Java - If-statement always returns true

Question

The if-statement in this code always returns true, no matter what I do. Even if I give no arguments to the program, I get a nullPointerException , indicating that the if-statement returned true. Can you help me?

import java.io.*;
import java.util.*;

public class Oblig4 {
    static Menu menu = new Menu();
public static void main(String[] args) {
    if(args[0].matches("^[a-zA-Z0-9]{1,40}\\\\.txt$") && args[1].matches("^[a-zA-Z0-9]{1,40}\\\\.txt$")){
        menu.menu(args[0], args[1]);
    }
    else{
        System.out.println("You have not given the correct arguments.");
    }
  }

}

Answer 1

Even if I give no arguments to the program, I get a nullPointerException, indicating that the if-statement returned true

This does not indicate that it will be true if it throws a NPE. First you should check for null for the args[0] and args[1] before going for that if statement that contains matches

Answer 2

You could avoid problems with parsing arguments using JSAP library. See how simple it is:

import java.io.File;
import java.util.regex.Pattern;

import com.martiansoftware.jsap.FlaggedOption;
import com.martiansoftware.jsap.JSAP;
import com.martiansoftware.jsap.JSAPException;
import com.martiansoftware.jsap.JSAPResult;
import com.martiansoftware.jsap.stringparsers.FileStringParser;

public class Oblig4 {

    private Pattern fileNamesPattern = Pattern
            .compile("^[a-zA-Z0-9]{1,40}\\.txt$");
    private JSAP jsap = new JSAP();

    public static void main(String[] args) throws JSAPException {
        Oblig4 oblig4 = new Oblig4();
        oblig4.init();
        JSAPResult config = oblig4.jsap.parse(args);
        if (!config.success()) {
            System.err.println();
            System.err.println("Usage: ");
            System.err.println("java -jar jar_file_name.jar "
                    + oblig4.jsap.getUsage());
            System.err.println(oblig4.jsap.getHelp());
            System.err.println();
            return;
        }
        File firstFile = config.getFile("firstFile");
        File secondFile = config.getFile("secondFile");
        if (!oblig4.fileNamesIsValid(firstFile, secondFile)) {
            System.out.println("File names is not valid. See pattern "
                    + oblig4.fileNamesPattern.pattern());
        }
        Menu menu = new Menu();
        menu.doSomething(firstFile, secondFile);
    }

    private void init() throws JSAPException {
        FileStringParser fileStringParser = FileStringParser.getParser();
        fileStringParser.setMustBeDirectory(false);
        fileStringParser.setMustBeFile(true);
        fileStringParser.setMustExist(false);
        FlaggedOption firstInputFile = new FlaggedOption("firstFile")
                .setStringParser(fileStringParser).setRequired(true)
                .setShortFlag('f');
        firstInputFile.setHelp("First text file");
        jsap.registerParameter(firstInputFile);

        FlaggedOption secondInputFile = new FlaggedOption("secondFile")
                .setStringParser(fileStringParser).setRequired(true)
                .setShortFlag('s');
        secondInputFile.setHelp("Second text file");
        jsap.registerParameter(secondInputFile);
    }

    private boolean fileNamesIsValid(File... files) {
        for (File file : files) {
            if (!fileNamesPattern.matcher(file.getName()).find()) {
                return false;
            }
        }
        return true;
    }
}

class Menu {
    public void doSomething(File firstFile, File secondFile) {

    }
}

If no arguments or arguments is not valid user will see:

Usage: 
java -jar jar_file_name.jar -f <firstFile> -s <secondFile>
  -f <firstFile>
        First text file

  -s <secondFile>
        Second text file