Thread: nullpointerexception bug

This program is to manipulate median of an array.it returns NullPointerException.i have tried it .plz any1 to rectify the bug.
thanxs in advance.

import java.io.*;
 
public class media {
 
	@SuppressWarnings("null")
	public static void main(String[] args) {
		int i=0,j=0,n=0;
 
	      float median;
		float t; 
	     int []a=null;
	     BufferedReader br=new BufferedReader(new InputStreamReader(System.in));                                                      
	       System.out.println("Enter the number of items\n");                  
	        try {
				n=Integer.parseInt(br.readLine());
			} catch (NumberFormatException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
	        System.out.println("Input values \n");  
	       for( i=1;i<n;i++)
			try {
				a[i]=Integer.parseInt(br.readLine());
			} catch (NumberFormatException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
 
	    /* Reading items in array a  */
 
 
	    /* Sorting begins */
	for (i = 1 ; i <= n-1 ; i++)                            
	       {     /* Trip-i begins  */
	for (j = 1 ; j <= n-i ; j++)                         
	          {                                                    
	              if (a[j] <= a[j+1])                              
	              { /* Interchanging values */
 
	                t = a[j];                                      
	                a[j] = a[j+1];                                 
	                a[j+1] = (int) t;                                    
	              }                                                
	              else continue ;                                     
	          }                                                    
	       } /* sorting ends */
	/* calculation of median  */
	if ( n % 2 == 0)                                        
	          median = (float) ((a[n/2] + a[n/2+1])/2.0) ;                   
	       else                                                    
	          median = a[n/2 + 1];                                 
 
	    /*  Printing */
	for (i = 1 ; i <= n ; i++)  
	{
	          System.out.println(a[i]); 
	}
 
 
 
	 System.out.println("\n\nMedian is " +median);  
	}
 
 
}
 
Exception in thread "main" java.lang.NullPointerException
	at media.main(media.java:26)

Wait to define the a[] array until you know how many elements it should have. Something like:

        System.out.println("Enter the number of items\n");                  
        try {
            n=Integer.parseInt(br.readLine());
        } catch (NumberFormatException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
 
        // define the array here with the appropriate number of elements
        int[] a = new int[n];
 
        System.out.println("Input values \n");

The output after modifcation as u suggested is:
plx see it nd suggest me
[highlight=code]
Enter the number of items

4
Input values

1
2
3
4

java.lang.NumberFormatException: For input string: ""
at java.lang.NumberFormatException.forInputString(Unk nown Source)
at java.lang.Integer.parseInt(Unknown Source)
at java.lang.Integer.parseInt(Unknown Source)
at media.main(media.java:27)
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 4
at media.main(media.java:44)
[\highlight]

The error, "Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 4
at media.main(media.java:44)" points at this code:

        /* Sorting begins */
        for ( i = 1 ; i <= n-1 ; i++ )
        {     /* Trip-i begins  */
            for ( j = 1 ; j <= n-i ; j++ )
            {
                 /* Interchanging values */
                if ( a[j] <= a[j + 1] )                              
                {

So you need to check the limits of the for loops:
For n = 4, i in the first for loop ranges from i = 1 to i = 3. In the second for loop, j ranges from j = 1 to j = 3. Then, in the last line of the code above, the if() statement, j = 3 could make the statement:

a[3] <= a[4]

but 4 is not a legal index of the array a[]. It's too large or "out of bounds."

Tracing the logic as I've done above is a demonstration of how you should find and then fix errors yourself. But when you need help, ask.

I've tried another approach to find the median but am getting correct ans for odd case not the desired one in case of even. plz have a glance at it .i have just simply used statistics median formula.
thanks alot in advance

[highlight=java]
import java.io.*;
import java.util.Arrays;

public class media {
public static float findMedian(int array[]) {
int i=0;
int length=array.length;
int[] sort = new int[length];
System.arraycopy(array, 0, sort, 0, sort.length);
Arrays.sort(sort);
for(i=0;i<sort.length;i++)
{
System.out.println("Sorted array" +sort[i]);

}
if (length % 2 == 0) {

return (sort[(sort.length / 2) - 1] + sort[(sort.length) / 2] / 2);
}
else {

return sort[sort.length / 2];
}
}

public static void main(String[] args) {

BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
int n=0;
int i=0;
System.out.println("Enter the no.of elements\n");
try {
n=Integer.parseInt(br.readLine());
} catch (NumberFormatException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
int[] ranNum = new int[n];
System.out.println("Input elements\n");
for(i=0;i<n;i++)
{
try {
ranNum[i]=Integer.parseInt(br.readLine());
} catch (NumberFormatException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}

float median=findMedian(ranNum);
System.out.println("Median= "+median);
}
}


[\highlight]

You just need to work through this formula to see if it is correct:

return (sort[(sort.length / 2) - 1] + sort[(sort.length) / 2] / 2)

So, for an even number of elements in the array, say '6', compute the formula's result:

sort.length = 6:
return (sort[(6 / 2) - 1] + sort[(6) / 2] / 2)
= return (sort[(3) - 1] + sort[3] / 2)
= return (sort[2] + sort[3] / 2)

Which isn't quite what you want, I think. How can you fix the original equation to resolve to the desired return?

plx do fix it for me