Introduction to Java syntax

Java - TP1
Author

Ludovic Deneuville

Objectives

In this first tutorial, we’re simply going to familiarize ourselves with Java syntax, without using object-oriented programming.

This is why methods are defined as static. They are class methods. You don’t need to instantiate an object to call them.

We’re going to do a few exercises on the subject of passwords.

1 Introduction

In this tutorial, we will use the INSEE SSPCloud Datalab.

1.1 Git setup

Caution

On the datalab, your services have a limited lifespan.

To save your programs, the best practice is to use a git repository. We will therefore create and use a token to communicate with GitHub.

To follow the process, you need to have a GitHub account. It is also possible to follow a similar process with GitLab.

Generate a GitHub Token

Already done?

If you have already generated and declared a GitHub token, there’s no need to repeat these 2 steps.

    • Fill in:
      • token name: Datalab GENES
      • expiration date ➡️ Custom ➡️ 1 year
    • ✅ Check the repo box
    • Click on Generate token
    • Copy the token starting with ghp_ and keep it safe for a few minutes
Warning
  • This token will only be visible once
  • If you lose it or if it expires, you must generate a new one

Declare Your Token

GitHub has provided you with a token. Now, you need to declare it on the Datalab:

    • Git username
    • email (the one used for your GitHub account)
Git Config

You can now exchange code between the Datalab services and your GitHub repositories. 🎉

1.2 Launch Vscode Java

A service is available with Visual Studio Code and Java.

Important

You won’t find this service in the basic catalog. You have to use the link above to launch it.

It uses a specific Docker image

    • enter : /home/onyxia/work/ENSAI-2A-Java-TP/ ➡️ OK
Shorcuts
  • SHIFT + ALT + A: Comment / Uncomment selected lines
  • SHIFT + ALT + O: Auto import

2 Exercices

    • Terminal > New Terminal (or CTRL + ù)

Compile

  • a single Java file: javac MyClass.java
  • all Java files: javac *.java
  • specify output directory for .class files (recommended): javac -d bin MyClass.java

Run

Important

To run a class, it must have a main() method.

  • java MyClass
  • when .class files are in the bin/ directory: java -cp bin MyClass
Static Methods

As mentioned at the beginning, the aim of this tutorial is not OOP, but simply basic Java syntax.

So all the methods defined here are static ➡️ They are class methods ➡️ They can be called without instantiating an object.

2.1 Brute force

You will write a Java method that attempts to brute-force a hashed 6-digit numeric password.

  • The method will iterate through all numbers from 000000 to 999999
  • For each number, Generate and compare hash with the given target hash
  • If a match is found, return the original number else return null
Tip
  • Use method hashPassword()
  • String.format("%06d", 123)➡️ “000123”

2.2 Strong password

You will write a Java method to check if a password is strong. A password is strong if:

  • ✅ It has at least 12 characters
  • ✅ It contains at least one uppercase letter
  • ✅ It contains at least one lowercase letter
  • ✅ It contains at least one digit
  • ❌ It does not contain whitespace
Tip

Usefull methods:

  • Character.isUpperCase(<char>)
  • Character.isDigit(<char>)
  • Character.isWhitespace(<char>)

2.3 Check password list

You will write a method that takes a list of passwords (ArrayList) and checks their strength.

The method should return a HashMap where:

  • The keys are the passwords
  • The values are true if the password is strong, false otherwise

2.4 Generate a Random Password

Objective: Implement a method to generate a secure random password that includes at least:

  • ✅ 1 uppercase letter
  • ✅ 1 lowercase letter
  • ✅ 1 digit
  • ✅ 1 special character
  1. Ensure length is at least 4
  2. Create a list of characters
  3. Add at least one character from each group
  4. Fill the remaining characters randomly from all groups
  5. Shuffle the password to mix character types Collections.shuffle(<myList>)
  6. Convert the list into a String and return it
Generate random int between 0 and 20 
SecureRandom random = new SecureRandom();
System.out.println(random.nextInt(21));

2.5 Login System

Complete file Login.java:

Bonus:

  • After entering username, give 3 chances to input the correct password
  • If all 3 attempts fail, restart username input
Use a Scanner 
Scanner scanner = new Scanner(System.in);

System.out.print("Enter username: ");
String username = scanner.nextLine();

scanner.close();