Argument Promotion and Casting

Another important feature of method calls is argument promotionconverting an argument's value to the type that the method expects to receive in its corresponding parameter. For example, a program can call Math method sqrt with an integer argument even though the method expects to receive a double argument (but, as we will soon see, not vice versa). The statement

 System.out.println( Math.sqrt( 4 ) );

correctly evaluates Math.sqrt( 4 ) and prints the value 2.0. The method declaration's parameter list causes Java to convert the int value 4 to the double value 4.0 before passing the value to sqrt. Attempting these conversions may lead to compilation errors if Java's promotion rules are not satisfied. The promotion rules specify which conversions are allowed, that is, which conversions can be performed without losing data. In the sqrt example above, an int is converted to a double without changing its value. However, converting a double to an int TRuncates the fractional part of the double valuethus, part of the value is lost. Converting large integer types to small integer types (e.g., long to int) may also result in changed values.

The promotion rules apply to expressions containing values of two or more primitive types and to primitive-type values passed as arguments to methods. Each value is promoted to the "highest" type in the expression. (Actually, the expression uses a temporary copy of each valuethe types of the original values remain unchanged.) Figure 6.5 lists the primitive types and the types to which each can be promoted. Note that the valid promotions for a given type are always to a type higher in the table. For example, an int can be promoted to the higher types long, float and double.

Figure 6.5. Promotions allowed for primitive types.

Type	Valid promotions
double	None
float	double
long	float or double
int	long, float or double
char	int, long, float or double
short	int, long, float or double (but not char)
byte	short, int, long, float or double (but not char)
boolean	None (boolean values are not considered to be numbers in Java)

Converting values to types lower in the table of Fig. 6.5 will result in different values if the lower type cannot represent the value of the higher type (e.g., the int value 2000000 cannot be represented as a short, and any floating-point number with digits after its decimal point cannot be represented in an integer type such as long, int or short). Therefore, in cases where information may be lost due to conversion, the Java compiler requires the programmer to use a cast operator (introduced in Section 4.9) to explicitly force the conversion to occurotherwise a compilation error occurs. This enables the programmer to "take control" from the compiler. The programmer essentially says, "I know this conversion might cause loss of information, but for my purposes here, that's fine." Suppose method square calculates the square of an integer and thus requires an int argument. To call square with a double argument named doubleValue, we would be required to write the method call as square( (int) doubleValue ). This method call explicitly casts (converts) the value of doubleValue to an integer for use in method square. Thus, if doubleValue's value is 4.5, the method receives the value 4 and returns 16, not 20.25.

Common Programming Error 6.9

Converting a primitive-type value to another primitive type may change the value if the new type is not a valid promotion. For example, converting a floating-point value to an integral value may introduce truncation errors (loss of the fractional part) into the result.