Compare line by line from a text file in Java

Question

I am trying to compare lines from a text file in Java. For example, there is a text file with these lines:

temp1 am 32.5 pm 33.5
temp2 am 33.5 pm 33.5
temp3 am 32.5 pm 33.5
temp4 am 31.5 pm 35

a b c d e

a is the name of the line, b is constant(am), c is a variable, d is constant(pm), e is another variable.

It will only compare the variables -> temp1(c) to temp2(c), temp1(e) to temp2(e) etc.

When there are two or more lines with the same c(s) and e(s), it will throw FormatException.

From the example text file above, because temp1's c is the same as temp3's c and temps1's e is the same as temp3's e, it will throw FormatException.

This is what I have so far:

public static Temp read(String file) throws FormatException {
        String line = "";
        FileReader fr = new FileReader(fileName);
        Scanner scanner = new Scanner(fr);

        while(scanner.hasNextLine()) {
            String line = scanner.nextLine();
            System.out.println(line);
        }
        scanner.close();

        if () {
            throw new FormatException("Error.");

How can I make this?

Answer 1

If I got your question right then you need to check line by line in order to find duplicates using c and e as criteria

this means, line n must be compared against all the other lines, if repeated then exception...

The suggestion will be:

Define a class that represent the element c and e of every line...

class LinePojo {

    private String c;
    private String e;
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((c == null) ? 0 : c.hashCode());
        result = prime * result + ((e == null) ? 0 : e.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        LinePojo other = (LinePojo) obj;
        if (c == null) {
            if (other.c != null)
                return false;
        } else if (!c.equals(other.c))
            return false;
        if (e == null) {
            if (other.e != null)
                return false;
        } else if (!e.equals(other.e))
            return false;
        return true;
    }

    @Override
    public String toString() {
        return "(c=" + c + ", e=" + e + ")";
    }

    public LinePojo(String c, String e) {
        this.c = c;
        this.e = e;
    }
}

then a list of that class where every line will be inserted and /or check if an element is there or not..

List<LinePojo> myList = new ArrayList<LinePojo>();

then iterate line by line

while(scanner.hasNextLine()) {
    String line = scanner.nextLine();
    String[] lineInfo = line.split(" ");
    LinePojo lp = new LinePojo(lineInfo[2], lineInfo[4]);
    if (myList.contains(lp)) {
        throw new IllegalArgumentException("there is a duplicate element");
    } else {
        myList.add(lp);
    }    
}

Answer 2

Here you got an example that basically includes:

• a collection in which store your lines
• simple pattern matching logic (see Java Regex Tutorial for more)
• a try-with-resource statement
• a recursive check method

First of all I would make a simple POJO representing a line info:

public class LineInfo {
    private String lineName;
    private String am;
    private String pm;

    public LineInfo(String lineName, String am, String pm) {
        this.lineName = lineName;
        this.am = am;
        this.pm = pm;
    }

    // getters and setters
}

Second I would need a pattern to validate each line and extract data from them:

//                                   group 1         group 2  group3           group 4  group 5
//                                        v               v    v                    v    v
private static final String LINE_REGEX = "(\\w+)\\s+am\\s+(\\d+(\\.\\d+)?)\\s+pm\\s+(\\d+(\\.\\d+)?)";
private static final Pattern LINE_PATTERN = Pattern.compile(LINE_REGEX);

Third I would rework the read method like this (I return void for simplicity):

public static void read(String fileName) throws FormatException {
    // collect your lines (or better the information your lines provide) in some data structure, like a List
    final List<LineInfo> lines = new ArrayList<>();

    // with this syntax your FileReader and Scanner will be closed automatically
    try (FileReader fr = new FileReader(fileName); Scanner scanner = new Scanner(fr)) {

        while (scanner.hasNextLine()) {
            final String line = scanner.nextLine();
            final Matcher matcher = LINE_PATTERN.matcher(line);

            if (matcher.find()) {
                lines.add(new LineInfo(matcher.group(1), matcher.group(2), matcher.group(4)));
            } else {
                throw new FormatException("Line \"" + line + "\" is not valid.");
            }
        }

        // recursive method
        compareLines(lines, 0);
    } catch (final IOException e) {
        e.printStackTrace();
        // or handle it in some way
    }
}

private static void compareLines(List<LineInfo> lines, int index) throws FormatException {
    // if there are no more lines return
    if (index == lines.size()) {
        return;
    }

    final LineInfo line = lines.get(index);
    for (int i = index + 1; i < lines.size(); i++) {
        final LineInfo other = lines.get(i);
        // do the check
        if (line.getAm().equals(other.getAm()) && line.getPm().equals(other.getPm())) {
            throw new FormatException(String.format("Lines #%d (%s) and #%d (%s) does not meet the requirements.",
                    index, line.getLineName(), i, other.getLineName()));
        }
    }

    // do the same thing with the next line
    compareLines(lines, index + 1);
}

Answer 3

You will need to split your lines to extract your variables and a Set to check for duplicates as next:

Set<String> ceValues = new HashSet<>();
while(scanner.hasNextLine()) {
    String line = scanner.nextLine();
    String[] values = line.split(" ");
    if (!ceValues.add(String.format("%s %s", values[2], values[4]))) {
        // The value has already been added so we throw an exception
        throw new FormatException("Error.");
    }
}

Answer 4

As I don't want to do your homework for you, let me get you started:

while(scanner.hasNextLine()) {
    String line = scanner.nextLine();
    String[] partials = line.split(" ");

    String a = partials[0]; 
    //...
    String e = partials[4];    
}

I'm splitting the line over a space as this is the only thing to split over in your case. This gives us 5 seperate strings (a through e). You will need to save them in a String[][] for later analysis but you should be able to figure out for yourself how to do this.

Try playing around with this and update your question if you're still stuck.