While reading the next and previous lines in a file I get a null pointer exception

Question

I know the way to read the next and previous lines while reading a file line by line is like

String prev = null;
String curr = null;
String next = null;

Scanner sc = new Scanner(new File("thefile.txt"));

while(sc.hasNextLine()) {
    prev = curr;
    curr = next;
    next = sc.nextLine();

But during the first iteration I get a null pointer exception as I have to work with the current element.

Whats the workaround for this?

    Scanner sc = new Scanner(file);

    String curr = null ;
    String next = null ;


      while (sc.hasNextLine()) {

            curr = next;
            next =  sc.nextLine();
            if(curr.length() > 0 && !curr.equals(". O O")) {
                lines+= curr.substring(0, curr.indexOf(" ")) + " ";

                for(String pat : patStrings)
                {
                // System.out.println(pat) ;
                    Pattern minExp = Pattern.compile (pat);
                    Matcher minM = minExp.matcher(curr);
                    while(minM.find())
                    {
                        String nerType = minM.group(2);
                        if(nerType.contains("B-NE_ORG")){
                            if (next.contains("I-NE_ORG"))
                                orgName+= minM.group(1)+" ";

Answer 1

You most likely need to read two lines first to get your variables setup properly

String prev = null;
String curr = null;

Scanner sc = new Scanner(new File("thefile.csv"));

if (sc.hasNextLine()) {
    curr = sc.nextLine();
}

while (sc.hasNextLine()) {
    prev = curr; 
    curr = sc.nextLine();
}

If you do need three lines at a time, then you should actually read three lines, then process them.

StringBuilder sb = new StringBuilder();
while (sc.hasNextLine()) {
    sb.setLength(0); // clear the stringbuilder

    String line = sc.nextLine(); 
    if (line.isEmpty()) continue; // if you want to skip blank lines
    else sb.append(line).append("\n");

    for (int i = 1; i < 3 && sc.hasNextLine(); i++) {
        line = sc.nextLine(); 
        sb.append(line).append("\n");
    }
    String[] lines = sb.toString().trim().split("\n");
    if (lines.length == 3) {
        String prev2 = lines[0];
        String prev1 = lines[1];
        String curr = lines[2];

        doStringThings(prev2, prev1, curr);
    }
}