In Eclipse, when you hover your mouse over a method, a window would appear with a description of what the method does, what the parameters mean and what it returns. Is there a way to get Android Studio to do the same thing?
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The answer is
Yes, you can. Go to File -> Settings -> Editor -> Show quick documentation on mouse move
Or, in Mac OS X, go to Android Studio - > Preferences -> Editor - > General > Show quick documentation on mouse move.
If you need only Parameter info then:
On Mac, it's assigned to Command+P
On Windows, it's assigned to Ctrl+P
If you need document info then:
On Mac, it's assigned to Command+Q
On Windows, it's assigned to Ctrl+Q
Android Studio 2.x.x
Moved under Editor -> General
Older Versions
Using windows 7 and Android Studio 1.0.x it took me a while to figure out the steps provided in the answer.
To help further visitors save some time, here is the way I did it:
Go to File -> Settings or press CTRL+ALT+S.
The following window will open and check Show quick doc on mouse move under IDE Settings -> Editor.
Or just press CTRL and hover your move over your method, class ...
I'm using Visual Studio too much and I want to see params when I click on Ctrl+Space that's why I'm using Visual Studio keys.
To change keymap to VS keymap:
Go to Preference->Editor->General
and choose show quick doc on mouse hover 
Macbook: ?J or fnF1 does the same.
Also, use the one from the editor definition as explained above.
On Mac the default key combination for showing the quick documentation is: CTRL + F1
The cursor has to be on the method when using this.
In addition to the answers here you might want to make sure that the documentation is downloaded. To do that go to SDK Manager
and then the SDK Tools tab and make sure Documentation for Android SDK is installed.
If it isn't check the box and click Apply.
On my Mac, CtrlF1 stopped working, but F1 on its own works.
Android Studio 1.2.2 has moved the setting into the General subfolder of Editor settings.
Many of the other answers are all well but if you want an informational tooltip instead of a fullblown window then do this: after enabling it using @Ahmad's answer then click on the little pin on the upper right corner:
After this the method information will appear on a tooltip like almost every other mainstream IDE.
Update Android Studio 4.1 & 4.2, this checkbox has moved under Editor/General/Code Completion 
If you just need a shortcut, then it is Ctrl + Q on Linux (and Windows). Just hover the mouse on the method and press Ctrl + Q to see the doc.
It comes very handy if you create a keymap for functionalities you use very frequently. By default (if you select OSX 10.5+ keymap) :
- Ctrl+P : To see what parameters are expected by the function
- Command+J : To see the documentation
- Ctrl+Space : To see the autocomplete suggestions
Source: Stackoverflow.com