1. Methods: Creation and Use Chapters 5 and 6
Basic Programming
Methods: Creation and Use
Chapters 5 and 6

2. Outline What are methods? Creating methods other than main
what are they for?
what do they look like?
how do we use them?
what are some useful ones?
Creating methods other than main
void, parameterless methods
void methods with parameters
value-returning methods

3. What are Methods? Methods are named bits of code
instructions for carrying out some (small) task
main is for doing a whole program
other methods not usually a whole program
also called procedures, subroutines, functions
We have been using them all along
main is a method
System.out.print and System.out.println are, too
so are all the kbd.nextWhatevers

4. What Are Methods For? Most methods do one thing
System.out.print prints what you give it
System.out.println prints it and ends the line
kbd.nextInt reads an integer value
Usually something you want to do many times
print lots of things; read lots of numbers
Often just one step of an algorithm
the low-level details of a high-level step
A few other reasons….

5. Calling a Method Telling the computer to do the instructions
to print this thing or read a number or whatever
Method calls look like this:
location.name(arguments)
location (e.g. System.out or kbd)
sometimes it can be left off entirely
name (e.g. println or nextDouble)
may be zero, one or more arguments
separated by commas
but parentheses are needed, even if no arguments

6. Named Instructions
Calling the method makes the computer run those instructions
running a console program = calling main
Someone wrote instructions for printing
saved them in a method called print
put that method into System.out
call System.out.print = run those instructions
Someone wrote instructions for reading an int
call kbd.nextInt = run those instructions

7. More Methods Many useful methods have been provided Math methods
find the square root or power, trig functions, …
y = Math.sqrt(x);
String methods
get part of it, find something in it, …
// everything before the comma is the person's last name
lastName = fullName.substring(0, fullName.indexOf(","));

8. Math Methods Standard functions available using Math
power function: pow
System.out.println("5 squared is " + Math.pow(5, 2));
maximum function: max
bestScore = Math.max(testScore, examScore);
square root: sqrt
root = (-b + Math.sqrt(b*b – 4*a*c)) / (2 * a);
Same form for all functions
Math.functionName(arguments…)

9. Arguments “Arguments” are given to the method
we also say that the method takes arguments
Math.sqrt(10) – 10 is the (only) argument
sqrt knows how to find the square root of a number
but need to tell it what number to find the square root of
Math.pow(5, 2) – 5 and 2 are both arguments
pow knows how to raise a number to a power
but need to tell it what number to raise to what power
arguments must be in the right order!
Math.pow(2, 5) is 25, not 52

10. Return Values Math methods return values
(some methods don’t)
we use the function to get the returned value
Math.sqrt(10) returns the square root of 10
Math.pow(2, 5) returns 2 to the 5th power
Math.pow(5, 2) returns 5 to the 2nd power
Math.sqrt(x) returns the square root of x
Name of method == what value is returned

11. Using Return Values Use the function in expressions
you can print it out
System.out.println("5 squared is " + Math.pow(5, 2));
you can save the value in a variable
bestScore = Math.max(testScore, examScore);
you can use it in a larger math expression
root = (-b + Math.sqrt(b*b – 4*a*c)) / (2 * a);
even as an argument to another Math method
x = Math.pow(10, Math.sqrt(5)); // x =

12. Using Return Values Use the function in expressions
return value used in the expression
System.out.println("5 squared is " + Math.pow(5, 2));
Math.pow(5, 2) returns 52 = 25.0
System.out.println prints “5 squared is 25.0”
root = (-b + Math.sqrt(b*b – 4*a*c)) / (2 * a);
suppose a is 1, b is 6 and c is 9
b*b – 4*a*c = 6*6 – 4*1*9 = 36 – 36 = 0
Math.sqrt(0) returns 0.0
root = ( ) / (2*1) = -6.0 / 2.0 = -3.0

13. Some Math Functions Trigonometric functions Other functions
sin(theta), cos(theta), tan(theta)
sine, cosine, tangent of an angle (in radians)
toDegrees(rad), toRadians(deg)
translate between radians and degrees
Other functions
exp(x), log10(x), log(x), abs(x)
ex, log10(x), ln(x),|x|

14. Exercise Write calls to Math functions to find the square root of 7
6 to the 3rd power
10 times the log (base 10) of x
the natural log of 10 times x
the number of radians equivalent to 45 degrees
the sine of 45 degrees

15. More Math Methods Larger & smaller; rounding off
max, min (each takes two numbers)
ceil, floor, round (each takes one number)
Random number (in range [0,1))
random
int dieRoll1 = 1 + (int)(Math.random() * 6);
int dieRoll2 = 1 + (int)(Math.random() * 6);
int dice = dieRoll1 + dieRoll2;
Note: random takes no arguments! That’s fine.

16. Rounding Off Math.round takes a double, returns a long
Math.round(3.0)  3L
long is like int, but bigger (up to ~10 quintillion)
can’t be saved in an int variable!
int rounded = Math.round(3.6);
Error: possible loss of precision
need to tell Java it’ll be smaller…
int rounded = (int)Math.round(3.6);
or just work with long variables!

17. Exercise
Write these expressions in Java
note: use Math.PI for π

18. String Methods Strings have methods, too
int and double don’t
String name = "Djenaba, Louise";
get its length: name.length()  15
get part of it: name.substring(0, 4)  "Djen"
find part of it: name.indexOf(",")  7
String surname = name.substring(0, name.indexOf(","));
get an UPPER CASE version: surname.toUpperCase()  "DJENABA"
NOTE: surname is still "Djenaba" (it doesn’t change)

19. String Positions
Position in a String is how many characters from the start of the String
"e" is 2 characters from the start of the String
after the D and the j
"D" is 0 characters from the start
"nab" starts 3 characters from the start
"a" is 4 characters from the start
it finds the first one D j e n a b 1 2 3 4 5 6 7

20. String Positions If it doesn’t exist, you get -1
name.indexOf("x")  -1
If there’s more than one, it finds the first…
name.indexOf("a")  4
…but you can tell it to start later in the String
name.indexOf("a", 5)  6
the first "a" at/after position 5 D j e n a b 1 2 3 4 5 6 7

21. String Positions Substrings go between the two numbers…
name.substring(0, 4)  "Djen"
name.substring(3, 6)  "nab"
name.substring(0, 7)  "Djenaba"
…or from the number to the end
name.substring(2)  "enaba"
name.substring(7)  "" D j e n a b 1 2 3 4 5 6 7

22. Argument Types Arguments must be the right type!
Math.pow("fred", true) makes no sense!
the two arguments must be numbers
doubles are OK: Math.pow(3.7, 1.98)
name.indexOf(7) makes no sense!
the argument must be a String: name.indexOf("7")
There must be the right number of them!
Math.pow(5) and Math.pow(1, 2, 3) make no sense!
and they must be in the right order

23. Exercise
Assume the calls below are correct. What argument type(s) does each method take?
Math.getExponent(3400.2)
Math.ulp(2.6)
Math.scalb(4.5, 2)
str.split(":", "one:two:three:four")
str.length()
thingy.doesItDo("2", 2, 2.0, "to", "too", "two")

24. Return Types Methods can return any kind of value
(or even none)
Math.sqrt(10) returns a double value
Math.max(3, 5) returns an int value
kbd.nextInt() returns an int value
kbd.next() returns a String value
str.toUpperCase() returns a String value
str.indexOf("n") returns an int value

25. Exercise
Assuming the code below is correct, what kind of value does each method return?
double x, y = 3.2;
int n = 42;
String name = "Mark";
boolean good;
name = name.replaceFirst("a", "o");
x = Math.exp(y);
n = Math.getExponent(x);
good = (name.equalsIgnoreCase("mork"));
No, I haven't mentioned boolean before. Next week.

26. Making a Method We have been making methods all along main is a method
it's the “main” method of a class
the method that runs when you run the program
main method has instructions in it
so will the other methods we make
our own methods will be in the same class as main
later we will make other classes with their own methods

27. Why Do We Make Methods? Code Re-use Code Hiding (Encapsulation)
Doing “same” thing in multiple places
we do a lot of printing!
Code Hiding (Encapsulation)
Secret
Implementation independence
Code Abstraction
Top-down design

28. How Do Methods Work Instructions written down in one place . . .
Scanner.java, String.java, Math.java, …
. . . call comes from some other place
MyProg.java, ExamGrades.java, …
Computer remembers where it was . . .
. . . follows instructions for the method . . .
. . . returns to where it left off

29. Flow of Control You ask System.out to print “Hello”
computer stops working on your program (for a moment) & takes “Hello” to the print method
the steps of the print method are followed to completion  Hello appears on the screen
your program starts up again, right where it left off
Recall: Something you send to a method (like “Hello”) is called an argument

30. Flow of Control You ask kbd to get the next int
computer stops working on your program (for a moment) & starts doing the nextInt method
the steps of the nextInt method are followed to completion, getting an int from the user
that int is brought back to your program
your program starts up again, right where it left off, with that int where kbd.nextInt is/was
Recall: Something a method sends back to you (like this int) is called a return value

31. Adding Methods to a Program
We’ll redo RectangleArea using methods
the version doing input
public class RectangleArea {
public static void main(String[] args) {
// create variables for length, width and area
// print an introduction for the user
// get length and width of rectangle
// calculate the area
// report dimensions and area to user }

32. Method Purpose Method has a job to do Method does its job
get an int from the user
print a string on the screen
translate a temperature in Celsius to Fahrenheit
Method does its job
how it does its job is not the caller’s business (recall: code hiding)
caller should not worry about it (nor have to)

33. Method Design What is the purpose of this method?
need to know what it’s supposed to do
be very clear on its job
write its job as a javadoc comment
put the javadoc comment after the end of main
/**
* Print the introduction for this program. */
choose a name that reflects that job
printIntroduction

34. Method Use Add the method call to main
don’t need a location
no location  the method is in this class
NetBeans will underline it in red
doesn’t know this method (yet!)
public class RectangleArea {
public static void main(String[] args) {
// create variables for length, width and area
// print an introduction for the user
printIntroduction();

35. Have NetBeans Create the Method
Click on red circle and choose the command
Red underlining disappears
Scroll down to bottom of class
there’s a new method there
move its javadoc comment to right above it
create method "printIntroduction()" in …
private static void printIntroduction() {
throw new UnsupportedOperationException(...); }

36. The Method Header Compare first lines of our two methods
public static void main(String[] args)
private static void printIntroduction()
private instead of public
static and void are the same
different names
empty parentheses instead of whatever that is
more about all this later

37. The Method Body We will delete the body NetBeans created
it means: throw a fit because I can’t cope with this
actually it’s an exception instead of a fit, but it’s close
Replace it with the commands for printing the introduction (same as in class last week)
yes, it’s only two lines; that’s fine
private static void printIntroduction() {
System.out.println("This program calculates …");
System.out.println(); }

38. Run the Program It’s not finished yet, but that’s OK
run the program  call main
main calls printIntroduction
printIntroduction prints some stuff
printIntroduction finished; go back to main
main is finished; program ends
run:
This program calculates the area of a rectangle.
BUILD SUCCESSFUL …

39. Next Method: Pause Follow same procedure as before
determine its purpose; write javadoc
prompt user to press enter; wait for user to press enter
add pause() to main right after printIntroduction()
have NetBeans create the method
replace its body with the required commands
private static void pause() {
System.out.print("Press enter …");
kbd.nextLine();
System.out.println(); }

40. Local Variables Problem: pause does not know symbol kbd
it hasn’t been declared
Problem: declaring kbd in main does no good
main and pause are different methods
each has its own variables
Declare kbd in pause method
private static void pause() {
Scanner kbd = new Scanner(System.in);
System.out.print("Press enter …");
kbd.nextLine();

41. Run Program Again Make sure what we just did works
main calls printIntroduction which prints stuff
main then calls pause which prints stuff and waits for the user to press enter
after user presses enter, pause ends, then main ends
run:
This program calculates the area of a rectangle.
Press enter…
BUILD SUCCESSFUL …

42. Programming Principles
Top-down design
write a high-level description of the program
write descriptions of each step involved
Modular design
pick a part and write the code for it
run your program to test the part you coded
you might need to add bits and pieces of code (take it out later) in order to call that part

43. Next Method: printReport
Purpose is to report findings to user
needs to print length, width and area
so it needs to be told those values
printReport(length, width, area);
those variables need to be created and given values
in main, because that’s who’s doing the telling!
/**
* Report findings to user. Include length, width and area of
* the rectangle in the report. */
Recall: the values given to methods are called arguments

44. Add Variables to main Declare the variables and give them values
remember: this is in main
just assign values; don’t even care if they’re right
Add call to printReport (at correct place)
int length, width, area;
length = 10;
width = 20;
area = 42;
// report dimensions and area to user
printReport(length, width, area);

45. Have NetBeans Create the Method
Same procedure as before
Result is a little bit different
three variables declared inside the parentheses
match the three variables we put in the method call
same types; same names
same types is important
same names is unimportant
private static void printReport(int length, int width, int area) {
throw new UnsupportedOperationException(...); }

46. Parameters Those variables are called parameters
have to be same type as arguments
are names this method will use for the arguments
same names as in main is fine
but they are different variables
get their values from the arguments
printReport(10, 20, 42);
printReport(int l, int w, int a)  l = 10, w = 20, a = 42
printReport(50, 77, -3);
printReport(int l, int w, int a)  l = 50, w = 77, a = -3

47. Method Body Have it print out the report
use names of parameters; not of arguments
private static void printReport(int length, int width, int area) {
System.out.print("The area of a " + length + "x" + width
+ " rectangle is " + area + ".");
System.out.println(); }
private static void printReport(int len, int wid, int ar) {
System.out.print("The area of a " + len + "x" + wid
+ " rectangle is " + ar + ".");
System.out.println(); }

48. Parameter Values Parameter values come from the caller
main is the only caller our methods have, yet
Do not read a value into a parameter!
the parameter already has a value; don’t change it!
main already asked the user for length and width
main (will have) already calculated the area
if printReport asks user for those values…
…user will be asked twice for length and width
…user will be asked for the area, which is what the user wanted from the program in the first place!

49. Run the Program New method: test it!
NOTE: those are the numbers you gave it
they are wrong, but method did what you told it
run:
This program calculates the area of a rectangle.
Press enter…
The area of a 10x20 rectangle is 42.
BUILD SUCCESSFUL …

50. Not My Job
printReport does not check whether the numbers you gave it make sense
that is NOT its job
its job is just to print the dang report
checking the numbers is somebody else’s problem
Programming principle: GIGO
garbage in; garbage out
if you give a method/program the wrong information, it will do the wrong thing
it is following its instructions; that is all it can do

51. Next method: calculateArea
Purpose of the method is to calculate the area of a rectangle of a given length and width
needs to be given the length and width
needs to return the area
we will save the result in the variable area
// calculate the area
area = calculateArea(length, width);
get rid of area = 42;
have NetBeans create the method for you
another small difference

52. Return Types New method has a return type: int replaces the word void
same as data type of area
the variable we saved the result in
NetBeans figured it out all by itself
same as with the parameters
says that this method will return an int value
programmer must tell it what value to return
private static int calculateArea(int length, int width) {
throw new UnsupportedOperationException(...); }

53. Returning a Value Area is product of length and width
Problem: area is not recognized
it belongs to main, not calculateArea
so declare it here…
private static int calculateArea(int length, int width) {
area = length * width; }
private static int calculateArea(int length, int width) {
int area;
area = length * width; }

54. Returning a Value Still got a problem you haven’t returned the value
need to use a return command
private static int calculateArea(int length, int width) {
int area;
area = length * width; }
private static int calculateArea(int length, int width) {
int area;
area = length * width;
return area; }

55. Last Method: readDimension
Recall our steps for reading length and width
same three commands, except length/width
one assignment command in each part
make method to prompt, read, tidy and return
System.out.print("Enter the length of the rectangle: ");
length = kbd.nextInt();
kbd.nextLine();
System.out.print("Enter the width of the rectangle: ");
width = kbd.nextInt();

56. Multiple Calls to a Method
Call once to read length; again to read width
need to tell the method what to prompt for
method reads one integer value each time
and returns one value each time
put what it returns into the appropriate variable
prompt for length  save length
prompt for width  save width
length = readDimension("length");
width = readDimension("width");

57. One Method Definition Have NetBeans create the method for one call
other call is to same method, so it’ll be good, too!
parameter type matches the argument (a String)
name of parameter taken from argument
"length"  length
"width"  width
we will give it a better name (e.g. dimension)
private static int readDimension(String length) {
throw new UnsupportedOperationException(...); }

58. Method Body Start with the code you'd use in main
just for reading the length
some errors to fix up
but we know how to do that!
private static int readDimension(String dimension) {
System.out.print("Enter the length of the rectangle: ");
length = kbd.nextInt();
kbd.nextLine(); }

59. Method Body Add kbd and length Add return command
remember, every method needs its own variables
Add return command
private static int readDimension(String dimension) {
Scanner kbd = new Scanner(System.in);
int length;
System.out.print("Enter the length of the rectangle: ");
length = kbd.nextInt();
kbd.nextLine();
return length; }

60. Run the Program Again It will almost work properly! run:
This program calculates the area of a rectangle.
Press enter…
Enter the length of the rectangle: 10
Enter the length of the rectangle: 20
The area of a 10x20 rectangle is 200.
BUILD SUCCESSFUL …

61. Fix the Prompt Argument says which dimension to ask for
include it in the prompt (instead of "length")
private static int readDimension(String dimension) {
Scanner kbd = new Scanner(System.in);
int length;
System.out.print("Enter the " + dimension " of the rectangle: ");
length = kbd.nextInt();
kbd.nextLine();
return length; }

62. Run the Program Again It works properly! run:
This program calculates the area of a rectangle.
Press enter…
Enter the length of the rectangle: 10
Enter the width of the rectangle: 20
The area of a 10x20 rectangle is 200.
BUILD SUCCESSFUL …

63. Variable Names Note that readDimension returns an int value
it does not decide what to do with that value
main decides what to do with that value
length = readDimension("…")  save in length
width = readDimension("…")  save in width
That number named length in readDimension
but it doesn’t matter to the computer
can call it length, or width, or area, or ludicrous
method will still do the same thing
but it does matter to other programmers

64. Rename the Variable It’s not just the length any more
it could be length or width
variable names should describe their content
Let’s call it answer (the user’s answer)
ask NetBeans to make the change!
double-click any occurrence of length in readDimension
select Rename command from Refactor menu
type the new name into the box that appears
other variables named length not changed
no chance of making different changes on different lines

65. Adding More Pauses Add calls to pause after input & report
OK to call a method multiple times
public static void main(String[] args) {
int length, width, area;
printIntroduction();
pause();
length = getDimension("length");
width = getDimension("width");
area = calculateArea(length, width);
printReport(length, width, area); }
I took out the pseudo-code comments and the blank lines so it would fit on the screen.
You should leave them in.

66. Updating the Javadocs
Should have been creating javadoc comments as you went along
purpose of the method
But now we have the program working, we should finalize them
document parameters and return values
NetBeans will help!

67. Creating Javadocs with NetBeans
Insert a blank line just above method header
Type /** on that line and press enter
NetBeans fills in a javadoc comment template
/** *
dimension */
private static int readDimension(String dimension) {
/**
private static int readDimension(String dimension) {
Scanner kbd = new Scanner(System.in);
int answer;
System.out.print("Enter the length of the rectangle: ");
answer = kbd.nextInt();
private static int readDimension(String dimension) {
Scanner kbd = new Scanner(System.in);
int answer;
System.out.print("Enter the length of the rectangle: ");
answer = kbd.nextInt();
kbd.nextLine();
private static int readDimension(String dimension) {
Scanner kbd = new Scanner(System.in);
int answer;
System.out.print("Enter the length of the rectangle: ");
answer = kbd.nextInt();

68. Filling in the Javadoc Comment
Type the purpose on the second line
add more lines as required; add a blank line after
Describe every parameter lines)
Describe the return value lines)
/**
* Read one dimension (length or width) of a rectangle. *
dimension the name of the dimension to read
the user-given value of that dimension */

69. NetBeans’ Navigation Pane
Bottom-left side of IDE window
double-click method to jump to its definition
hover to see its javadoc
remind you in case you forgot what it’s for

70. Methods Summary Many useful methods available Can create our own
Scanners, Strings, Math, …
Can create our own
private static methods
void methods just do something
value-returning methods return a value (duh!)
type of value returned replaces void
arguments saved into parameters
use local variables and parameters
don’t change parameter values

71. Exercise Write methods to:
print a given line and a blank line after it.
“givenˮ means given to the method
calculate f(x) where f(x) = 2x3 – 3x + 5.
print the sum of three given numbers.
sum three given numbers.
what’s different from the previous question?

72. Questions