Comparing two string arrays in java using string.equals

Question

I have a method that works fine and gives desired result. Here is it's code:

public class arraycomparison {
public static void main(String args[]) {
 try {
 Scanner sc1 = new Scanner(new FileInputStream("/Users/esna786/File1.txt"));
 Scanner sc2 = new Scanner(new FileInputStream("/Users/esna786/File2.txt"));
 List<String> File1Content = new ArrayList<String>();
 List<String> File2Content = new ArrayList<String>();
 List<String> commonWords = new ArrayList<String>();
 //Getting the contents of File1
 while (sc1.hasNext()) {
 File1Content.add(sc1.next());
 }
 String arr1[] = File1Content.toArray(new String[File1Content.size()]);
 //Getting the contents of File1
 while (sc2.hasNext()) {
 File2Content.add(sc2.next());
 }
 String arr2[] = File2Content.toArray(new String[File2Content.size()]);
 int count = 0;
 for (int i = 0; i < arr1.length; i++) {
 for (int j = 0; j < arr2.length; j++) {
 if (arr1[i].equals(arr2[j])) {
 commonWords.add(arr1[i]);
 count++;
 }
 }
 }
 String[] comWords = commonWords.toArray(new String[commonWords.size()]);
 System.out.println("Total Number of Common Words are: "+comWords.length);
 for (int i = 0; i < comWords.length; i++) {
 System.out.println(i + 1 + ": " + comWords[i]);
 }
 } catch (Exception e) {
 e.printStackTrace();
 }
}

}

it's out put is,

Total Number of Common Words are: 7
1: apple
2: ball
3: cat
4: duck
5: elephant
6: fan
7: goat
BUILD SUCCESSFUL (total time: 0 seconds)

Using the same files when I run the program code below almost same as above:

//Counting the number of matched words
public int getCommonWords(File File1, File File2) throws IOException {
 try {
 Scanner sc1 = new Scanner(new FileInputStream(File1));
 Scanner sc2 = new Scanner(new FileInputStream(File1));
 List<String> File1Content = new ArrayList<String>();
 List<String> File2Content = new ArrayList<String>();
 List<String> commonWords = new ArrayList<String>();
 //Getting the contents of File1
 while (sc1.hasNext()) {
 File1Content.add(sc1.next());
 }
 String arr1[] = File1Content.toArray(new String[File1Content.size()]);
 //Getting the contents of File1
 while (sc2.hasNext()) {
 File2Content.add(sc2.next());
 }
 String arr2[] = File2Content.toArray(new String[File2Content.size()]);
 int count = 0;
 for (int i = 0; i < arr1.length; i++) {
 for (int j = 0; j < arr2.length; j++) {
 if (arr1[i].equals(arr2[j])) {
 commonWords.add(arr1[i]);
 count++;
 }
 }
 }
 String[] comWords = commonWords.toArray(new String[commonWords.size()]);
 System.out.println("Total Number of Common Words are: " + comWords.length);
 for (int i = 0; i < comWords.length; i++) {
 System.out.println(i + 1 + ": " + comWords[i]);
 }
 } catch (Exception e) {
 e.printStackTrace();
 }
 return 0;
}

it displays following in the output area:

run:
Total words of File1.txt are: 11
Total Number of Common Words are: 11
1: apple
2: ball
3: cat
4: duck
5: elephant
6: fan
7: goat
8: a
9: b
10: c
11: d

it is even displaying the unmatched words as well, why is it so? Please help.

• Reimeus: Initialise commonWords as the stacktrace indicates...
• Mohsin Ali: I initialized it to null, it should later be occupied with other values in loops.
• Hovercraft Full Of Eels: Heck no. You initialize it to null, to nothing. You can't assign items to a null array. You're confusing the array with the array items.
• PM 77-1: null is not an array. null[i] produces an NPE.

Answer 1

You are getting the null pointer exception for an obvious reason, the variable commonWord is not pointing to an array (you didn't allocate memory for storing the array strings).

String commonWords[]=null;

You have 2 solutions:

• Use fixed memory allocation:

  String commonWords[]= new String[Math.min(arr1.length, arr2.length)];
  int commonCount = 0;
  ....
  commonWords[commonCount++] = arr2[i]
  

• Use dynamic memory allocation:

  ArrayList<String> commonWords = new ArrayList<>();
  ...
  commonWords.add(arr2[i]);
  

Answer 2

If you want to preserve your logic and data structures, then use something like:

String commonWords[] = new String [Math.min(arr1.length, arr2.length)];

It's far from perfect, but will allow you to assign values to your array.

Then to print:

for(String str : commonWords) {
 if (str != null) {
 System.out.println(str);
 }
}