Run the application in Landscape mode only?

I want to run a Hello World sample application only in landscape mode on my Device.

If the device changed to portrait, I would then like to raise one toaster. For example "please change to landscape mode to run your application".

How should I do this?

You can go for both programmatically as follows:

setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);

or also in manifest file you can give here as

<activity android:name=".YourActivity"  android:screenOrientation="landscape" android:configChanges="keyboardHidden|orientation"/>

add this to your activity tag in your manifest file:

android:screenOrientation="landscape"

It will fix your orientation to landscape mode and you need not to show any toast to user.

in your Manifestfile(xml)

<activity android:name=".ActivityName" android:screenOrientation="landscape">
    </activity>

refer this also Is that possible to check was onCreate called because of orientation change?

You may try something like this

Display display = getWindowManager().getDefaultDisplay(); 
int width = display.getWidth();
int height = display.getHeight();

Log.v("log_tag", "display width is "+ width);
Log.v("log_tag", "display height is "+ height);

if(width<height){
    Toast.makeText(getApplicationContext(),"Device is in portrait mode",Toast.LENGTH_LONG ).show();
} else {
    Toast.makeText(getApplicationContext(),"Device is in landscape  mode",Toast.LENGTH_LONG ).show();
}

You can use the configChanges-attribute for the Activity you want to catch the screen-orientation change for.

After that, the onConfigurationChanged()-method in your Activity will be called when the orientation changes.

To check which orientation is currently displayed, check this older post: Check orientation on Android phone

After that, you can show off your message or do whatever you like (of course you can also set it to only show in landscape).