Parameter passing mechanism:
pass-by-value
Introduction
• In the last webpage, we discussed how to pass information
to a method
• I have kept it (deliberately) simple by using constant values
as parameters:
r = ToolBox.min( 1.0, 4.0 );
• In this webpage, we will discuss how to pass information
stored in variables to a method.
Specifically, we will study the pass-by-value mechanism
Example: passing information stored in
variables
• Consider the following program:
Example: passing information stored in
variables (cont.)
• Question to ponder:
• How can we pass (give) the information stored
inside the variables x and y to the method
ToolBox.min
There are quite a few ways to allow (enable) you to
accomplish this "passing"
The possible answers ranges from simple to pretty weird
Parameter passing mechanisms
• Definition:
• Parameter passing mechanism = agreement between the
calling method and the called method on how a parameter is
passed between them
Parameter passing mechanisms (cont.)
• Important note:
• Both the calling method and the called method must
agree to use the same passing mechanism (or else, the
information will be passed incorrectly)
Parameter passing mechanisms (cont.)
• Most commonly used parameter passing mechanisms:
Pass-by-value
The calling method passes the
information stored inside a variable by
passing (= copying) the value contained
inside a variable into the parameter
variable.
Parameter passing mechanisms (cont.)
This is the most obvious way to pass information...
Example:
• if you want to give you phone number
of your home to someone, you make a
copy of the information (in the parameter
variable)
Parameter passing mechanisms (cont.)
Pass-by-reference
• The calling method passes the
information stored inside a variable by
passing (= copying) the address
(location) of a variable into the parameter
variable.
Parameter passing mechanisms (cont.)
This is a less obvious but more powerful way to pass
information...
Example:
• if you want to give you phone number of
your home to someone, you make a copy of
the address of your home (in the parameter
variable)
He/she can find the phone number by visiting
that address !!!
Parameter passing mechanisms (cont.)
• Terminology:
• A reference in Computer Science is the location
(or address) (of a variable or a method)
Terminology: formal parameters and actual
parameters
• Definitions:
• Formal parameter = a parameter variable
• Actual parameter = a variable whose value is to be
passed to some formal parameter
Terminology: formal parameters and actual
parameters (cont.)
• Illustrated example:
Terminology: formal parameters and actual
parameters (cont.)
• Explanation:
• The parameter variables a and b in the definition of
the ToolBox.min method are formal parameters
• The variables x and y used in the method invocation
ToolBox.min(x, y) are actual parameters
The Pass-by-value mechanism - the
agreement
• Recall:
• Parameter passing mechanism = agreement between
the calling method and the called method on how a
parameter is passed between them
The Pass-by-value mechanism - the
agreement (cont.)
• The agreement used in the Pass-by-value mechanism:
• For the calling method:
• The calling method creates the parameter
variables for the called method, .... and
• The calling method copies the value of the
actual parameter into the formal parameter
The Pass-by-value mechanism - the
agreement (cont.)
• For the called method:
• The called method obtains the information
directly from the parameter variables
The Pass-by-value mechanism - an example
• Example program:
The Pass-by-value mechanism - an example
(cont.)
• When main starts running, it will first create its local
variables:
The Pass-by-value mechanism - an example
(cont.)
• When execution reaches the method call
ToolBox.min(x,y), the Pass-by-value mechanism first
creates the parameter variables:
The Pass-by-value mechanism - an example
(cont.)
• Then the Pass-by-value mechanism copies the value of the
actual parameter to the corresponding formal parameter:
The Pass-by-value mechanism - an example
(cont.)
• The method invocation mechanism is completed as usually
with the following steps:
• Save the return address on the stack:
The Pass-by-value mechanism - an example
(cont.)
• Jump to the called method:
The Pass-by-value mechanism - an example
(cont.)
• When the min method executes, it will create its local
variable m:
The Pass-by-value mechanism - an example
(cont.)
• Notice how the called method uses the parameter
variables:
• When the called method uses a parameter variable, the
information is obtained directly from the parameter
variable:
A quiz on the Pass-by-value mechanism
• Consider the following program:
A quiz on the Pass-by-value mechanism
(cont.)
• Questions:
• What value is printed by the statement
System.out.println(x); ?
• What value is printed by the statement
System.out.println(y); ?
• What value is printed by the statement
System.out.println(r); ?
A quiz on the Pass-by-value mechanism
(cont.)
• Example Program: (Demo above code)
– Prog file:
http://www.mathcs.emory.edu/~cheung/Courses/170/Syllabus/08/P
rogs/pass-by-value/quiz/MyProgram.java
- Prog file:
http://www.mathcs.emory.edu/~cheung/Courses/170/Syllabus/08/P
rogs/pass-by-value/quiz/ToolBox.java
• How to run the program:
• Right click on links and save in a scratch directory
• To compile: javac MyProgram.java
• To run:
java MyProgram
A quiz on the Pass-by-value mechanism
(cont.)
• Output of the program:
1.0
(the value in x is UNCHANGEDD !)
4.0
(the value in y is UNCHANGEDD !)
8.0
(= 2.0 + 6.0)
Did you understand why the update statements "a = a + 1"
and "b = b + 2" did not update the actual parameters x and
y ???
The quiz explained
• Notice the similarities between the ToolBox.min and the
ToolBox.fun methods:
public static double min
( double a, double b )
{
double m = 0;
if ( a < b )
{
m = a;
}
else
{
m = b;
}
return(m);
}
public static double fun
( double a, double b )
{
double m = 0;
a = a + 1;
b = b + 2;
m = a + b;
return(m);
}
The quiz explained (cont.)
• Both methods have 2 parameter variables and 1 local
variable
• I have constructed the quiz in such a way that I can re-use
the diagrams from the Pass-by-value example above.
The quiz explained (cont.)
• So according to the Pass-by-value example above, when
the ToolBox.min method starts running, the following
variables have been created on the System Stack:
The quiz explained (cont.)
• Notice that:
• The local variables x and y in the main
method
and
• The parameter variables a and b in the fun method
are different variables (they occupy different memory cells !)
The quiz explained (cont.)
• The assignment statements:
a = a + 1;
b = b + 2;
will change the values of the parameter variables:
The quiz explained (cont.)
The quiz explained (cont.)
• Notice that:
• The values in the actual parameters (x and y) are
unchanged !!!
The quiz explained (cont.)
• That's why the statements
System.out.println(x); ---> prints 1.0
System.out.println(y); ---> prints 4.0
The quiz with an additional twist...
• Now, consider the following program:
The quiz with an additional twist... (cont.)
• We use the same names for actual and formal parameters !!!
• Questions:
• What value is printed by the statement
System.out.println(a); ?
• What value is printed by the statement
System.out.println(b); ?
• What value is printed by the statement
System.out.println(r); ?
The quiz with an additional twist... (cont.)
• Example Program: (Demo above code)
– Prog file:
http://www.mathcs.emory.edu/~cheung/Courses/170/Syllabus/08/P
rogs/pass-by-value/quiz2/MyProgram.java
- Prog file:
http://www.mathcs.emory.edu/~cheung/Courses/170/Syllabus/08/P
rogs/pass-by-value/quiz2/ToolBox.java
• How to run the program:
• Right click on links and save in a scratch directory
• To compile: javac MyProgram.java
• To run:
java MyProgram
The quiz with an additional twist... (cont.)
• Output of the program:
1.0
(the value in x is UNCHANGEDD !)
4.0
(the value in y is UNCHANGEDD !)
8.0
(= 2.0 + 6.0)
Did you understand why the update statements "a = a + 1"
and "b = b + 2" (that updates the formal parameters) did
not update the actual parameters a and b ???
The quiz explained
• Recall that:
• Different method scopes are always disjoint
scopes
• You can define different variables with the same
name in disjoint scopes (See:
http://www.mathcs.emory.edu/~cheung/Courses/17
0/Syllabus/08/scope.html#disjoint)
The quiz explained (cont.)
• In other words:
• The local variables named a and b defined inside the
main method
and
• The parameter variables named a and b defined
inside the fun method
are different variables
The quiz explained (cont.)
• The following diagram shows the fact that there are 2
different variables with the same name created created on
the System Stack:
The quiz explained (cont.)
• Notice that:
• The blue colored variables named a and b are inside
the scope of the main method
and
• The magenta colored variables named a and b are
inside the scope of the fun method
are different variables --- it's possible because of the scopes
are non-overlapping (furthermore, they use different
memory cells !)
The quiz explained (cont.)
• The assignment statements:
a = a + 1;
b = b + 2;
will change the values of the parameter variables:
The quiz explained (cont.)
The quiz explained (cont.)
• Notice that:
• The values in the actual parameters (a and b) are
unchanged !!!
The quiz explained (cont.)
• That's why the statements
System.out.println(a); ---> prints 1.0
System.out.println(b); ---> prints 4.0