Q:

Literal Assignment in Java

what's the difference in defining 
double example = 23.1d

or 
double example = 23.1

Why long, float and double can end with  l,  f,  d? 

A:

There is no difference between double example = 23.1d; and double example = 23.1; because a floating point literal without a type suffix is always interpreted as a double.
The type suffixes are necessary in order to avoid ambiguities in certain scenarios.
For example, java supports method overloading.  This means that you can have void x( float f ); and void x( double d );  Both methods are called x; which one will be selected depends on the type that you pass; if you pass a variable which is already known to be either float or double, things are clear; but if you want to pass a literal, like this: x( 5 ); then you have to be able to specify whether you mean this 5 to be a float or a double, so as to select the right method.
There are a few other very nuanced situations where the type of the literal matters.  For example, the following code:
System.out.println( "" + (2/3.3333) );
System.out.println( "" + (2/3.3333f) );

Yields the following output:
0.6000060000600006
0.600006

...because the first number is a double, while the second number is a float.
Similar disambiguation concerns make the "L" type suffix necessary for long integer literals.

