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How to prevent extraction of android apk source code from attackers?

I have an existing released Android app in google play. But some hacker extract source code from apk and use our services.I have already implement proguard + Base64 but even through hacker extract source code from apk. How can i prevent my apk from extraction?

Cant prevent extraction.
Can do proguard but a good attacker it wont stop, because he can
decompile your and reverse engineer it. Then he can listen to your
LogCat, Binder, sniff communication...
On the other hand u can do a lot of stuff to make your code better.
Use SSL learn how to use proguard better so it ill be harder for
them, dont write anything to log, encrypt data.

Android Parse SDK is it safe to use Application ID and Client Key within the app?

I am new to Parse SDK. Is it safe to use Application ID and Client Key within the app? as reverse engineering the APK file might reveal the keys. Is there any other workaround to pull them in the Parse.initialize() function.

Like everything else contained in your APK it is only safe if you obfuscate your code. Make sure that you are using ProGaurd when you build your application and have configured it for gradle if you use Android Studio.
If you use ProGuard to obfuscate your code then you wont have to worry about people being able to unpack your APK and retrieve your Parse credentials.
If you want to test that your ProGaurd configuration is correct you can try and unpack your own APK to make sure everything is obfuscated and hidden as expected. This question will show you the process.

This is something that I have always asked myself. In addition to this you could also use a random keygen to connect to random parse database if you were trying to hack data. I tried to find the SO.com post about this but was unable to do so, regardless, I saw someone post that they had their key strings stored in AWS so that they werent in the APK package. In my opinion you shouldn't have to do this but whatever... Parse is pretty sweet when you cut back on development time and their online data portal is nice as well.

Is there a possibility to upload an reverse engineered APK?

We are developing an Android application. We know that using tools like APKTool, dex2jar can get the source code of an APK.
1) Can they get complete source code so that they can rebuild the same APK, with very minimal effort?
2) After getting the source code, is there a possibility that others can upload the rebuilt APK under their own name?
3) If possible, how to prevent this?
Our clients are keen about this.

It is not possible to prevent your application from being reverse engineered. However, you can make it harder using tools loke proguard.
Yes, it is possible that others can upload the reverse engineered APK. Nevertheless, they need to change the package name.
It cannot be prevented.
Have a good look here for a more detailed explaination: How to secure my app against piracy

1) They can, but NOT WITH A MINIMAL effort.
2) Sure, they can.
3) You can't. You can just make their work a bit harder, trying to obfuscate your code as much as possible and crypting your dbs (which is the most important thing you and your users should worry about).

Not sure what you mean by APK? You can certainly extract and decompile an apk, repackage and resign. It would be the same but would have a different signature and so couldn't be installed over the an existing installation. I'd argue this can be done with minimal effort using Apktool there 100's of articles on how to do it. But even easier with APK2Java which turns converting apk to java into a point and click experience.
Yes, to other appstores with no code changes. If the attacker changes the package name which is simple there are automated scripts that can so this it could be uploaded to Playstore.
100% preventable?, no. But you can make it allot harder and raise the difficulty and effort level required. As others have mentioned Proguard is a good start, I recommend Dexguard it's not free but really adds to your apk hardening. You could also add tamper checks to break functionality or alert user.
I've talked about Android app hardening at Droidcon UK here's the slides they may help. The idea is to add several levels of security to raise the time/effort it takes so most attackers will just move on to another app.

Verify Android apk has not been repackaged?

Looking to improved the security of my Android app to flag if the .apk has been extracted, modified, repacked and resigned. Here's article from Zdnet noting the issue link1.
The concern is if the app is targeted by hackers they could add malicious code and upload to an alternate app store and dupe users in to downloading it.
So I'm thinking code to verify a checksum of the apk or signing certificate?
I appreciate the app code could be repacked and any security code removed, but it does increase the difficulty of repacking it, maybe enough for them to try another app.
[update]I know the Google Play store licensing module offers something similar but I'm looking for something for non paid apps and other/non marketplaces.

I ended up using Dexguard (paid obfuscator for Android). It offers a module that preforms apk verification. It is simple to implement and offers better than average protection.
Here's the code to do the check:
dexguard.util.TamperDetection.checkApk(context)
The main issue is where to store the checksum of the apk to verify against given that it could to be replaced. The dexguard way is to check it locally but using other features like class/string encryption and api hiding obscure this call.

Here are some of the articles that could help you out.
Retrieving APK signature during runtime.
Self checking an APK signature.
How to check APK signature.

Use the Google licensing service It will connect with the play store to make sure the user purchased the app every few days. (you can set the amount) Also loook at ProGuard. It strips all the class, method, and variable names out of your code making it really hard to understand once its decompiled.

How to protect android app from pirates [duplicate]

There's a good report here of a recent scam perpetrated through the Android market.
http://www.theregister.co.uk/2011/12/12/android_market_malware/
Someone copied some popular game .apk files off a rooted phone, decompiled them with smali, added code to make them use an expensive premium SMS service, re-signed them with his own key, and put them back in the Android market. Over 10K users downloaded them in the 24 hours before Google booted the apps from the market.
How can app developers protect against their work being stolen, modified, and remarketed in this way? I suppose you could match the MD5 hash of your app as it was when you shipped it (perhaps kept with your app in the google market, or on one of your servers), with the MD5 hash of the app as it is now. But we don't have tools for checking the MD5 hash of an .apk file, and the underlying files are not visible to app software.
Ensuring identity integrity of an app you run, surely, is a solved problem. What is the best practice?

This kind of black hat practice are impossible to avoid. Whatever you find, a lucky, ingenious, patient hacker will undo it.
You can nevertheless hope to work hard enough to make it harder for most hackers to easily break your protections. That's what people often call IT security btw :)
Fields to investigate are :
obfscation
CRCs and MD5 as you mentionned it
secure networked verification
a second app to check the first
a custom checking class loader (can you do that on android ?)

Not the be-all-to-end-all but there is ProGuard:
The ProGuard tool shrinks, optimizes, and obfuscates your code by
removing unused code and renaming classes, fields, and methods with
semantically obscure names. The result is a smaller sized .apk file
that is more difficult to reverse engineer. Because ProGuard makes
your application harder to reverse engineer.

If you are willing to spend some money then go for some professional help such as Arxan. We have used this in our enterprise level app. Basically it recompiles the .apk with its own encryption and creates a new apk which will enable you to know whether tampering was done with apk and other such instances. You may refer to this url : https://www.arxan.com/ for more details and do your research accordingly.