It has been a month since I studied Java. I have simply built a student information management system with my knowledge. Because I haven't started to learn database yet, I use ArrayList set to store student information data and HashSet to store user information.

Note: Student id must be int data. It is added from 1, otherwise, an error will be reported when deleting. This is a small bug, which needs to be improved.

The code is as follows:

Student.java

package com.briup.login;

public class Student {
	private int id;
	private String name;
	private int age;
	private double source;

	public Student() {
		super();
	}

	public Student(int id, String name, int age, double source) {
		super();
		this.id = id;
		this.name = name;
		this.age = age;
		this.source = source;
	}

	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	public double getSource() {
		return source;
	}

	public void setSource(double source) {
		this.source = source;
	}

	@Override
	public String toString() {
		return "Student [id=" + id + ", name=" + name + ", age=" + age + ", source=" + source + "]";
	}

}

Customer.java

package com.briup.login;

public class Customer {
	private String username;
	private String password;

	public Customer() {
		super();
	}

	public Customer(String username, String password) {
		super();
		this.username = username;
		this.password = password;
	}

	public String getUsername() {
		return username;
	}

	public void setUsername(String username) {
		this.username = username;
	}

	public String getPassword() {
		return password;
	}

	public void setPassword(String password) {
		this.password = password;
	}

	@Override
	public String toString() {
		return "Customer [username=" + username + ", password=" + password + "]";
	}

}

Login.java

package com.briup.login;

import java.util.HashSet;
import java.util.Scanner;
import com.briup.login2.Customer;

public class Login {
	public static HashSet<Customer> set = new HashSet<>();;

	public Login() {
		super();
	}

	// Judge whether the login is successful
	public static void login(String username, String password) {
		if (set.size() == 0) {
			System.out.println("*Login failed, maybe not registered, please try again!*");
			return;
		}
		for (Customer cust : set) {
			if (cust.getUsername().equals(username)) {
				if (cust.getPassword().equals(password)) {
					System.out.println("*Login succeeded!*");
					StudentMessages sm = new StudentMessages();
					sm.index();
				} else {
					System.out.println("*Login failed! Wrong password!*");
					break;
				}
			} else {
				System.out.println("*Login failed, user name does not exist!*");
				break;
			}
		}
	}

	// Judge whether the registration is successful
	public static boolean register(String username, String password) {
		if (set.size() == 0) {
			System.out.println("*Registration succeeded!!*");
		}
		for (Customer cust : set) {
			if (cust.getUsername().equals(username)) {
				System.out.println("*User name already exists!!*");
				return false;
			} else {
				System.out.println("*Registration succeeded!!*");
				return true;
			}
		}
		return true;
	}

	// Registration main interface
	public void loginindex() {
		while (true) {
			System.out.println("******Main interface******");
			System.out.println("*     1.Sign in          *");
			System.out.println("*     2.register          *");
			System.out.println("*     0.Sign out          *");
			System.out.println("*Please choose:");
			@SuppressWarnings("resource")
			int num = new Scanner(System.in).nextInt();
			while (true) {
				if (num == 1) {
					System.out.println("*enter one user name:");
					@SuppressWarnings("resource")
					String name = new Scanner(System.in).nextLine();
					System.out.println("*Please input a password:");
					@SuppressWarnings("resource")
					String password = new Scanner(System.in).nextLine();
					login(name, password);
					break;
				} else if (num == 2) {
					System.out.println("*enter one user name:");
					@SuppressWarnings("resource")
					String name = new Scanner(System.in).nextLine();
					System.out.println("Please input a password:");
					@SuppressWarnings("resource")
					String password = new Scanner(System.in).nextLine();
					if (register(name, password) == false) {
						break;
					}
					Customer cust = new Customer(name, password);
					set.add(cust);
					break;
				} else if (num == 0) {
					System.out.println("***System exited***");
					System.exit(0);
					break;
				} else {
					System.out.println("The format entered is incorrect, please try again!!");
				}
			}
		}
	}
}

StudentMessages.java

package com.briup.login;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.Scanner;
import com.briup.login2.Student;

public class StudentMessages {
	public static ArrayList<Student> list = new ArrayList<>();

	public StudentMessages() {
		super();
	}

	// Query all student information
	public void findAll() {
		if (list.size() == 0) {
			System.out.println("*No student information!!*");
			return;
		} else {
			Iterator<Student> iter = list.iterator();
			while (iter.hasNext()) {
				System.out.println(iter.next());
			}
		}
	}

	// Add students to judge whether the student number exists
	public boolean add(int id) {
		if (list.size() == 0) {
			return true;
		}
		for (Student stu : list) {
			if (stu.getId() == id) {
				System.out.println("*Student ID already exists*");
				return false;
			}
		}
		return true;
	}

	// Modify student information to determine whether student number exists
	public boolean update(int id, String name, int age, double source) {
		if (list.size() == 0) {
			return false;
		}
		for (Student stu : list) {
			if (stu.getId() == id) {
				stu.setName(name);
				stu.setAge(age);
				stu.setSource(source);
				list.set(id - 1, stu);
				return true;
			}
		}
		if (true) {
			System.out.println("There is no information about the student in the system!!");
		}
		return false;
	}

	// Delete student information with specified id
	public boolean delete(int id) {
		if (list.size() == 0) {
			return false;
		}
		for (Student stu : list) {
			if (stu.getId() == id) {
				list.remove(id - 1);
				System.out.println("Delete successful");
				return true;
			}
		}
		System.out.println("This student number does not exist!!");
		return true;
	}

	// Design the main interface of student address book management system
	public void index() {

		StudentMessages sm = new StudentMessages();
		s: while (true) {
			System.out.println("*********Student information management system*********");
			System.out.println("*        1.Query student information                   *");
			System.out.println("*        2.Add student information                   *");
			System.out.println("*        3.Modify student information                   *");
			System.out.println("*        4.Delete student information                   *");
			System.out.println("*        0.Exit management system                   *");
			System.out.println("*Please input:");
			Scanner scanner = new Scanner(System.in);
			int num = scanner.nextInt();
			s2: while (true) {
				if (num == 1) {
					sm.findAll();
					break;
				} else if (num == 2) {
					System.out.println("*Please input the student's id: ");
					@SuppressWarnings("resource")
					int id = new Scanner(System.in).nextInt();
					if (sm.add(id) == false) {
						break s2;
					}
					System.out.println("*Please enter the student's name:");
					@SuppressWarnings("resource")
					String name = new Scanner(System.in).nextLine();
					System.out.println("*Please enter the age of the student:");
					@SuppressWarnings("resource")
					int age = new Scanner(System.in).nextInt();
					System.out.println("*Please enter the student's score:");
					@SuppressWarnings("resource")
					double source = new Scanner(System.in).nextDouble();
					Student stu = new Student(id, name, age, source);
					list.add(stu);
					System.out.println("Added successfully!!");
					break;
				} else if (num == 3) {
					System.out.println("*Please enter the student's id: ");
					@SuppressWarnings("resource")
					int id = new Scanner(System.in).nextInt();
					System.out.println("*Please enter the student's name:");
					@SuppressWarnings("resource")
					String name = new Scanner(System.in).nextLine();
					System.out.println("*Please enter the age of the student:");
					@SuppressWarnings("resource")
					int age = new Scanner(System.in).nextInt();
					System.out.println("*Please enter the student's score:");
					@SuppressWarnings("resource")
					double source = new Scanner(System.in).nextDouble();
					if (sm.update(id, name, age, source) == false) {
						break s2;
					}
					System.out.println("Modification succeeded!!");
					break;
				} else if (num == 4) {
					System.out.println("*Please input the student's id: ");
					@SuppressWarnings("resource")
					int id = new Scanner(System.in).nextInt();
					if (sm.delete(id) == false) {
						break s2;
					}
					break;
				} else if (num == 0) {
					System.out.println("****System exited!****");
					System.exit(0);
					break s;
				} else {
					System.out.println("Input error, please input again!!");
				}
			}
		}
	}
}

Logintest.java (test)

package com.briup.login;

public class loginTest {
	public static void main(String[] args) {
		Login l = new Login();
		l.loginindex();
	}
}

Operation result: