Error: Selection does not contain a main type
Right click on the folder where you put your main class then click on Build Path --> Use as Source Folder.
Finally run your main file as java application. Hope this problem will be solved.
The other answers are all valid, however, if you are still having a problem you might not have your class inside the src folder in which case Eclipse may not see it as part of the project. This would also invoke the same error message you have seen.