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Easy Java Question

  1. Sep 27, 2011 #1
    Hello, I am just starting to learn Java. I have a small file that needs reading in, line by line, with different arguments. Such a line looks like this:

    Code (Text):
    123 Midterm .25 100
    456 TakehomeExam .25 200
    0 Quiz03 .10 25
    1 Final .40 100
    I need to read these in as separate variables. I already have part of my code:

    Code (Text):
    import java.io.*;

    public class Program {
        public static void main(String args[]) {
            double id = 0.0;
            String label = "";
            double weight = 0.0;
            double graduate = 0.0;
            double undergrad = 0.0;
            int expectedArg = 5;
           
            try {
                // Open the file that is the first
                // command line parameter
                FileInputStream fstream = new FileInputStream("exams.dat");
                // Get the object of DataInputStream
                DataInputStream in = new DataInputStream(fstream);
                BufferedReader br = new BufferedReader(new InputStreamReader(in));
                String strLine;
                //Read File Line By Line
                while ((strLine = br.readLine()) != null)   {
                // Print the content on the console
                    //System.out.println (strLine);
                    //Try to put the args into the variables.
                    if (strLine == expectedArg) {
                    try {
                        id = Double.parseDouble(args[0]);
                        label = args[1];
                        weight = Double.parseDouble(args[2]);
                        graduate = Double.parseDouble(args[3]);
                        undergrad = Double.parseDouble(args[4]);
                    } catch (Exception e) {
                        System.out.println("Error: ");
                        e.printStackTrace();
                        System.exit(1);
                    }
                    }
                }
                //Close the input stream
                in.close();
            }catch (Exception e){//Catch exception if any
                System.err.println("Error: " + e.getMessage());
            }
        }
    }
     
    The "if (strLine == expectedArg)" part returns an error. I know it should be strLine.something but I'm not sure what. Any suggestions? Also, will this work in assigning the file arguments to variables?
     
  2. jcsd
  3. Sep 27, 2011 #2
    A couple of things to point out.

    Consider using FileReader instead of FileInputStream. FileReader is designed to work with text-based files. Also, you wrap the FileInputStream with a DataInputStream followed by a BufferedReader. Is there any particular reason why you are doing this? I don't think it is really necessary. No big deal, just more work than you really need to do.

    So you get your strLine from the BufferedReader via getLine. That contains your data, say:

    Code (Text):

    123 Midterm .25 100
     
    Now I'm assuming you want to parse the line and place the values in your id, label, weight, etc variables, right? The next thing you need to do is tokenize the String contained in strLine. This will turn it into an array of Strings. There's a convenience method on the String class called split. Check it out, it will tokenize the string for you.

    http://download.oracle.com/javase/6/docs/api/java/lang/String.html#split(java.lang.String)

    So basically you'd do something like:

    Code (Text):
    String parts[] = strLine.split(regex);
    Here's a hint. The regex will be the character that divides up the fields of the string.

    Regarding your question about the if conditional, you are wanting to check to make sure you have the right number of columns, right? Tokenizing the string will help you accomplish that. Hint: the length of an array such as 'parts' can be had by accessing the 'length' property (e.g. parts.length).

    Hope that helps. There are some other parts that need to be fixed, but hopefully this will get you on the right track.
     
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