Methods Parameters ICS 111 Introduction to Computer Science

Methods & Parameters • ICS 111: Introduction to Computer Science I – William Albritton • Information and Computer Sciences Department at the University of Hawai‘i at Mānoa • “I have traveled the length and breadth of this country and talked with the best people, and I can assure you that data processing is a fad that won't last out the year. ” – The editor in charge of business books for Prentice Hall, 1957 2/12/2022 © 2007 William Albritton 1

Initials Problem • We want to prompt the user for their first & last names, then print their initials – Since this is a fairly common problem, let’s make a static method that will do this • This will organize the program into several distinct parts, so its easier to read, debug, and maintain – We can also use the same method in other programs without having to rewrite it! • Saves time, because we don’t have to “reinvent the wheel” every time we write a new program! – See Initials. java 2/12/2022 © 2007 William Albritton 2

Method Call Terminology Initials. display. Initials(first, last); “actual” parameters Class name 2/12/2022 method name © 2007 William Albritton 3

Method Definition Terminology public static void display. Initials(String first. Name, String last. Name){ {. . . } body prototype public static void display. Initials(String first. N… return type access modifier name of method parameter’s Class “formal” parameter use modifier 2/12/2022 original slide by Jan Stelovsky, modified by William Albritton 4

Terminology & Syntax • Actual parameter – The value passed to a method call – Also called “argument” – Syntax of a static method call • Class. Name. method. Name(actual. Parameter. Name); • Formal parameter – Variable in method prototype that receives the value – Syntax of static method prototype • public static void method. Name(Param. Type param. Name) 2/12/2022 © 2007 William Albritton 5

Actual & Formal Parameters • Within a method, formal parameters have the same scope as local variables – Formal parameters cannot be used outside that particular method • Usually we just say “parameter” instead of “actual parameter” or “formal parameter” – We use the context of the conversation to determine whether it is an “actual parameter” or “formal parameter” • “argument” is another word for “parameter” 2/12/2022 © 2007 William Albritton 6

Parallel Parameters • The formal and actual parameters have a one-toone correspondence – The Class, order, & number of parameters in the calling method must match those in the method definition • Initials. display. Initials(first, last); • public static void display. Initials (String first. Name, String last. Name){ } – Note that the actual parameter (first & last) can have a different name than the formal parameter (first. Name & last. Name) 2/12/2022 © 2007 William Albritton 7

Parameters & Variables • Remember that a parameter is a category of variable – An actual parameter is a variable that is placed within the parenthesis of a method call – A formal parameter is a variable declaration that is placed within the parenthesis of a method prototype (part of the method definition) • Usually, variables are initialized using the assignment operator, but formal parameters are initialized to the actual parameter using a method call 2/12/2022 © 2007 William Albritton 8

How Does It Work? • The value of the formal parameter is a copy of the value of the actual parameter – For example, if the first variable (actual parameter) has the address of String "Nami", then the first. Name variable (formal parameter) will also have the address of String "Nami" • Initials. display. Initials(first, last); • public static void display. Initials (String first. Name, String last. Name){ } 2/12/2022 © 2007 William Albritton 9

Example of How It Works • In other words, using parameters in method calls is equivalent to assigning values to a variable using an assignment statement – Using parameters • Initials. display. Initials(first, last); • public static void display. Initials (String first. Name, String last. Name){ } – Is the functional equivalent of the following code • String first. Name = first; • String last. Name = last; 2/12/2022 © 2007 William Albritton 10

Order of Methods • The order the methods are listed in the program has no influence on program control – The main() method, constructor, and other methods can be in any order – What does matter is the order in which they are called in the program • You can switch the main() method & the display. Initials() method in reverse order, but the program output will be the same 2/12/2022 © 2007 William Albritton 11

Class Exercise 1 • Trace through the following Java program & write the output – See Exercise 1. java 2/12/2022 © 2007 William Albritton 12

Terminology • Program control (flow of control, control) – Order of execution of statements in a program • CPU (Central Processing Unit) only executes one instruction at a time • Usually, the instructions in the code are executed from the top to the bottom • However, if different methods are invoked (called), control “jumps around” the program code • Information about each active method is stored in memory on the runtime stack 2/12/2022 © 2007 William Albritton 13

Program Control main() method main(String [] args){ } method 1(); other methods method 1(){ } method 2(); See Program. Control. java 2/12/2022 © 2007 William Albritton 14

Terminology • The method’s signature is combination of the method identifier (name), the class of its parameters, and the order of its parameters – Like a person’s signature, a method’s signature must be unique – Cannot have two methods with same signature, or compiler will give error message • For example, if we make another method 2() in the Program. Control. java file, then we get this error message: method 2() is already defined 2/12/2022 © 2007 William Albritton 15

Class Exercise 2 • Write a Java static method that has a String parameter & an Integer parameter – The method should output the substring of the string from 0 to the integer in uppercase letters – Write a main method to test your new method 2/12/2022 © 2007 William Albritton 16

Methods & Scope • Just like local variables, the scope of formal parameters is limited to that method – See Initials. java – variable name • • 2/12/2022 argument. Ts first last exception first. Name last. Name initials variable category scope formal parameter local variable formal parameter local variable © 2007 William Albritton 8 -19 9 -19 10 -19 16 -18 23 -28 25 -28 17

Methods & Graphics • We can also use static methods in our graphics program to draw shapes – For example, let’s use a static method to draw the sun part of the Japanese flag (red circle on white background) • See Hinomaru. Applet. java for the source code • See display. html for the graphical output – Note: to draw on the applet window, you need to use a Graphics parameter in your static methods – Note: to access the applet itself, you need to use a Applet parameter in your static methods 2/12/2022 © 2007 William Albritton 18

Methods & Multiple Characters • Methods are particularly handy when we want to draw multiple versions of the same graphical character – Compare this program using methods • See Kurochans. Applet. java for the source code • See display. html for the graphical output – Note: to draw on the applet window, you need to use a Graphics parameter in your static methods 2/12/2022 © 2007 William Albritton 19

Class Exercise 3 • What is the scope (line numbers) & category (local variable or parameter) of the variables in the two applet programs? – The scope is the line numbers where the variables can be used in the program 2/12/2022 © 2007 William Albritton 20