Android Flashlight

A minimal Android flashlight app built as a hands-on exercise in Android development, camera2 API usage, runtime permission handling, and APK decompilation.

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What it does

A single-screen app with a toggle button that switches the rear camera torch on and off using the camera2 API. On first use, it requests the CAMERA permission at runtime and retries the toggle once granted.

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Implementation notes

Torch control is handled via CameraManager.setTorchMode(). The app iterates the camera ID list, checks LENS_FACING_BACK to identify the rear camera, and calls setTorchMode(cameraId, isChecked). This avoids opening a full camera session just to drive the flash, which is the correct approach on API 23+.

Runtime permissions - the CAMERA permission is declared in AndroidManifest.xml and requested via ActivityCompat.requestPermissions() on first button press. The result is handled in onRequestPermissionsResult, which resets the toggle state on denial and re-fires the click on grant.

UI is a ConstraintLayout with a custom-styled ToggleButton (pill shape, themed orange) and two TextView headers. All strings are externalised to strings.xml; colours to colors.xml. The background is a density-bucketed drawable covering ldpi through xxxhdpi.

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Building

Open the project in Android Studio (Electric Eel or later) and run on a physical device - the emulator has no torch hardware.

Minimum SDK: 24 (Android 7.0)
Target SDK: 29 (Android 10)

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APK decompilation

The repo includes a pre-built debug APK and the toolchain used to inspect it, as an exercise in understanding how Android packages the compiled output.

Step 1 - convert the DEX bytecode to a JAR

chmod +x apk_decompiler/dex2jar/*.sh
apk_decompiler/dex2jar/d2j-dex2jar.sh apk_release/flashligh-app-debug.apk -o flashligh-app-debug-dex2jar.jar

Step 2 - open the JAR in the Java decompiler

java -jar apk_decompiler/jd-gui-1.6.6.jar flashligh-app-debug-dex2jar.jar

Expand com.example.flashlight in the left panel to browse the decompiled MainActivity source.

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Project structure

app/src/main/
├── java/com/example/flashlight/
│   └── MainActivity.java       - torch toggle + permission handling
├── res/
│   ├── layout/activity_main.xml
│   ├── drawable/button.xml     - pill-shaped button background
│   ├── values/colors.xml
│   └── values/strings.xml
└── AndroidManifest.xml         - CAMERA permission + hardware feature declarations
apk_release/
│   ├── flashligh-app-debug.apk
│   └── flashlight_demo.gif
apk_decompiler/
    ├── dex2jar/                - DEX-to-JAR conversion toolchain
    └── jd-gui-1.6.6.jar       - Java decompiler GUI