[This post is by Reto Meier AKA @retomeier, who wrote the book on Android App development. — Tim Bray]

As a developer, I’m excited by Android’s potential as a single development platform that can make my apps available on a wide range of devices. From smartphones to televisions, Android is now being used on an increasingly diverse collection of hardware.

Last year’s Android SDK 1.6 release was the first to introduce support for variations in device hardware, paving the way for devices like the HTC Tattoo — a small screen device with a non-autofocus camera. Future devices, like Google TV, may not include some of the hardware features that we now expect, such a accelerometers and telephony.

We all want our apps available on as many devices as possible, but on some hardware they might just not make sense, so it’s important that apps are available only on the devices where they do.

Android Market Rule #1: Don't let existing applications break on new devices

As curators of the Android Market, one of our most important responsibilities is ensuring consumers and developers can trust the Market to only deliver applications to devices capable of running them.

The Android SDK includes built-in support for specifying which hardware features your application needs, ensuring that when we see more hardware variations, the Market will make sure your apps are available everywhere (and only where) they make sense.

Specify the hardware your app needs using the application Manifest

That includes the target and minimum SDK versions, supported screen sizes, and the required hardware features without which your app will “break”. You can specify the hardware features your app requires by adding a uses-feature node to your manifest.

<uses-feature android:name="android.hardware.microphone" />

By updating your manifest now to include all the hardware features you require, you effectively opt out of future hardware that won’t be capable of properly supporting your app.

Android Market Rule #2: Don't let existing applications break on new devices

In extreme cases — such as the introduction of small screen sizes in Android 1.6 — developers will be required to explicitly opt in their apps before they will be made visible in the Market on these new devices.

In other cases the Android Market will analyze the permissions requested by an app to determine if it implies a dependence on any particular hardware. For example, requiring the CALL_PHONE permission strongly implies the need for telephony hardware.

Until we provide a more convenient tool, you can use AAPT in the SDK to analyze your apps (2.2 SDK required) and see which device requirements are being implicitly added to your application:

aapt dump badging myApp.apk

Where your app uses a particular hardware feature, but you know (and have tested) that it will still work without it, you can specify it as optional by setting the required attribute to false.

<uses-feature android:name="android.hardware.telephony" android:required="false" />

Ensure your application manifest correctly identifies what hardware your app needs, and what is optional

With the uses-feature name strings now available, you can ensure right now that your app appears in the Market, where appropriate, on current and future hardware devices rather than waiting for the devices to be released.

It's in your interest as a developer to ensure your apps work well, and are available, on as many devices as possible and appropriate. Now is the time to test your applications and update your Manifest to opt in to all hardware configurations which you support, and opt out of those that don’t make sense.