想要学习Android，需要先下载安装Android SDK，然后在编译软件中安装插件，如在Eclipse中安装ADT。
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Android官网为：www.android.com
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打开官网，拉到网页底部点击“
Developers”，进入developer.android.com。
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?在上方点击SDK标签。
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?在右侧表格里选择相应系统的SDK下载。
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（也可以直接点击本页的链接下载。）
PlatformPackageSizeMD5 Checksum Windows android-sdk_r15-windows.zip 33895447 bytes cc2aadf7120d12b574981461736a96e9 installer_r15-windows.exe?(Recommended) 33902520 bytes ee8481cb86a6646a4d963d5142902c5c Mac OS X (intel) android-sdk_r15-macosx.zip 30469921 bytes 03d2cdd3565771e8c7a438f1c40cc8a5 Linux (i386) android-sdk_r15-linux.tgz 26124434 bytes f529681fd1eda11c6e1e1d44b42c1432
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下载安装完成后，在安装目录下找到“SDK Manager.exe”，双击打开：
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选择你需要的项目安装。对于这些项目，官网上有相关说明：EnvironmentSDK?ComponentComments Basic SDK Tools If you've just installed the SDK starter package, then you already have the latest version of this component. The SDK Tools component is required to develop an Android application. Make sure you keep this up to date. SDK Platform-tools This includes more tools that are required for application development. These tools are platform-dependent and typically update only when a new SDK platform is made available, in order to support new features in the platform. These tools are always backward compatible with older platforms, but you must be sure that you have the latest version of these tools when you install a new SDK platform. SDK platform You need to download?at least one platform?into your environment, so that you will be able to compile your application and set up an Android Virtual Device (AVD) to run it on (in the emulator). To start with, just download the latest version of the platform. Later, if you plan to publish your application, you will want to download other platforms as well, so that you can test your application on the full range of Android platform versions that your application supports. + ? Recommended
(plus Basic)Documentation The Documentation component is useful because it lets you work offline and also look up API reference information from inside Eclipse. Samples The Samples components give you source code that you can use to learn about Android, load as a project and run, or reuse in your own app. Note that multiple samples components are available — one for each Android platform version. When you are choosing a samples component to download, select the one whose API Level matches the API Level of the Android platform that you plan to use. Usb Driver The Usb Driver component is needed only if you are developing on Windows and have an Android-powered device on which you want to install your application for debugging and testing. For Mac OS X and Linux platforms, no special driver is needed. + ? Full
(plus Recommended)Google APIs The Google APIs add-on gives your application access to the Maps external library, which makes it easy to display and manipulate Maps data in your application. Additional SDK Platforms If you plan to publish your application, you will want to download additional platforms corresponding to the Android platform versions on which you want the application to run. The recommended approach is to compile your application against the lowest version you want to support, but test it against higher versions that you intend the application to run on. You can test your applications on different platforms by running in an Android Virtual Device (AVD) on the Android emulator.
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简单地来说，Tools下的项目全部都要安装；Tools下的项目，个人建议全部安装；剩下的是各版本的Android，1和2打头的是手机的Android版本，3打头的是平板的Android版本，4.0则是同时兼容手机和平板，不过不太成熟，不建议日常开发之用，不过体验下还是不错的。
各版本里，SDK Platform是Android官方的平台，Documentation是Android的帮助文档，Samples是一些样品的源代码，Google APIs是google公司定制的平台，其他项目类似。
个人建议安装Tools、Tools、4.0中的Documentation和SDK Platform、3.2中的SDK Platform、2.3.3中的SDK Platform和Google APIs。
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好了，只要你把这些安装完成，那么你的Android SDK就安装完成啦~~