#AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.gokisoft.c1812l">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity"></activity>
        <activity android:name=".LoginActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".RegisterActivity"></activity>
    </application>

</manifest>

#RegisterActivity.java

package com.gokisoft.c1812l;

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class RegisterActivity extends AppCompatActivity {
    EditText txtFullname, txtUsername, txtPassword, txtConfirmPwd;
    Button btnBack, btnSave;

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

        txtFullname = findViewById(R.id.ar_txt_fullname);
        txtUsername = findViewById(R.id.ar_txt_username);
        txtPassword = findViewById(R.id.ar_txt_password);
        txtConfirmPwd = findViewById(R.id.ar_txt_confirm_password);
        btnBack = findViewById(R.id.ar_btn_back);
        btnSave = findViewById(R.id.ar_btn_save);

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

        btnSave.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String fullname = txtFullname.getText().toString();
                String username = txtUsername.getText().toString();
                String pwd = txtPassword.getText().toString();
                String confirmPwd = txtConfirmPwd.getText().toString();

                if(username.isEmpty() || !pwd.equals(confirmPwd)) {
                    Toast.makeText(RegisterActivity.this, "Check inputs again!!!", Toast.LENGTH_SHORT).show();
                    return;
                }

                Intent intent = new Intent();
                intent.putExtra("fullname", fullname);
                intent.putExtra("username", username);
                intent.putExtra("pwd", pwd);
                intent.putExtra("confirmPwd", confirmPwd);

                setResult(LoginActivity.REQUEST_CODE_REGISTER, intent);
                finish();
            }
        });
    }
}

#MainActivity.java

package com.gokisoft.c1812l;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    TextView txtHello;
    Button btnTest;

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

        txtHello = findViewById(R.id.txt_hello);
        btnTest = findViewById(R.id.button_test);

        txtHello.setText("OKOK");

        btnTest.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Toast.makeText(MainActivity.this, "TEST",
                        Toast.LENGTH_SHORT).show();
            }
        });
    }
}

#LoginActivity.java

package com.gokisoft.c1812l;

import android.content.Intent;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class LoginActivity extends AppCompatActivity {
    public static final int REQUEST_CODE_REGISTER = 1;

    EditText txtUsername, txtPassword;
    Button btnRegister, btnLogin;

    String username = "", password = "", fullname = "";

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

        //mapping fields
        txtUsername = findViewById(R.id.al_txt_username);
        txtPassword = findViewById(R.id.al_txt_password);
        btnRegister = findViewById(R.id.al_btn_register);
        btnLogin = findViewById(R.id.al_btn_login);

        //dang ky su kien khi click regsiter & login
        btnRegister.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //lam the de start activity tu activity nay
                Intent intent = new Intent(LoginActivity.this, RegisterActivity.class);
                startActivityForResult(intent, REQUEST_CODE_REGISTER);
            }
        });

        btnLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
//                Toast.makeText(LoginActivity.this, "Check login", Toast.LENGTH_SHORT).show();
                String uname = txtUsername.getText().toString();
                String pwd = txtPassword.getText().toString();
                if(uname.equals(username) && pwd.equals(password)) {
                    Toast.makeText(LoginActivity.this, "Welcome " + fullname, Toast.LENGTH_SHORT).show();
                } else {
                    Toast.makeText(LoginActivity.this, "Login failed!!!", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        switch (requestCode) {
            case REQUEST_CODE_REGISTER:
                //du lieu duoc tra ve tu RegisterActivity
                if(data != null) {
                    fullname = data.getStringExtra("fullname");
                    username = data.getStringExtra("username");
                    password = data.getStringExtra("pwd");
                }
                break;
        }
    }
}