From the Java Tutorials:
Floating-Point Literals
A floating-point literal is of type float if it ends with the letter F or f; otherwise its type is double and it can optionally end with the letter D or d.
The floating point types (float and double) can also be expressed using E or e (for scientific notation), F or f (32-bit float literal) and D or d (64-bit double literal; this is the default and by convention is omitted).
double d1 = 123.4;
// same value as d1, but in scientific notation
double d2 = 1.234e2;
float f1 = 123.4f;
This tells you that:
- Both D and d have the same meaning - just as you observed
- "By convention [D or d are] omitted" because they are the default
Questions of the type "what is better" are offtopic on StackOverflow since they can't have a correct or incorrect answer, only opinions.
But if you ask for a convention I would tend to follow what I've found above - omit the suffix.