First of all, you will need some gradle dependencies. So in your build.gradle file, add the following dependencies.

dependencies {
    compile 'com.android.support:design:25.3.1'
    compile 'com.google.android.gms:play-services-vision:10.2.1'
}

Then, you need to have the following five classes in your project. I am adding the classes here. Please import the missing classes if necessary.

1. CameraSource.java
2. CameraSourcePreview.java
3. BarcodeCaptureActivity.java
4. BarcodeGraphicTracker.java
5. BarcodeTrackerFactory.java

Now BarcodeCaptureActivity has a layout which you need to put in your layout folder as well.

Here's the barcode_capture.xml layout that you need.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/topLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:keepScreenOn="true"
    android:orientation="vertical">

    <!-- Do not forget to replace with your package name where the class is located -->
    <com.example.yourpackage.camera.CameraSourcePreview
        android:id="@+id/preview"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

You will need some strings and an activity entry in manifest for the BarcodeCaptureActivity. Here are the strings that you need to put in your strings.xml.

<!-- QR Code related strings -->
<string name="permission_camera_rationale">Access to the camera is needed for detection</string>
<string name="no_camera_permission">This application cannot run because it does not have the camera permission.  The application will now exit.</string>
<string name="low_storage_error">Face detector dependencies cannot be downloaded due to low device storage</string>
<string name="ok">OK</string>

And the AndroidManifest.xml should have a new entry for the BarcodeCaptureActivity like the following.

<activity
    android:name=".util.scanner.BarcodeCaptureActivity"
    android:theme="@style/Theme.AppCompat" />

Now your setup is done and you are ready to open your camera for scanning a barcode or QR code. Just call the following initiateScan function where necessary.

public static final int RC_BARCODE_CAPTURE = 9001;

public void initiateScan() {
    Intent intent = new Intent(YourActivity.this, BarcodeCaptureActivity.class);
    startActivityForResult(intent, RC_BARCODE_CAPTURE);
}

Please note that, you need to ask for camera permission to the user before you call initiateScan function. On granting the permission of the camera you will call the initiateScan function.

The initiateScan function will open the scanner and then after a successful scan, it will return to the calling Activity or Fragment. So you need to have a onActivityResult function in the calling Activity or Fragment.

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    if (resultCode == CommonStatusCodes.SUCCESS && requestCode == RC_BARCODE_CAPTURE) {
        if (data == null) return;
        Barcode barcode = data.getParcelableExtra(BarcodeCaptureActivity.BarcodeObject);
        final String scanResult = barcode.displayValue;
        if (scanResult == null) return;

        doSomethingWithTheScanResult(scanResult);
    }
}

Do not forget to add the CAMERA permission in your AndroidManifest.xml file as well.

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

Hope that helps for easier integration of QR code scanner using Google Vision API.