什么是异常错误，我该如何解决？

Question

I am extremely new to Java and need your help. I was given the class MD5Digest by my instructor, and when I run the file by itself, it compiles without an issue. But when I copy and paste the class over to my file, it doesn't work. Can someone please help me? I read that it has something to do with a try catch loop, but I am not sure how to set one up for this particular class, being that it was given to me and I don't know what half of it means. Thanks in advance!

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
import java.security.MessageDigest;

public class Login{
    public static void main(String []args){
        Scanner user_input = new Scanner(System.in);

        String entered_username = Get_Credentials.get_username(user_input);
        String entered_password = Get_Credentials.get_password(user_input);
        MD5Digest.encrypt(entered_password);
        user_input.close();
        User[] all_users = File_Scanner.create_users();
    }
}

class Get_Credentials{
    public static String get_username(Scanner user_input){
        System.out.println("Username: ");
        return user_input.next();
    }

    public static String get_password(Scanner user_input){
        System.out.println("Password: ");
        return user_input.next();
    }

}

class File_Scanner{
    public static User[] create_users(){
        User users[] = new User[6];
        int index_counter = 0;

        try {
            File credentials_file = new File("credentials.txt");
            String pattern = "[^\"\\s]+|\"(\\\\.|[^\\\\\"])*\"";
            Scanner file_reader = new Scanner(credentials_file);

            while (file_reader.hasNextLine()) {
                users[index_counter] = new User();
                users[index_counter].username = file_reader.findInLine(pattern);
                users[index_counter].encrypted_password = file_reader.findInLine(pattern);
                users[index_counter].password = file_reader.findInLine(pattern);
                users[index_counter].role = file_reader.findInLine(pattern);
                file_reader.nextLine();
                index_counter++;
            }

            file_reader.close();
        }
        catch (Exception e) {
          System.out.println(e.getClass());
        }
        return users;
    }
}

class User {
    String username;
    String password;
    String encrypted_password;
    String role;
}

class MD5Digest {
    public static void encrypt(String original) throws Exception {
        MessageDigest md = MessageDigest.getInstance("MD5");
        md.update(original.getBytes());
        byte[] digest = md.digest();
        StringBuffer sb = new StringBuffer();
        for (byte b : digest) {
            sb.append(String.format("%02x", b & 0xff));
        }

        System.out.println("original:" + original);
        System.out.println("digested:" + sb.toString());
    }

}

error thrown:

Login.java:12: error: unreported exception Exception; must be caught or declared to be thrown
        MD5Digest.encrypt(entered_password);
                         ^
1 error

Answer 1

What IDE are you using? You should be able to easily solve this error without needing any outside help. On IntelliJ pressing Alt + Enter will give you the option to enclose the encrypt method call in a try/catch block, like such:

    try {
        MD5Digest.encrypt(entered_password);
    } catch (Exception e) {
        e.printStackTrace();
    }

The method throws an exception, which basically means it can run into an error and the try catch block is a way to handle that error without your program crashing.

The encrypt method is located at the bottom, inside the MD5Digest class. Looking at the first line:

public static void encrypt(String original) throws Exception

It's telling the compiler that it can possibly run into an Exception (error), and that you should be prepared to handle that possibility. Hence the try/catch is required. It will try what's in the try brackets, and then if it runs into an error the code in the catch block will get executed.

Answer 2

Change your main method to this: public static void main(String []args) throws Exception { .
Since you're calling the encrypt method though the main method, you also need to add the throws Exception to the main method as well. Hope this helps!

Answer 3

Basically you just need to give a sign to your method about the possible exceptions, called checked exceptions in this case. So you use MessageDigest abstract class and the method that you access getInstance has a signature as throws NoSuchAlgorithmException . As @Dymas said, IDE can be helpful to see it more clearly.

So as encrypt method uses getInstance method, you should let encript know about this checked exception. Also since main calls encript , which brings the exception from the getInstance , you should also let the main know about this. Which leads you to put:

public static void main(String []args) throws NoSuchAlgorithmException
public static void encrypt(String original) throws NoSuchAlgorithmException

This page also explains it quite well, it might be helpful for further read:

https://www.geeksforgeeks.org/checked-vs-unchecked-exceptions-in-java/

You can use signature of throws NoSuchAlgorithmException or as @K.Kretz says throws Exception , this is about the exception hierarchy which you could also have a look here with a picture:

https://itblackbelt.wordpress.com/2015/02/17/checked-vs-unchecked-exception-in-java-example/