1. Michele Weigle - COMP 14 - Spr 04 Catie Welsh February 21, 2011
COMP 110 Classes
Catie Welsh
February 21, 2011

2. Michele Weigle - COMP 14 - Spr 04
Announcements
Lab 4 due on Friday

3. Michele Weigle - COMP 14 - Spr 04
Questions?
Any other questions?

4. Michele Weigle - COMP 14 - Spr 04
Today in COMP 110
Briefly go over Strings and Loops Worksheet
Briefly go over Program 2
Classes

5. Program 2
Operand 1
Operator
Operand 2 1 2 . 7 + 3 4 5 6 8 9

6. Program 2 1 2 . 7 + 3 4 5 6 8 9
Assume the String above is in the variable calculation.
int firstSpace = calculation.indexOf(‘ ’);
int lastSpace = calculation.lastIndexOf(‘ ’);
double operand1 = Double.parseDouble(calculation.substring(0, firstSpace));
double operand2 = Double.parseDouble(calculation.substring(lastSpace + 1));
char operator = calculation.charAt(firstSpace + 1);

7. Program 2
Now you can determine which calculation to perform using a switch or if/else
double result = 0.0;
switch (operator) {
case ‘+’:
result = operand1 + operand2;
break;
case ‘-’:
result = operand1 – operand2;
// and so on }

8. Exponent ^ double result = 0.0; switch (operator) { case ‘^’:
if(operand2 > 0)
for (int count = 0; count < (int) operand2; count++)
result *= operand1; }
else if(operand2 < 0)
for (int count = 0; count > (int) operand2; count--)
result = 1.0 / result;
break;
case ‘-’:
result = operand1 – operand2;
// and so on

9. == generally bad for floating-point
Floating-point numbers are imprecise
After doing many computations, value may not be exactly the same
5.0 * 3.0 might be different from
Okay for this assignment for the divide by 0 error checking

10. num++ vs. ++num num++ is NOT num + 1 num++ does num = num + 1
So does ++num, BUT, there is a difference
int num1 = 5;
System.out.println(num1++);
// outputs num1 (5), then increments num1
int num2 = 5;
System.out.println(++num2);
// increments num2, then outputs num2 (6)

11. Classes and Objects
Java programs (and programs in other object- oriented programming languages) consist of objects of various class types
Objects can represent objects in the real world
Automobiles, houses, employee records
Or abstract concepts
Colors, shapes, words

12. Class A class is the definition of a kind of object
A blueprint for constructing specific objects
Class Name: Automobile
Data:
amount of fuel
speed
license plate
Methods (actions):
accelerate:
How: Press on gas pedal.
decelerate:
How: Press on brake pedal.

13. Objects, Instantiation
Object Name: patsCar
amount of fuel: 10 gallons
speed: 55 miles per hour
license plate: “135 XJK”
Object Name: suesCar
amount of fuel: 14 gallons
speed: 0 miles per hour
license plate: “SUES CAR”
Object Name: ronsCar
amount of fuel: 2 gallons
speed: 75 miles per hour
license plate: “351 WLF”
Instantiations, or instances, of the class Automobile

14. UML (Universal Modeling Language)
Automobile
fuel: double
speed: double
license: String
+ accelerate(double pedalPressure): void
+ decelerate(double pedalPressure): void
Class name
Data
Methods (actions)

15. Objects
Important: classes do not have data; individual objects have data
Classes specify what kind of data objects have

16. Class files and separate compilation
Each Java class definition goes in its own, SEPARATE .java file
ClassName  save the file as ClassName.java
Student.java includes the class Student

17. Class files and separate compilation
What happens when you compile a .java file?
.java file gets compiled into a .class file
Contains Java bytecode
Same filename except for .class instead of .java
You can compile a Java class before you have a program that uses it

18. class Student Class Name: Student Name Year GPA Major Credits GPA sum
+ getName
+ getMajor
+ printData
+ increaseYear
How: increase year by 1
+ calcGpa
How: average grades

19. class Student Class Name: Student name: String year: int gpa: double
major: String
credits: int
gpaSum: double
+ getName(): String
+ getMajor(): String
+ printData(): void
+ increaseYear(): void
+ calcGpa(double grade): void

20. Defining a class Class name Data (instance variables) Methods
public class Student {
public String name;
public int classYear;
public double GPA;
public String major;
// ...
public String getMajor()
return major; }
public void increaseYear()
classYear++;
Class name
Data
(instance variables)
Methods

21. Creating an object
Create an object jack of class Student Student jack = new Student(); Scanner keyboard = new Scanner(System.in); Create an object keyboard of class Scanner
Assign memory address of object to variable
Return memory address of object
Create an object

22. Instance variables
Data defined in the class are called instance variables
public String name;
public int classYear;
public double GPA;
public String major;
variables
public: no restrictions on how these instance variables are used (more details later – public is actually a bad idea here)
type: int, double, String…

23. Using instance variables: inside the class definition
public class Student {
public String name;
public int classYear;
public double GPA;
public String major;
// ...
public String getMajor()
return major; }
public void increaseYear()
classYear++;

24. Using public instance variables outside a class
public static void main(String[] args) {
Student jack = new Student();
jack.name = “Jack Smith”;
jack.major = “Computer Science”;
System.out.println(jack.name + “ is majoring in ” + jack.major);
Student apu = new Student();
apu.name = “Apu Nahasapeemapetilon”;
apu.major = “Biology”;
System.out.println(apu.name + “ is majoring in ” + apu.major); }
jack.name and apu.name are two different instance variables because they belong to different objects

25. Methods Two kinds of methods Methods that return a value
Examples: String’s .substring() method, String’s .indexOf() method, etc.
Methods that return nothing
Example: System.out.println()

26. Methods returns a String return type returns nothing
public String getMajor() { return major; } public void increaseYear() classYear++;
returns a String
return type
returns nothing

27. Defining methods that return nothing
Method heading: keywords
public: no restriction on how to use the method (more details later)
void: the method returns nothing
Method body: statements executed when the method is called (invoked)
Must be inside a pair of braces
public void increaseYear() {
classYear++; }

28. Method printData
As usual, inside a block (defined by braces), you can have multiple statements
public void printData() {
System.out.println(“Name: ” + name);
System.out.println(“Major: ” + major);
System.out.println(“GPA: ” + gpa); }

29. Calling methods that return nothing
object, followed by dot, then method name, then ()
Use them as Java statements
Student jack = new Student();
jack.classYear = 1;
jack.increaseYear();
System.out.println(“Jack’s class year is ” + jack.classYear);

30. Defining methods that return a value
Method heading: keywords
public: no restriction on how to use the method (more details later)
Type: the type of value the method returns
Method body: statements executed
Must be inside a pair of braces
Must have a return statement
public String getMajor() {
return major; }

31. return statement
A method that returns a value must have at least one return statement
Terminates the method, and returns a value to the caller
Syntax:
return Expression;
Expression can be any expression that produces a value of type specified by the return type in the method heading

32. Methods that return a value
public String getClassYear() { if (classYear == 1) return “Freshman”; else if (classYear == 2) return “Sophomore”; else if ... }

33. Calling methods that return a value
object, followed by dot, then method name, then () (same as before)
Use them as a value of the type specified by the method’s return type
Student jack = new Student();
jack.major = “Computer Science”;
String m = jack.getMajor();
System.out.println(“Jack’s full name is ” + jack.getName());
System.out.println(“Jack’s major is ” + m);

34. return statement Can also be used in methods that return nothing
Terminates the method
Syntax:
return;
public void increaseYear() {
if (classYear >= 4)
classYear++; }

35. Michele Weigle - COMP 14 - Spr 04
Wednesday
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