Description
Use 2 Methods
• Use two methods for developing your solutions
– Main method:
• interacts with the user in terms of input/output.
• Calls the problem solving method to perform a task.
– Problem solving method:
• Receives some parameters from the main method,
does some computation and returns a result to the
main method.
2
Program Template
class Template // Replace ‘Template’ with your own program name.
{
// the main method contains all interactions with the user
public static void main (String[] args)
{
// DECLARE VARIABLES/DATA DICTIONARY
// READ IN GIVENS
Problem_Solving(); // Calling the Problem_Solving method
// PRINT OUT RESULTS
}
// the ‘main’ method calls the Problem_Solving method
public static ? Problem_Solving(?)
{
// DECLARE VARIABLES
// BODY OF ALGORITHM
}
/*Replace this with a descriptive comment for each method.*/
} // Don’t remove this brace bracket!
4
Getting ready to write programs…..
• You may use Template.java as a guide for your
programs
1. Copy Template.java (from Lab 3 on Blackboard
Learn into your working directory.
2. Start up Dr. Java
3. Click on “open” and select Template.java
4. Start a “new” file.
5. Copy and paste the contents of the
Template.java file into the (empty) unnamed file.
6. Close the file Template.java
• In your own program, do not forget to
replace the two “?” in the Template.java
with whatever is appropriate.
5
Exercise 1
• Write the complete Java program
• Use the two methods
• Main method:
• Main gets 3 numbers from the user
• Invokes the problem solving method with
those 3 numbers
• Gets the average back
• Problem solving method:
• Receives 3 numbers
• Computes the average
• Returns average
Exercise 2
/* Lab 3, Exercise 1. */
class Average
{
// the main method contains all interac=ons with the user
public sta=c void main (String[] args)
{
// Declare Variables
double n1, n2, n3; // numbers to averages
double average; // the averagage
// prompt the user to enter 3 numbers
System.out.print( “Enter three numbers: ” );
n1 = ITI1120.neadDouble(); // read first number
n2 = ITI1120.readDouble(); // read second number
n3 = ITI1120.readDouble(); // read third numre
// call problem solving method
double result = average( n1, n2, n3 );
// display maximum value
System.out.println( “The maximum is: ” + result );
}
public sta=c void averagedouble num1, double num2,double num3)
{
// Declare variables
double sum; // sum of the numbers
double avg; // RESULT: average of the numbers
// BODY
sum = num1 + num2 + num3 ;
avg = sum / 3;
// RETURN RESULTS
return(avg);
}
}
Program Memory Terminal/Output Screen
n1
n2
n3
average
Working Memory
num1
num2
num3
sum
avg
7
Using the Debugger
• Using Dr. Java’s
“debug mode”, you can trace your
Java program as it runs.
– You can go through the program one step at a time.
– You can stop the program at “break points” of your
choosing.
– You can check the values of variables.
• Try this for the program you wrote for Exercise 1,
the average of 3 numbers.
• Use the programming mode to follow the execution of
the program.
8
Break Points
• Select a line of your program, and under the debug menu,
choose “toggle break point on this line”.
– The first System.out.println statement is a good
choice
– This will change the colour of the chosen line of code to
red.
• You can also right-click on a line and select “Toggle
BreakPoint”.
– Many lines can be (de)selected this way.
• When you run the program, the program will stop just
before this line is going to be executed.
• In the interactions window, the debugger will tell you
where the program is, and the current line of code will be
coloured light blue.
9
Watches
• To keep track of the values of variables as they
change, use a “watch”
– Double-click on an empty area in the “
name
”
column, then type in the name of a variable, and hit
‘enter’.
– If the variable already has a value, it will be shown.
If the variable does not yet have a value, the value
will say
• Try this for all of the variables you use in your
program for example 1.
• As the program executes, each time the program
stops in the debugger, the current values of the
variables will be shown.
10
Controlling Execution
• With the debugger, there are four ways to advance
through a program
• Resume
– The program will run up to the next break point, or
the end of the program if there are no more break
points.
• Step into
– Use this for the most detailed debugging
– The program will move to the next statement –
even if that statement is in another method.
– This will not go into methods in the Java software
development kit.
11
Controlling Execution
• Step over
– Most commonly used
– Use this to move to the next statement in the current
method.
– If the current line of the program calls one or more
methods, all of those methods will be invoked, and returned
from.
• Step out
– Often used when you have stepped into a method and you
want to go back quickly to the previous method.
– Use this to run as far as the end of the current method.
• Try using “Step over” to go through your Exercise 1 program one statement at a time.
• But use “Step into” when you arrive at the call of the problem solving method (computeAverage).
12
Exercise 3
Write a complete Java program that asks a user for
temperature in Fahrenheit and converts it to Celsuis.
Your program should have
1. the main method that communicates with the user
2. and a method caled calculateCelsuis that converts
temperature expressed in Fahrenheit to Celsius,
according to the following formula:
C ← ( F – 32) * 5 / 9
and returns the computed value to the main.
13
Exercise 4
Write a complete Java program that given a two digit
positive integer prints that number in reverse.
– For example: Your program will transform the two
digit integer 12 into 21 (and print 21).
• Hints:
– The first digit is the result of dividing the integer by 10
(integer division)
– The second digit is the remainder of the division by 10
• e.g.:original integer: 12
– first digit is 12 / 10 = 1
– second digit is 12 % 10 = 2
• Use two methods model – as you did in Exercise 3).
• Complie and Test your program
14
Built-in math functions
• The Math class
– Automatically loaded: no import required.
• Math.abs() – absolute value | x |
• Math.pow() – exponentiation
• Math.sqrt() – square root √ x
• Examples
– Math.abs(-3) Result: 3 | -3 | = 3
– Math.pow(2,5)Result: 32.0 25 = 32
– Math.sqrt(49)Result: 7.0 √ 49 = 7
• See other math functions in Section 5.9 of the textbook
• On line description at
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Math.html
15
Exercise 5
• Consider the following problem: Given coordinates of
2 points in the plane (xa,ya) and (xb, yb), compute
the distance between the two points, according to the
following formula:
Write a complete Java program for the above problem.
(Use two methods – as you did in Exercises 3 and 4).
Compile and test your code.
(xa − xb)
2 +(ya − yb)
2
16
Note
• According to standard convention, the class names in
Java start with Upper-case and names for variables
and methods start with lower case.
• Use indentation to make your programs readable
– HINT: in Dr Java, if you type Cntrl-A (all your
code will be selected) and then type Tab, Dr Java
will organize your code using standard indentation
convention.
