import java.io.BufferedWriter;
import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.file.OpenOption;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Scanner;

//Replacing class
public class WriteAndUpdate {

    void replace(String nameOfFile, String current, String update, String FileToUpdate) throws IOException {
        Path newPath = Paths.get(nameOfFile);
        Path updatePath = Paths.get(FileToUpdate);
        Charset characterSet = Charset.forName("UTF-8");
        BufferedWriter write = Files.newBufferedWriter(updatePath, characterSet);
        Scanner scan = new Scanner(newPath, characterSet.name());
        String passwordLine;
        while(scan.hasNextLine()) {
            passwordLine = scan.nextLine();
            passwordLine = passwordLine.replace(current, update);
            write.write(passwordLine);
        }
        scan.close();
        write.close();

    }
}

private JPasswordField currPass;
private JPasswordField newPass;
static String npass;

String passd = currPass.getText();
npass = newPass.getText();

JOptionPane.showMessageDialog(contentPane, "Password updated!", "Password Accepted", JOptionPane.PLAIN_MESSAGE);
                try {
                    updateFile.replace("Password.txt", passd, npass, "Password.txt");

                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

/**
 * If the file already exists and it is opened for {@link #WRITE}
 * access, then its length is truncated to 0. This option is ignored
 * if the file is opened only for {@link #READ} access.
 */

public static void main(String[] args) throws Exception {
    Path newPath = Paths.get("/Users/nfn8y/Documents/Code/abc.txt");
    Charset characterSet = Charset.forName("UTF-8");
    BufferedWriter write = Files.newBufferedWriter(newPath, characterSet,
                                StandardOpenOption.TRUNCATE_EXISTING);
    String passwordLine;
        write.write("Overwritten from Java");
    write.close();
}