Transcript notes - Department of Computer Science

COM S 207
Method
Instructor: Ying Cai
Department of Computer Science
Iowa State University
[email protected]
Repeated Code
int hours = 0;
while (hours < 1 || hours > 12)
{
System.out.println(“Enter Hours (1-12):”);
hours = in.nextInt();
}
int minutes = 0;
while (minutes < 0 || minutes > 59)
{
System.out.println(“Enter Minutes (0-59):”);
minutes = in.nextInt();
}
int seconds = 0;
while (seconds < 0 || seconds > 59)
{
System.out.println(“Enter Seconds (0-59):”);
seconds = in.nextInt();
}
Turn Repeated Code into a Method
public static int readValueBetween(String s, int low, int high)
{
int input = low-1;
while (input < low || input > high)
{
System.out.println(“Enter ” + s + “ (“ + low + “-” + high +
“):”);
input = in.nextInt();
}
return input;
}
int hours = readValueBetween(“hours”, 1, 12);
int minutes = readValueBetween(“minutes”, 0, 59);
int seconds = readValueBetween(“seconds”, 0, 59);
A method is a named sequence of instructions
Math.pow(2, 3) is a method to compute 2^3
public static void main(String[] args)
{
double z = Math.pow(2, 3);
}
2
3
math.pow
black box
8
The implementation is hidden from the users. Users need to know
only the specification of the methods, no the implementation:
If you provide parameter values X and Y, the method returns X^Y
What is the return value of Math.pow(Math.pow(2, 2), 2)?
Implementing methods
Example: volume of a cube
type of return value
name of
method
type of
parameter
variable
Name of
parameter
variables
public static double cubeVolume(double sideLength)
{
double volume = sideLength * sideLength * sideLength;
return volume;
}
method body: executed
when the method is called
public class Cubes
{
public static void main(String[] args)
{
double result1 = cubeVolume(2.0);
double result2 = cubeVolume(10.0);
System.out.println(“Cube with side 2.0 has volume “ + result1);
System.out.println(“Cube with side 10.0 has volume “ + result1);
}
public static double cubeVolume(double sideLength)
{
double volume = sideLength * sideLength * sideLength;
return volume;
}
}
What will be printed out?
public class Mystery
{
public static void main(String[] args)
{
double number = mystery(2, 5);
System.out.println(“The mystery function returns: “ + number);
double cube = cubeVolume(5.0);
System.out.println(“The cubeVolume function returns: “ + cube);
}
public static double cubeVolume(double sideLength)
{
double volume = sideLength * sideLength * sideLength;
return volume;
}
public static int mystery(int x, int y)
{
int result = (x + y) / (y – x);
return result;
}
}
Exercise
Write java code to do the following tasks:
1)1 + 2 + 3 + … + 1000
2)231 + 232 + 233 + … + 999
3)30 + 31 + … + 8989
4)41 + 42 + … + 2345
Exercise
Write java code to do the following tasks:
1)1 + 2 + 3 + … + 1000
2)231 + 232 + 233 + … + 999
3)30 + 31 + … + 8989
4)41 + 42 + … + 2345
Parameter Passing
public class Mystery
{
public static void main(String[] args)
{
for (int x=1; x<=5; x++)
{
System.out.println(mystery(x));
}
•
}
public static int mystery(int x)
{
int result = 0;
for (int i=0; i<=x; i++)
{
result = result + i;
}
return result;
}
}
Memory space of a parameter
• Valid scope of variables
• Modification of a parameter
within a method
• Return value must match (if it
is void, no return value)
Excercise
Write a method to compute the volume of a pyramid
whose base is a square
height
base length
volume = 1/3 * base length^2 * height
Write a method that does this work
• Take two parameters, double a and integer b,
and return a^b
• Take three integer numbers as its arguments,
and return the smallest of the numbers
• Take three doubles as its arguments, and
return the average of them
• Take three integers as its arguments and
return true if they are sorted
Write the following methods
• int lastDigit(int n) //return the last digit of n
• int firstDigit(int n) //returning the first digit of n
• int digits(int n) //return the number of digits of n
Telephone Number Converter
Write a program that translates a telephone
number with letters in it (such as 1-800-FLOWERS)
into the actual phone number. Use the standard
letters on a phone pad.
Password Checking
Write a program that asks for a password and then
asks again to confirm it, if passwords don’t match
or the following rules are not fullfilled, prompt
again:
1.The password must be at least 8 characters long.
2.The password must have at least one uppercase
and one lowercase letter
3.The password must have at least one digit