Transcript Parameter variables-..

Parameter variables: definition
(how to recognize them), life time
and scope.
Previously discusses: different kinds of
variables
• Java has 4 different kinds of variables (See:
http://download.oracle.com/javase/tutorial/java/nutsandbol
ts/variables.html)
• Class variables
• Instance variables
• Local variables
• Parameter variables
Previously discusses: different kinds of
variables (cont.)
• We have just learned about the local variables
• In this webpage, we will discuss the parameter variables
• Specifically:
• How to identify (and define)
parameter variables
• The life time of parameter variables
• The scope of parameter variables
Purpose of parameter variables
• Parameter variables:
• A parameter variable is used to store information
that is being passed from the location of the method
call into the method that is called
Purpose of parameter variables (cont.)
• Illustration:
Purpose of parameter variables (cont.)
• Explanation:
• Method 1 is about to call (invoke) method 2
• Method 1 has some information (stored in some variable
inside method 1) that it want to pass (convey) to method 2
• The mechanism used to accomplish this passing of
information are parameter variables
Purpose of parameter variables (cont.)
• Example:
Purpose of parameter variables (cont.)
• Explanation:
• Method main is calling (invoke) the method min
• Method main has some information (the values 1.0 and
4.0) that it want to pass (convey) to method min
• The mechanism used to accomplish this passing of
information uses the parameter variables a and b
Defining (recognizing) parameter variables
• How to the define (and recognize) a parameter variable:
• A parameter variable is defined inside the brackets ( ... )
of the header of a method
Defining (recognizing) parameter variables
(cont.)
• Example:
public class MyProgram
{
public static void main(String[] args)
{ // Body of method "main"
public class ToolBox
{
public static double min ( double a, double b )
{ // Body of method "min"
double m = 0; // *** Local variable
double r; // *** Local variable
if ( a < b )
{
m = a; // a is the smaller value
}
else
{
m = b; // b is the smaller value
}
r = MyProgran.min( 1.0, 4.0 );
System.out.println(r);
r = MyProgram.min( 3.7, -2.9 );
System.out.println(r);
r = MyProgram.min( -9.9, 3.8 );
System.out.println(r);
return(m);
}
}
}
}
Defining (recognizing) parameter variables
(cont.)
• Comments:
• The variable args in method main is a parameter
variable
Its type is String[] (will be explained much later...)
• The variables a and b in method min are 2 parameter
variable
Their type is double (you should know what that
means...)
Life time and scope of parameter variables
• Simple rule to go by:
• A parameter variable behaves like a local variable that is
defined at the start of the method
• In addition:
• The value of the parameter variable is always
initialized by the method invocation mechanism
using the values inside the call
Life time and scope of parameter variables
(cont.)
• Example:
• The method call ToolBox.min(1.0, 4.0):
Life time and scope of parameter variables
(cont.)
It is as if the ToolBox.min method adds the following
definition of parameter variables:
Life time and scope of parameter variables
(cont.)
• Now you can understand completely why the ToolBox.min
method will return the value 1.0 !!!
(Because it used parameter variable with values 1.0 and 4.0
- so it will return the smaller value which is 1.0)
Summary: life time and scope of parameter
variables
• Life time of a parameter variable:
• is the entire body of the method
Summary: life time and scope of parameter
variables (cont.)
• It is as if as follows:
Summary: life time and scope of parameter
variables (cont.)
• Scope of a parameter variable:
• is also the entire body of the method
Summary: life time and scope of parameter
variables (cont.)
• It is as if as follows: