I m getting error -- Exception in thread "main" java.lang.NullPointerException at java.lang.StringBuffer.(Unknown Source)

Question

In an array of strings 'a' having 'n' strings i have to select the Palin Pairs from the given strings .for ex for input 3 bba abb abb Output=2 what m i doing wrong

import java.util.*;
class Test {
  public static void main(String args[]) {
    int i,j,k=0;
    Scanner h=new Scanner(System.in);
    int n=h.nextInt();
    String a[]=new String[n];

    for(i=0;i<n;i++) {
      if(a[i]!="") {
        String rev = (new StringBuffer(a[i])).reverse().toString();
        for(j=i+1;j<n;j++) {
          if(rev.equals(a[j])) {
            k++;
            a[j]="";
          }
        }
      }
    }

    h.close();
  }
}

Please don't use StringBuffer as it was replaced by StringBuilder more than ten years ago. — Peter Lawrey, Jul 24 '15 at 20:25

Andy Thomas · Accepted Answer · 2015-07-24T21:21:40.993

4

You're creating a String array, but never initializing its members.

The default values of the members are null.

Hence, when you call new StringBuffer(a[i]), you're passing a null value.

You could avoid this by:

Filling the array with something.
Checking for null in addition to your existing check for empty string.

By the way, you're attempting to check for empty string with the equality operator !=. This will check that the references are different, not that the String objects to which they point contain different strings. You'll want to check instead with either:

  (null != a[i]) && ! a[i].isEmpty()

or

  ! "".equals( a[i] )

or, if you're using Google's guava library:

  ! Strings.isNullOrEmpty( a[i] )

edited Jul 24 '15 at 21:21

answered Jul 24 '15 at 20:14

Andy Thomas

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thanks that solved it n i m getting output :-),just one more question why cant i check empty string with equality operator – Shivangi Awasthi Jul 24 '15 at 20:22
@ShivangiAwasthi - You caught me mid-edit. That's included above now. The equality operator checks that the *references* are equal. You can see more detail about that in this question: http://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java – Andy Thomas Jul 24 '15 at 20:25
@Andy Thomas A better way to check whether a[I] is empty is "a[i].isEmpty()" – FredK Jul 24 '15 at 21:13
@FredK - You're right, that's higher-level than the explicit length check -- which is how `isEmpty()` is implemented. I'll add it above. The `"".equals()` check works whether the reference is null or not. There's also guava's Strings.isNullOrEmpty(). – Andy Thomas Jul 24 '15 at 21:20

score 1 · Answer 2 · answered Jul 24 '15 at 20:27

1

Most likely you are expecting to read the words from the console which you are not doing. I suggest you use Scanner.nextLine().

int n=h.nextInt();
h.nextLine(); // discard the rest of the line.
String a[]=new String[n];

for(int i = 0; i < n; i++) {
   // read a word.
   a[i] = h.nextLine();

answered Jul 24 '15 at 20:27

Peter Lawrey

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Or with `Scanner.next()`, if the words are on the same line. You're right, if the count is coming from stdin, likely the data is as well. – Andy Thomas Jul 24 '15 at 21:31

I m getting error -- Exception in thread "main" java.lang.NullPointerException at java.lang.StringBuffer.(Unknown Source)

2 Answers2