Expressions, Variables and Assignments - A Concise and Practical Introduction to Programming Algorithms in Java - page 16

double x=2; // implicit casting int->double
double x=( double )2; // explicit casting
long x=2.0; // implicit casting double->long
double x=2.0d; // The constant 2.0 is declared of type double
// add a 'd' after the number

The implicit casting rules of primitive types are summarized in Figure 1.1.

double

float

long

char

int

short

byte

Figure 1.1 Implicit casting rules of primitive types

For example, consider the following code snippet that implicitly casts a
character (of type char ) into its corresponding ASCII integer code:
char c='X';
int code=c;
System.out.println(code);

We get the ASCII code 88 for the capital letter 'X'.
There are slight restrictions on the names of variables that should not begin
with a digit, nor bear the name of a reserved keyword of the language. Trying
to use a reserved keyword for the name of a variable will result in a compiler
error as illustrated in the following code:

Program 1.11 Variable names should not belong to the list of reserved

keywords
class ReservedKeyword
{ public static void main ( String arg [ ] ) {

double x,y;
// Generate a syntax error:
// "not a statement"
int import ;
}
}