activity_home.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    tools:context=".HomeActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="מסך הבית"
        android:textSize="30sp" />

</LinearLayout>

activity_sign_in.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical"
    android:padding="20sp"
    tools:context=".SignInActivity">

    <EditText
        android:id="@+id/email_et"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="10sp"
        android:gravity="right"
        android:hint="אימייל:"
        android:inputType="textEmailAddress" />

    <EditText
        android:id="@+id/password_et"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="10sp"
        android:gravity="right"
        android:hint=":סיסמה"
        android:inputType="textPassword" />

    <Button
        android:id="@+id/sign_in_btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="התחברות" />

</LinearLayout>

activity_sign_up.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical"
    android:padding="20sp"
    tools:context=".SignUpActivity">

    <EditText
        android:id="@+id/email_et"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="10sp"
        android:gravity="right"
        android:hint="אימייל:"
        android:inputType="textEmailAddress" />

    <EditText
        android:id="@+id/password_et"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="10sp"
        android:gravity="right"
        android:hint=":סיסמה"
        android:inputType="textPassword" />

    <EditText
        android:id="@+id/confirm_password_et"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="10sp"
        android:gravity="right"
        android:hint=":סיסמה שוב"
        android:inputType="textPassword" />

    <Button
        android:id="@+id/sign_up_btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="הרשמה" />

</LinearLayout>

DBHelper.java

package com.codix.app;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

import androidx.annotation.Nullable;

public class DBHelper extends SQLiteOpenHelper {

    public static final String USERS_DATABASE = "users.db";

    public static final String LOGIN_TABLE = "login";

    public static final String COL_ID = "id";
    public static final String COL_EMAIL = "email";
    public static final String COL_PASSWORD = "password";

    public DBHelper(@Nullable Context context) {
        super(context, USERS_DATABASE, null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
        sqLiteDatabase.execSQL("CREATE TABLE " + LOGIN_TABLE + " (id INTEGER PRIMARY KEY AUTOINCREMENT, email TEXT, password TEXT)");
    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {
        sqLiteDatabase.execSQL("DROP TABLE IF EXISTS " + LOGIN_TABLE);
        onCreate(sqLiteDatabase);
    }

    public boolean signUp(String email, String password) {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues cv = new ContentValues();
        cv.put("email", email);
        cv.put("password", password);
        long rowId = db.insert(LOGIN_TABLE, null, cv);
        db.close();
        return rowId != -1;
    }

    public boolean signIn(String email, String password) {
        SQLiteDatabase db = this.getReadableDatabase();
        String[] columns = {COL_ID};
        String selection = COL_EMAIL + "=?" + " and " + COL_PASSWORD + "=?";
        String[] selectionArgs = {email, password};
        Cursor cursor = db.query(LOGIN_TABLE, columns, selection, selectionArgs, null, null, null);
        int count = cursor.getCount();
        db.close();
        cursor.close();
        return count >= 1;
    }
}

SignInActivity.java

package com.codix.app;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.util.Patterns;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class SignInActivity extends AppCompatActivity {

    EditText emailEt, passwordEt;
    Button signInBtn;

    DBHelper db;

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

        emailEt = findViewById(R.id.email_et);
        passwordEt = findViewById(R.id.password_et);
        signInBtn = findViewById(R.id.sign_in_btn);

        db = new DBHelper(this);

        signInBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                signIn();
            }
        });
    }

    public void signIn() {
        String email = emailEt.getText().toString();
        String password = passwordEt.getText().toString();

        if (isValidEmail(email)) {

            if (password.length() >= 4) {

                boolean isSuccessful = db.signIn(email, password);
                if (isSuccessful)
                    startActivity(new Intent(getApplicationContext(), HomeActivity.class));
                else
                    Toast.makeText(getApplicationContext(), "הפרטים לא נכונים", Toast.LENGTH_SHORT).show();

            } else
                Toast.makeText(getApplicationContext(), "הסיסמה קצרה מדי", Toast.LENGTH_SHORT).show();
        } else
            Toast.makeText(getApplicationContext(), "האימייל לא תקין", Toast.LENGTH_SHORT).show();
    }

    public boolean isValidEmail(String email) {
        return Patterns.EMAIL_ADDRESS.matcher(email).matches();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        menu.add("הרשמה");
        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected(@NonNull MenuItem item) {

        if (item.getTitle().equals("הרשמה"))
            startActivity(new Intent(getApplicationContext(), SignUpActivity.class));

        return super.onOptionsItemSelected(item);
    }
}

SignUpActivity.java

package com.codix.app;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.util.Patterns;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class SignUpActivity extends AppCompatActivity {

    EditText emailEt, passwordEt, confirmPasswordEt;
    Button signUpBtn;

    DBHelper db;

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

        emailEt = findViewById(R.id.email_et);
        passwordEt = findViewById(R.id.password_et);
        confirmPasswordEt = findViewById(R.id.confirm_password_et);
        signUpBtn = findViewById(R.id.sign_up_btn);

        db = new DBHelper(this);

        signUpBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                signUp();
            }
        });
    }

    public void signUp() {
        String email = emailEt.getText().toString();
        String password = passwordEt.getText().toString();
        String confirmPassword = confirmPasswordEt.getText().toString();

        if (isValidEmail(email)) {

            if (password.length() >= 4) {

                if (password.equals(confirmPassword)) {

                    boolean isSuccessful = db.signUp(email, password);
                    if (isSuccessful) {
                        startActivity(new Intent(getApplicationContext(), HomeActivity.class));
                        Toast.makeText(getApplicationContext(), "נרשמת בהצלחה", Toast.LENGTH_SHORT).show();
                    } else
                        Toast.makeText(getApplicationContext(), "שגיאה בהרשמה", Toast.LENGTH_SHORT).show();

                } else
                    Toast.makeText(getApplicationContext(), "הסיסמאות לא תואמות", Toast.LENGTH_SHORT).show();
            } else
                Toast.makeText(getApplicationContext(), "הסיסמה קצרה מדי", Toast.LENGTH_SHORT).show();
        } else
            Toast.makeText(getApplicationContext(), "האימייל לא תקין", Toast.LENGTH_SHORT).show();
    }

    public boolean isValidEmail(String email) {
        return Patterns.EMAIL_ADDRESS.matcher(email).matches();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        menu.add("התחברות");
        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected(@NonNull MenuItem item) {

        if (item.getTitle().equals("התחברות"))
            startActivity(new Intent(getApplicationContext(), SignInActivity.class));

        return super.onOptionsItemSelected(item);
    }
}