Log in/Sign up

package com.example.yourapp;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.*;

public class DatabaseHelper extends SQLiteOpenHelper {

    public static final String DATABASE_NAME = "UserDB.db";
    public static final String TABLE_NAME = "users";
    public static final String COL_1 = "ID";
    public static final String COL_2 = "USERNAME";
    public static final String COL_3 = "PASSWORD";

    public DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE " + TABLE_NAME + " (ID INTEGER PRIMARY KEY AUTOINCREMENT, USERNAME TEXT, PASSWORD TEXT)");
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
        onCreate(db);
    }

    public boolean insertUser(String username, String password) {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues contentValues = new ContentValues();
        contentValues.put(COL_2, username);
        contentValues.put(COL_3, password);
        long result = db.insert(TABLE_NAME, null, contentValues);
        return result != -1;
    }

    public boolean checkUser(String username, String password) {
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.rawQuery("SELECT * FROM " + TABLE_NAME + " WHERE USERNAME=? AND PASSWORD=?", new String[]{username, password});
        return cursor.getCount() > 0;
    }

    public boolean userExists(String username) {
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.rawQuery("SELECT * FROM " + TABLE_NAME + " WHERE USERNAME=?", new String[]{username});
        return cursor.getCount() > 0;
    }
}

package com.example.yourapp;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.*;
import androidx.appcompat.app.AppCompatActivity;

public class LoginActivity extends AppCompatActivity {

    EditText username, password;
    Button loginBtn, signupRedirectBtn;
    DatabaseHelper db;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        username = findViewById(R.id.etUsername);
        password = findViewById(R.id.etPassword);
        loginBtn = findViewById(R.id.btnLogin);
        signupRedirectBtn = findViewById(R.id.btnSignupRedirect);

        db = new DatabaseHelper(this);

        loginBtn.setOnClickListener(v -> {
            String user = username.getText().toString();
            String pass = password.getText().toString();

            if (db.checkUser(user, pass)) {
                Toast.makeText(LoginActivity.this, "Login Successful", Toast.LENGTH_SHORT).show();
                startActivity(new Intent(LoginActivity.this, MainActivity.class));
                finish();
            } else {
                Toast.makeText(LoginActivity.this, "Invalid credentials", Toast.LENGTH_SHORT).show();
            }
        });

        signupRedirectBtn.setOnClickListener(v -> startActivity(new Intent(LoginActivity.this, SignupActivity.class)));
    }
}

package com.example.yourapp;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.*;
import androidx.appcompat.app.AppCompatActivity;

public class SignupActivity extends AppCompatActivity {

    EditText username, password, confirmPassword;
    Button signupBtn;
    DatabaseHelper db;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_signup);

        username = findViewById(R.id.etUsernameSignup);
        password = findViewById(R.id.etPasswordSignup);
        confirmPassword = findViewById(R.id.etConfirmPassword);
        signupBtn = findViewById(R.id.btnSignup);

        db = new DatabaseHelper(this);

        signupBtn.setOnClickListener(v -> {
            String user = username.getText().toString();
            String pass = password.getText().toString();
            String cpass = confirmPassword.getText().toString();

            if (user.equals("") || pass.equals("") || cpass.equals("")) {
                Toast.makeText(SignupActivity.this, "All fields are required", Toast.LENGTH_SHORT).show();
            } else if (!pass.equals(cpass)) {
                Toast.makeText(SignupActivity.this, "Passwords do not match", Toast.LENGTH_SHORT).show();
            } else if (db.userExists(user)) {
                Toast.makeText(SignupActivity.this, "User already exists", Toast.LENGTH_SHORT).show();
            } else {
                if (db.insertUser(user, pass)) {
                    Toast.makeText(SignupActivity.this, "Signup Successful", Toast.LENGTH_SHORT).show();
                    startActivity(new Intent(SignupActivity.this, LoginActivity.class));
                    finish();
                } else {
                    Toast.makeText(SignupActivity.this, "Signup Failed", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }
}

if (db.checkUser(user, pass)) {
    Toast.makeText(LoginActivity.this, "Login Successful", Toast.LENGTH_SHORT).show();
    Intent intent = new Intent(LoginActivity.this, MainActivity.class);
    intent.putExtra("username", user); // optional
    startActivity(intent);
    finish(); // prevent back navigation to login
}

String username = getIntent().getStringExtra("username");
TextView welcome = findViewById(R.id.tvWelcome);
welcome.setText("Welcome, " + username + "!");

Button logoutBtn = findViewById(R.id.btnLogout);
logoutBtn.setOnClickListener(v -> {
    Intent intent = new Intent(MainActivity.this, LoginActivity.class);
    intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK); // clear back stack
    startActivity(intent);
});