untuk membuatnya pertama-tama masukan despendency ini di module.gradle 

compile 'com.google.android.gms:play-services:10.0.1'

<?xml version="1.0" encoding="utf-8"?>

<RelativeLayout 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"

    tools:context="com.giviews.qrcodescanner.MainActivity">



    <Button

        android:id="@+id/scanbtn"

        android:layout_marginTop="40dp"

        android:layout_width="match_parent"

        android:layout_height="wrap_content"

        android:text="scan"

        android:layout_centerHorizontal="true" />



    <TextView

        android:id="@+id/result"

        android:layout_width="match_parent"

        android:layout_height="wrap_content"

        android:layout_below="@+id/scanbtn"

        android:text="RESULT"

        android:textSize="25dp"

        android:layout_marginTop="20dp"

        android:gravity="center"/>



</RelativeLayout>

package com.giviews.qrcodescanner;



import android.Manifest;

import android.content.Intent;

import android.content.pm.PackageManager;

import android.support.v4.app.ActivityCompat;

import android.support.v4.content.ContextCompat;

import android.support.v7.app.AppCompatActivity;

import android.os.Bundle;

import android.view.View;

import android.widget.Button;

import android.widget.TextView;



import com.google.android.gms.vision.barcode.Barcode;



public class MainActivity extends AppCompatActivity {

    private Button scanbtn;

    private TextView result;

    public static final int REQUEST_CODE = 100;

    public static final int PERMISSION_REQUEST = 200;



    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        if (ContextCompat.checkSelfPermission(this, Manifest.permission.CAMERA) != PackageManager.PERMISSION_GRANTED) {

            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.CAMERA}, PERMISSION_REQUEST);

        }

        scanbtn = (Button) findViewById(R.id.scanbtn);

        result = (TextView) findViewById(R.id.result);

        scanbtn.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View view) {

                Intent intent = new Intent(getApplicationContext(), ScanActivity.class);

                startActivityForResult(intent, REQUEST_CODE);

            }

        });

    }



    @Override

    protected void onActivityResult(int requestCode, int resultCode, Intent data) {

        if (requestCode == REQUEST_CODE && resultCode == RESULT_OK) {

            if (data != null) {

                final Barcode barcode = data.getParcelableExtra("barcode");

                result.post(new Runnable() {

                    @Override

                    public void run() {

                        result.setText(barcode.displayValue);

                    }

                });

            }

        }

    }

}

<?xml version="1.0" encoding="utf-8"?>

<RelativeLayout 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"

    tools:context="com.giviews.qrcodescanner.ScanActivity">



    <SurfaceView

        android:id="@+id/cameraView"

        android:layout_width="match_parent"

        android:layout_height="match_parent" />



</RelativeLayout>

package com.giviews.qrcodescanner;



import android.*;

import android.Manifest;

import android.content.Intent;

import android.content.pm.PackageManager;

import android.support.v4.content.ContextCompat;

import android.support.v7.app.AppCompatActivity;

import android.os.Bundle;

import android.util.SparseArray;

import android.view.SurfaceHolder;

import android.view.SurfaceView;

import android.widget.Toast;



import com.google.android.gms.vision.CameraSource;

import com.google.android.gms.vision.Detector;

import com.google.android.gms.vision.barcode.Barcode;

import com.google.android.gms.vision.barcode.BarcodeDetector;



import java.io.IOException;



import static com.giviews.qrcodescanner.MainActivity.REQUEST_CODE;



public class ScanActivity extends AppCompatActivity {

    SurfaceView cameraView;

    BarcodeDetector barcode;

    CameraSource cameraSource;

    SurfaceHolder holder;



    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_scan);

        cameraView = (SurfaceView) findViewById(R.id.cameraView);

        cameraView.setZOrderMediaOverlay(true);

        holder = cameraView.getHolder();

        barcode = new BarcodeDetector.Builder(this)

                .setBarcodeFormats(Barcode.QR_CODE)

                .build();

        if (!barcode.isOperational()) {

            Toast.makeText(getApplicationContext(), "Sorry couldnt setup the detector", Toast.LENGTH_LONG).show();

            this.finish();

        }



        cameraSource = new CameraSource.Builder(this, barcode)

                .setFacing(CameraSource.CAMERA_FACING_BACK)

                .setRequestedFps(24)

                .setAutoFocusEnabled(true)

                .setRequestedPreviewSize(1920,1024)

                .build();

        cameraView.getHolder().addCallback(new SurfaceHolder.Callback() {

            @Override

            public void surfaceCreated(SurfaceHolder surfaceHolder) {

                try {

                    if (ContextCompat.checkSelfPermission(ScanActivity.this, Manifest.permission.CAMERA) == PackageManager.PERMISSION_GRANTED) {

                        cameraSource.start(cameraView.getHolder());

                    }

                } catch (IOException e) {

                    e.printStackTrace();

                }

            }



            @Override

            public void surfaceChanged(SurfaceHolder surfaceHolder, int i, int i1, int i2) {



            }



            @Override

            public void surfaceDestroyed(SurfaceHolder surfaceHolder) {



            }

        });

        barcode.setProcessor(new Detector.Processor<Barcode>() {

            @Override

            public void release() {



            }



            @Override

            public void receiveDetections(Detector.Detections<Barcode> detections) {

                final SparseArray<Barcode> barcode = detections.getDetectedItems();

                if (barcode.size() > 0) {

                    Intent intent = new Intent();

                    intent.putExtra("barcode", barcode.valueAt(0));

                    setResult(RESULT_OK, intent);

                    finish();

                }

            }

        });

    }

}

<uses-permission android:name="android.permission.CAMERA" />

<meta-data

    android:name="com.google.android.gms.vision.DEPENDENCIES"

    android:value="barcode" />

jika scan berhasil

package com.giviews.giviewsmessenger;



import android.app.ProgressDialog;

import android.content.Intent;

import android.support.annotation.NonNull;

import android.support.v7.app.AppCompatActivity;

import android.os.Bundle;

import android.support.v7.widget.Toolbar;

import android.text.TextUtils;

import android.util.Log;

import android.view.View;

import android.widget.Button;

import android.widget.EditText;

import android.widget.Toast;



import com.google.android.gms.tasks.OnCompleteListener;

import com.google.android.gms.tasks.Task;

import com.google.firebase.auth.AuthResult;

import com.google.firebase.auth.FirebaseAuth;

import com.google.firebase.auth.FirebaseAuthInvalidCredentialsException;

import com.google.firebase.auth.FirebaseAuthUserCollisionException;

import com.google.firebase.auth.FirebaseAuthWeakPasswordException;

import com.google.firebase.auth.FirebaseUser;

import com.google.firebase.database.DatabaseReference;

import com.google.firebase.database.FirebaseDatabase;

import com.google.firebase.iid.FirebaseInstanceId;



import java.util.HashMap;



public class RegisterActivity extends AppCompatActivity {



    private EditText mDisplayName;

    private EditText mEmail;

    private EditText mPassword;

    private Button mCreateBtn;

    private Toolbar mToolbar;

    private ProgressDialog mProgress;



    private FirebaseAuth mAuth;

    private DatabaseReference mDatabase;



    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_register);



        mAuth = FirebaseAuth.getInstance();

        mDatabase = FirebaseDatabase.getInstance().getReference().child("Users");

        mToolbar = (Toolbar) findViewById(R.id.main_page_toolbar);

        setSupportActionBar(mToolbar);

        getSupportActionBar().setTitle("Register");

        getSupportActionBar().setDisplayHomeAsUpEnabled(true);



        mDisplayName = (EditText) findViewById(R.id.reg_name);

        mEmail = (EditText) findViewById(R.id.reg_email);

        mPassword = (EditText) findViewById(R.id.reg_password);

        mCreateBtn = (Button) findViewById(R.id.reg_create_btn);

        mProgress = new ProgressDialog(this);



        mCreateBtn.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View view) {

                String displayname = mDisplayName.getText().toString().trim();

                String email = mEmail.getText().toString().trim();

                String password = mPassword.getText().toString().trim();



                if (!TextUtils.isEmpty(displayname) && !TextUtils.isEmpty(email) && !TextUtils.isEmpty(password)) {

                    mProgress.setTitle("Registering User");

                    mProgress.setMessage("Please wait while we create your account...");

                    mProgress.setCanceledOnTouchOutside(false);

                    mProgress.show();



                    register_user(displayname, email, password);

                }

            }

        });

    }



    private void register_user(final String displayname, String email, String password) {

        mAuth.createUserWithEmailAndPassword(email, password)

                .addOnCompleteListener(this, new OnCompleteListener() {

                    @Override

                    public void onComplete(@NonNull Task task) {



                        // If sign in fails, display a message to the user. If sign in succeeds

                        // the auth state listener will be notified and logic to handle the

                        // signed in user can be handled in the listener.

                        if (!task.isSuccessful()) {

                            mProgress.hide();



                            String error = "";

                            try {

                                throw task.getException();

                            } catch (FirebaseAuthWeakPasswordException e) {

                                error = "Weak Password";

                            } catch (FirebaseAuthInvalidCredentialsException e) {

                                error = "Invalid Email";

                            } catch (FirebaseAuthUserCollisionException e) {

                                error = "Existing account";

                            } catch (Exception e) {

                                error = "Unknown Error!";

                                e.printStackTrace();

                            }

                            Toast.makeText(getApplicationContext(), error, Toast.LENGTH_SHORT).show();

                        }else {

                            FirebaseUser currentUser = mAuth.getCurrentUser();

                            String uid = currentUser.getUid();



                            mDatabase = FirebaseDatabase.getInstance().getReference().child("Users").child(uid);

                            String device_token = FirebaseInstanceId.getInstance().getToken();



                            HashMap, String> userMap = new HashMap, String>();

                            userMap.put("name", displayname);

                            userMap.put("status", "Hi there, I'm using giviews");

                            userMap.put("image", "default");

                            userMap.put("thumb_image", "default");

                            userMap.put("device_token", device_token);



                            mDatabase.setValue(userMap).addOnCompleteListener(new OnCompleteListener() {

                                @Override

                                public void onComplete(@NonNull Task task) {

                                    mProgress.dismiss();

                                    Intent mainIntent = new Intent(RegisterActivity.this, MainActivity.class);

                                    mainIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);

                                    startActivity(mainIntent);

                                }

                            });



                        }

                    }

                });

    }

}