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Thread: nullpointerexception bug

  1. #1
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    Default 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)


  2. #2
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    Default Re: nullpointerexception bug

    Wrap your code in code tags. Post the exact error message you get.

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    Default Re: nullpointerexception bug

    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");

  4. #4
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    Default Re: nullpointerexception bug

    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]

  5. #5
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    Default Re: nullpointerexception bug

    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.

  6. The Following User Says Thank You to GregBrannon For This Useful Post:

    Jedi khan (July 11th, 2013)

  7. #6
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    Default Re: nullpointerexception bug

    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]

  8. #7
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    Default Re: nullpointerexception bug

    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?

  9. The Following User Says Thank You to GregBrannon For This Useful Post:

    Jedi khan (July 11th, 2013)

  10. #8
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    Default Re: nullpointerexception bug

    plx do fix it for me

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