How to change the game code on Android: an expert guide

Modifying mobile games is an exciting process that opens up access to hidden functions, changes the balance, or adds new content to your favorite projects. Users often look for ways how to change the code of a game on Androidto remove ads, get endless resources, or unlock paid levels. However, this path requires not only enthusiasm, but also a deep understanding of the architecture of Android applications.

At the heart of any game is a file .apk, which is a zipped container with code, resources and a manifest. Direct editing of this file is not possible using standard methods. You will need specialized decompilation tools to convert the binary code into a readable format. Without this knowledge, any attempts to interfere with the structure of the application are doomed to failure or will lead to a crash of the app upon startup.

The process of changing the logic of an application is called reverse engineering. It involves reverse engineering: you take the finished product apart to understand how it was created and make your own changes. This is a legal method of software research for personal use, as long as you do not violate license agreements or distribute cracked versions for commercial purposes. In this article we will analyze the technical side of the issue in detail.

Preparing the working environment and choosing tools

Before you start modding, you need to prepare a reliable software base. Working with the code on the device is possible, but it is extremely inconvenient due to the limitations of the screen and the performance of the mobile processor. A professional approach involves using a personal computer running Windows, macOS or Linux.

The key tool in the modder's arsenal is APKTool. This utility allows you to decompile an APK file by extracting resources and converting the file classes.dex (Dalvik bytecode) into a readable format smali. Smali is an assembly-like language that intermediates between Java code and the machine code of the Android virtual machine. Understanding smali syntax is critical to making changes.

In addition to APKTool, you will need a code editor with syntax highlighting. A regular notepad will not work, as it will not help track down errors in the file structure. Experienced specialists often use Notepad++, VS Code or specialized IDEs such as Android Studioif a deep reworking of the project is planned. Also, don't forget to install Java Development Kit (JDK)as most modding tools are written in Java and require a runtime environment.

To work with resources (pictures, sounds, XML interface markup), additional utilities are often used, such as NP Manager or built-in archiver tools. It is important to understand that different versions of Android and different types of encryption in games may require updating the toolkit. Old versions of APKTool may not correctly process applications compiled for new versions of the SDK.

Decompilation process and analysis of the project structure

The first step in changing the code is decompilation. You place the target APK file in your tools folder and run the extract command. The result is a directory containing many subfolders and files. The structure of a typical project looks like this:

• 📂 res/ — a folder with application resources (images, interface layouts, localization strings).
• 📂 smali/ (or smali_classes2, smali_classes3) — the source code of the game in smali format is stored here.
• 📄 AndroidManifest.xml — a manifest file describing permissions, components and version of the application.
• 📄 apktool.yml — a configuration file of APKTool itself, containing metadata about the SDK version and package names.

The file AndroidManifest.xml after decompilation becomes a readable XML document. Here you can find information about application entry points, activity names, and required permissions. Changing the manifest is often used to add new permissions or change the name of the package in order to install a modified version of the game next to the original.

The most difficult part of the work is concentrated in the folder smali. The code here is divided into many files corresponding to the classes of the Java source code. Navigating these files requires an understanding of object-oriented programming. You should be able to find methods, class fields, and control flow statements. Finding the desired piece of code is often done through string search, since text error messages or button names are easier to find than abstract variables.

When analyzing the structure, pay attention to the files .dex. In modern heavy games, the code can be divided into several dex files (multidex). APKTool automatically distributes them into folders smali, smali_classes2 and so on. The game logic may be scattered between these folders, making it difficult to find the necessary functions. Sometimes developers use obfuscation, renaming classes into meaningless character sets like a.b.c, which makes analysis extremely labor-intensive.

Editing Smali code and game logic

The Smali language may seem intimidating to a beginner due to its brevity and lack of familiar high-level constructs. However, once you understand the basic instructions, you will be able to control the logic of the game. The code consists of methods that perform a sequence of actions: loading data, mathematical operations, calling other methods, and returning values.

Consider a typical task: changing the amount of currency in the game. You need to find a method that is responsible for accruing or debiting resources. Often such methods are called addCurrency, updateScore or contain keywords buy, sell. Inside the method, you will see registers and operations instructions.

.method public addCoins(I)V
.registers 3
.param p1, "amount"    # I

iget-object v0, p0, Lcom/game/model/User;->coins:Ljava/lang/Integer;
invoke-virtual {v0, p1}, Ljava/lang/Integer;->intValue()I
move-result v1
add-int/2addr v1, p1
invoke-static {v1}, Ljava/lang/Integer;->valueOf(I)Ljava/lang/Integer;
move-result-object v1
iput-object v1, p0, Lcom/game/model/User;->coins:Ljava/lang/Integer;
return-void
.end method

In this example, the code takes the current coin value, adds the passed argument to it, and stores the result. To make a currency infinite, modders often replace the addition logic with assigning a maximum value, or simply force the method to return success without actually debiting. To do this, an instruction is used const/high16 or changing the transition conditions if-eq, if-nez.

What are registers in Smali?

Registers (v0, v1, p0, p1) are temporary memory cells inside the method. 'v' denotes local variables and 'p' denotes parameters passed to the method. Errors in working with registers (for example, using an uninitialized register) lead to an immediate application crash (Force Close).

Changing check conditions is another popular modding method. If the game checks to see if the player has enough money to make a purchase, it uses the comparison instruction. By replacing the condition or inverting it (for example, replacing if-lt by if-ge), you can fool the check. However, modern games often duplicate checks on the server, so local code changes may not work in online projects.

⚠️ Attention: Never change the length of the call stack or the number of registers without recalculating the method header (.registers). A discrepancy between declared registers and those actually used will lead to an error in the Android verifier, and the application simply will not start.

Modification of resources and interface

Not all changes concern logic. Often users need to Russify the game, remove logos or replace textures. Resources are stored in a folder res and compiled into binary format resources.arsc. APKTool decompiles them back into readable XML, which allows you to edit strings and values.

To change the interface text, you need to find the files in the directory res/values/. The file strings.xml contains all text constants of the application. You can find the required string by its value or identifier. After editing the text, the game will automatically pick up the new value during the next build.

Resource type	Location	What can be changed
Text lines	res/values/strings.xml	Button names, messages, dialogues
Images	res/drawable-*	Icons, backgrounds, character sprites
UI layout	res/layout/*.xml	Arrangement of elements on the screen
Colors	res/values/colors.xml	Color interface diagram

Replacing graphics requires compliance with formats and sizes. If you are replacing an image in a folder drawable-hdpi, the new file must be in a compatible resolution and format (usually PNG). Violation of proportions can lead to interface distortion or a crash when trying to render a resource. Complex texture replacements in 3D games may require editing assets inside files assetswhich often requires separate utilities for specific engines (Unity, Unreal Engine).

Sometimes you need to change the configuration files in the folder assets. Databases, Lua scripts or JSON files with game balance are stored here. If the game does not encrypt these files, you can open them with any text editor and change the parameters of damage, speed or item cost. This is much simpler than editing smali code, and often gives the same result in offline games.

Building, signing and installing a modified APK

After making all the necessary edits, the assembly stage begins. You must compile the modified files back into the APK container. To do this, use the build command in APKTool. The process checks the syntax of smali code and packages resources. If there are errors in the code, the assembly will be interrupted, indicating the line number and type of error.

The assembled file cannot be immediately installed on the phone. Android requires all apps to be digitally signed with a developer certificate. The original signature is lost during decompilation, so you need to create your own. To do this, use a tool jarsigner (included with the JDK) or utilities like Uber Apk Signer.

jarsigner -verbose -keystore mykey.keystore -signedjar game_mod.apk game_unsigned.apk alias_name

This command will create a new file game_mod.apksigned with your key. Only after this the file is ready for installation. Before installing, be sure to remove the original version of the game from the device, since the signatures do not match, and the system will block the update from above.

⚠️ Attention: If you plan to test multiple versions of the mod, use the command adb install -r to replace the application without losing data, but remember that this will only work if you did not change the package name in the manifest.

In some cases, especially when working with applications that use Integrity Check, a simple rebuild may not help. When the game starts, it checks the checksums of files or signatures. To bypass such protections, you need to patch the verification code, which is a task of increased complexity and often requires the use of a debugger.

Debugging, error detection and security

It is rare that a mod works perfectly the first time. The most common problem is "Force Close" at startup or on a specific screen. To find reasons it is used Logcat —Android system log. By connecting your phone to the PC and running adb logcat, you will see a stream of messages in real time.

In the logs you need to look for lines with the tag AndroidRuntime or FATAL EXCEPTION. It will indicate the type of error (for example, NullPointerException or VerifyError) and a call stack pointing to the specific class and method where the failure occurred. This information is invaluable for correcting errors in smali code.

• 🐞 VerifyError — means that you have violated the bytecode rules (incorrect use of registers, data types).
• 💥 NullPointerException — an attempt to access an object that is null (often happens with incorrect initialization).
• 🔒 SecurityException —an access violation or check signatures.

Security when modding also plays an important role. By downloading ready-made mods from unknown authors, you risk infecting your device with a Trojan. Changing the code yourself ensures that there is no malware built into the game unless you add it yourself. However, download modding tools only from official repositories such as GitHub.

Remember that online games with server authorization are almost impossible to hack by changing the client code. Currency and progress data is stored on a remote server, and any local substitutions will be rejected during synchronization. In such cases, modding is limited only to cosmetic changes to the interface.

Is it possible to change the game code directly on your phone without a computer?

Technically, this is possible using applications like MT Manager or NP Manager. They allow you to decompile, edit smali and resources, and then build and sign the APK directly on the device. However, the smartphone screen is inconvenient for working with a large amount of code, and performance can be low when processing large games.

What should you do if the game crashes immediately after installing the mod?

Most likely, there is an error in the smali syntax or a violation of the integrity of resources. Connect your phone to your PC, launch adb logcat and play the crash. Study the last lines of the log before departure - the reason will be indicated there. Often it helps to roll back the last changes or check the number of registers in the changed methods.

Is it safe to change the code of games for online mode?

No, it is not safe. Anti-cheat systems for online games easily detect a modified client (by signature, file hash or behavior). Using mods in online games is almost guaranteed to lead to account blocking (ban). Modding is intended primarily for offline projects.

Do you need to know the Java language to edit Smali?

Deep knowledge of Java greatly simplifies the process, since Smali is essentially an assembler for the Java virtual machine. Understanding how classes, methods, and loops work in Java helps you quickly recognize the corresponding constructs in Smali. Without basic programming knowledge, modding will be extremely difficult.

Is it possible to restore the original game after modding?

Yes, if you saved the original APK file. Just uninstall the modified version and install the original. If you did not make a backup, you will have to download the game again from Google Play or another source. Modding does not make changes to the system partitions of the phone, so deleting the application completely clears traces of interference.