3[1]+grade3[2]+grade3[3];
		    			x= x/3;
		    			x= Math.round(x);
      	System.out.print("\t"+x);
		    
		    
		    
		}
		
}

		

  
  

hey everyone, i was wondering if someone could help me out with my code... i know some of it may be confusing and could probably be cut down a little bit but im ok with that.... my problem is that i need to sort my names array and as you can see i already have that coded and it compiles fine, but when i run the code i get the following errors:
Exception in thread "main" java.lang.NullPointerException
at java.util.Arrays.mergeSort(Arrays.java:1144)
at java.util.Arrays.sort(Arrays.java:1079)
at grades.main(finalfinal.java:59)
if anyone could help me out with this and help explain what i can do to fix this, my computer science teacher was completley stumped when i showed him this, because it works completely fine when the sorting code is taken out, so again any and all help would be appreciated and thank you in advanced

[Steve Luke]

You get that error because the first element in the array is null (has no value). You must not have any null values in the array in order to avoid a NullPointerException.

In Java, arrays are indexed from 0. So the first element in the names array is names[0]. However, when you fill the array you start looping from an index of 1:

            for (i = 1; i< names.length; i++)
            {	
                System.out.println("What is your student's name?");
                names[i] = br.readLine();
            }
            //sorting  names
            Arrays.sort(names);

Since you never assign to the first index in the array (names[0]) the value remains null. You should start the for loop at 0 rather than 1:

            for (i = 0; i< names.length; i++)
            {	
                System.out.println("What is your student's name?");
                names[i] = br.readLine();
            }
            //sorting  names
            Arrays.sort(names);

Note that you make the same mistake for all your other arrays. Also, if you want only 3 names (as it appears you do) you should change the array sizes to 3, rather than 4:

            String names[];
            //allocate array
            names = new String[3];

[Campbell Ritchie]

Welcome to JavaRanch

Once I found out where the Exception occurred, it became obvious why. Insert the following just before line 95, which appears to be where the Exception occurs:

/* Test block follows */
for (int i = 0; i < names.length; i++)
{
   System.out.printf("names[%d] = %s%n", i, names[i]);
}//end test block

Then go back to where those objects are initialised and see if you can work out why it went wrong.

[Campbell Ritchie]

I see Steve has beaten me to it.

It is not good design to have several parallel arrays; you ought to have a Student class incorporating the name and marks, and use a Student[] array.