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: 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 r = ToolBox.min( 1.0, 4.0 );

Example: passing information stored in variables Consider the following program:

Example: passing information stored in variables (cont.) Question to ponder: There are quite a few ways to allow (enable) you to accomplish this "passing" The possible answers ranges from simple to pretty weird How can we pass (give) the information stored inside the variables x and y to the method ToolBox.min

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: rogs/pass-by-value/quiz/MyProgram.java -Prog file: 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: Did you understand why the update statements "a = a + 1" and "b = b + 2" did not update the actual parameters x and y ??? 1.0 (the value in x is UNCHANGEDD !) 4.0 (the value in y is UNCHANGEDD !) 8.0 (= )

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: are different variables (they occupy different memory cells !) The local variables x and y in the main method and The parameter variables a and b in the fun method

The quiz explained (cont.) The assignment statements: will change the values of the parameter variables: a = a + 1; b = b + 2;

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: rogs/pass-by-value/quiz2/MyProgram.java -Prog file: 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: 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 ??? 1.0 (the value in x is UNCHANGEDD !) 4.0 (the value in y is UNCHANGEDD !) 8.0 (= )

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: 0/Syllabus/08/scope.html#disjoint)

The quiz explained (cont.) In other words: are different variables 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

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: are different variables --- it's possible because of the scopes are non-overlapping (furthermore, they use different memory cells !) 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

The quiz explained (cont.) The assignment statements: will change the values of the parameter variables: a = a + 1; b = b + 2;

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