Show message based on
You determine if it is in the first interval, and then all other intervals depends on the upper limit. So you can make it even shorter:
String greeting = null;
if(hours>=1 && hours<=11){
greeting = "Good Morning";
} else if(hours<=15){
greeting = "Good Afternoon";
} else if(hours<=20){
greeting = "Good Evening";
} else if(hours<=24){
greeting = "Good Night";
}
Toast.makeText(this, greeting, Toast.LENGTH_SHORT).show();