Java Strings Loops Arrays Null

There are several issues with your code.

• You use a scanner to read from the file, which I've rarely seen and never done myself. The FileInputStream class is more optimised for that purpose.

• You use a fixed size array to store an unknown number of Strings

• You use a for-loop for an unknown number of loop iterations. This is not wrong, but a while-loop is more appropriate.

• You create a new HashSet from an array. Again not wrong, but why have you not used the HashSet in your loop? You would have avoided using indices alltogether.

I'm assuming you are a beginning programmmer who just whipped up some code to automate something you didn't want to do manually. Nothing wrong with that of course, and in that case, just use an ArrayList, as all String objects are guaranteed to be unique in your case anyway:

public class Duplicate {
 public static void main(String [] args){

Scanner keyboard = new Scanner(System.in);
System.out.println("Enter file name: ");
String fileName = keyboard.nextLine();
if(fileName.equals("")){
    System.out.println("Error: User did not specify a file name.");
}
  else{Scanner inputStream = null;

try{inputStream = new Scanner(new File(fileName));
}
catch(FileNotFoundException e){
    System.out.println("Error: "+ fileName + " does not exist.");
    System.exit(0);
}


ArrayList<String> addresses = new ArrayList<String>();

for(int i=0;inputStream.hasNextLine();i++){
    String email = inputStream.nextLine();
    address.add(email.toLowerCase());
    //System.out.println(email);
}

for(String email:addresses){
    System.out.println(email);
}

With appropriate imports. Note: that's with minimal changes, there are some things that can still be improved, see above. Good luck!

Just do this:

Set<String> addresses = new HashSet<String>(); // use a Set

while (inputStream.hasNextLine()) {
    addresses.add(inputStream.nextLine().toLowerCase()); // in-line unused variable
}

for (String email : mail) {
    System.out.println(email);
}

String [] address = new String[100];  

change on

SortedSet<String> address = new TreeSet<String>();   

and

address[i]=email.toLowerCase();  

change on

address.add(email.toLowerCase());

You are trying to read fixed number [100] elements from the file.

If there are more e-mails, you will miss some, if there are less - you will have some nulls left [which the Set will collapse into a single null].

Try using an ArrayList instead of an array, or using a Set from the first place..

I assume you are reading less than 100 addresses. The remaining elements in the array address are null. This is the cause for the null value.

Replace the fixed size array with an ArrayList<String>:

List<String> address = new ArrayList<String>();
//...
    address.add(email.toLowerCase());
    // ...

You must also replace the construction of the set:

Set<String> mail = new HashSet<String>(address);