问题是 SDK Platform 没有安装在我的电脑上，所以我打开了 Window> Android SDK Manager ，然后点击 SDK Platform 然后点击安装1包然后问题解决了！

I want to start programming for android so I downloaded Eclipse and installed ADT Plugin on it and downloaded the Android SDK Now when I want to create a new android project in "File->New->Android Application Project" it directs me to a page that wants "Application name" & "Project name" etc. so I filled the fields like this:

Application name: Hello

Project name: Hello

Package name: test.example.hello

Minimum required SDK: API8: Android 2.2(Froyo)

Target SDK: API16: Android 4.1(Jelly Bean)

Compile with:

theme: none

But the "Next" button is disabled! in the top of the page it says: " A package name cannot start or end with a dot" and there is a tiny red thing(cross) near the theme field so I cannot create a new project what should I do ?

The problem was that The SDK Platform was not installed on my computer, so I opened Window>Android SDK Manager and then clicked on SDK Platform and then clicked Install 1 package and then the problem was solved!