D.7. Argument Promotion and Casting

Another important feature of method calls is argument promotion—converting an argument’s value, if possible, to the type that the method expects to receive in its corresponding parameter. For example, a program can call Math method sqrt with an int argument even though a double argument is expected. 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 method sqrt. Such conversions may lead to compilation errors if Java’s promotion rules are not satisfied. These rules specify which conversions are allowed—that is, which ones 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 value—thus, part of the value is lost. Converting large integer types to small integer types (e.g., long to int, or int to short) may also result in changed values.