如何在JAVA中读取.txt文件的特定部分

Question

I have been trying to figure out how to read from a .txt file. I know how to read a whole file, but I am having difficulties reading between two specific points in a file. I am also trying to use the scanner class and this is what I have so far:

public void readFiles(String fileString)throws FileNotFoundException{

        file = new File(fileString);
        Scanner scanner = null; 
        line=""; 


        //access file
        try {
            scanner = new Scanner(file);
        } 
        catch (FileNotFoundException e) {
            System.out.println("File not found.");
        }


            // if more lines in file, go to next line
           while (scanner.hasNext())
            {
               line = scanner.next();


               if (scanner.equals("BGSTART")) //tag in the txt to locate position
               {
                   line = scanner.nextLine();
                   System.out.println(line);
                   lb1.append(line); //attaches to a JTextArea.
                   window2.add(lb1);//adds to JPanel
                }
            }

.txt file looks something like this:

BGSTART //content BGEND

Nothing is posted onto the panel when I run the program.

I am trying to read it between those two points.I don't have a lot of experience in reading from txt file. Any suggestions? Thank You.

Answer 1

Assuming that BGSTART and BGEND are on seperate lines, as per @SubOptimal's question, you would need to do this:

boolean tokenFound = false;
while (scanner.hasNextLine())
{
   line = scanner.nextLine();

   //line, not scanner. 
   if (line.equals("BGSTART")) //tag in the txt to locate position
   {
       tokenFound = true;
   }
   else if (line.equals("BGEND"))
   {
       tokenFound = false;
   }

   if(tokenFound)
   {                   
       System.out.println(line);
       lb1.append(line); //attaches to a JTextArea.
       window2.add(lb1);//adds to JPanel
   }
}

Some improvements:

try {
        Scanner scanner = new Scanner(new FileInputStream(file));

        //Moved the rest of the code within the try block.
        //As it was before, if there where any problems loading the file, you would have gotten an error message (File not found)
        //as per your catch block but you would then have gotten an unhandled null pointer exception when you would have tried to
        //execute this bit: scanner.hasNextLine()
        boolean tokenFound = false;
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine().trim();

            //line, not scanner.
            if (line.equals("BGSTART")) //tag in the txt to locate position
            {
                tokenFound = true;
            } else if (line.equals("BGEND")) {
                tokenFound = false;
            }

            if ((tokenFound) && (!line.equals("BGSTART"))) {
                System.out.println(line);
               //I am not sure what is happening here.
                //lb1.append(line); //attaches to a JTextArea.
                //window2.add(lb1);//adds to JPanel
            }
        }
    } catch (Exception e) {
        System.out.println("File not found.");
    }

File content:

do not show line one
do not show line two
BGSTART 
this is a line
this is another line
this is a third line
BGEND
do not show line three
do not show line four

Answer 2

Why don't you use substring? once you have located your BGSTART and BGEND you can capture the string between it somewhere in the lines of the below code:

 StringBuilder sb= new StringBuilder();
 while (scanner.hasNextLine()) {
        String line = scanner.nextLine();
        sb.append(line);
 }
String capturedString = sampleString.substring(sb.toString().indexOf("BGSTART"),
          sb.toString().indexOf("BGEND"));

hope this helps

Answer 3

Almost good, but you are doing:

 if (scanner.equals("BGSTART")) //tag in the txt to locate position

You should look for file content in line variable not scanner .
So try this:

 if (line.equals("BGSTART")) //tag in the txt to locate position