1. Announcements
Lab 5 due Wednesday at noon

2. Questions? Classes, Instance Vars, & Methods visibility
accessors/mutators
parameters
interface versus implementation

3. Today in COMP 110 Review Some common mistakes (recent)
Programming Demo
Objects & References

4. Review Started with simple arithmetic Needed choice Needed repetition
variables and expressions
Needed choice
Perform different actions based on different inputs
if statements
switch statements
Needed repetition
Can do known number of iterations without loops
Just copy and paste code (Triangle)
For variable number of iterations:
while, do-while, for loops

5. Review Duplicating code is hazardous
prone to mistakes, tedious to change (Sandwiches)
consolidate replicated code into a method
may want the method to take in a value (parameter)
may want the method to return a value (return type)
Duplicating data is also hazardous
finite amount must be known at compile-time
run out of variable names!
code becomes too long and not readable
Use classes instead

6. Programming Constructs
Variables/Expressions
Conditionals
Loops
Methods
Classes
(also enumerations)
More to come...

7. If-Statement Example What is the output? int x = 7; int y = 5;
if(y > x)
x = x + y;
System.out.println("y > x");
System.out.println("y <= x");
Output
y > x
y <= x

8. If-Statement
Remember to include curly braces for if-statement bodies that include multiple statements
int x = 7;
int y = 5;
if(y > x) {
x = x + y;
System.out.println("y > x"); }
System.out.println("y <= x");

9. Local Variables What is the output of greet()? Output hello
public class Example {
private String str = “hello”;
public void foo() {
String str = “goodbye”; }
public void greet() {
foo();
System.out.println(str);
Output
hello

10. Local Variables What is the output of greet()? Output goodbye
public class Example {
private String str = “hello”;
public void foo() {
str = “goodbye”; }
public void greet() {
foo();
System.out.println(str);
Output
goodbye 10

11. Accessors & Mutators public class Example {
private double data; //private, can not be accessed directly //from OUTSIDE the class
public double getData() { //this is an accessor
return data; //this method is the ONLY way to access
} //“data”
public void setData(double newData) { //this is a mutator
data = newData; //this method is the ONLY way to
} //change “data” }

12. Accessors & Mutators public class ExampleTest {
public static void main(String[] args) {
Example e = new Example();
e.data = 6.5; //not allowed, data is private
e.setData(6.5); //ok, only way to set data
double d = e.getData(); //ok, only way to get data }

13. Programming Demo Grading Program Functionality
Two quizzes – 10 points each
A midterm and final – 100 points each
Final Exam – 50%
Midterm – 25%
Quizzes – 25%
Functionality
Read in a students score and display record

14. Programming Demo Approach Instance variable for each score
Method to read input
Method to display student record
Including total score and final grade(A-F)
Use two helper methods
One to calculate total score
Other to get letter grade

15. Programming Demo
Programming

16. Variables of a Primitive Type
Variables of primitive type hold a value
int a = 6;
double d = 6.55;
boolean b = a > d;
We can say
The value of a is 6
The value of d is 6.55
The value of b is false

17. Variables of a Class Type
Classes can have multiple data members
public class Student {
public String name;
public int year;
public double GPA;
public String major;
//… }
What is the “value” of a variable of a class type?
Student jack = new Student();

18. Variables of a Class Type
The value of a variable of a class type is a memory address
The address of the object it refers to
The address to this other location is called a reference to the object
Class types are also called reference types

19. Example: Books
public class Book { private String name; private int page; public void setPage(int page) { this.page = page; } public void setName(String name) { this.name = name;

20. Example: Books Assume we have a class named Book vs.
Book jacksBook = new Book();
Book samsBook = new Book();
vs.
Book samsBook = jacksBook;
//samsBook now refers to the same object as jacksBook

21. Objects in Memory Memory jacksBook samsBook 2078 1056 2078 ? 2078 ? ?
Book jacksBook;
Book samsBook;
jacksBook = new Book();
jacksBook.setName("Java");
samsBook = new Book();
samsBook.setName("Java");
jacksBook.setPage(137);
samsBook.setPage(253);
samsBook = jacksBook;
samsBook.setPage(509);
jacksBook is now on p. 509!
jacksBook
samsBook
2078
1056
2078 ?
2078 ? ?
2078 ?
1056
2078
Java ?
137 ? ?
Java
Java ?
Java
253
137
Java
253
509

22. Remember Variables of a class type contain memory addresses
NOT objects themselves

23. == Operator on Objects
The == operator checks whether the values of two variables are the same
The value of class variable is a memory address
When using the == operator to compare two objects, you are checking whether they have the same address in memory

24. == vs. equals() for Strings Explained
String is a class type
What happens when you have
String s1 = new String("Hello");
String s2 = new String("Hello");
boolean strEqual = (s1 == s2);
strEqual is false! Why?
s1 and s2 store different addresses!

25. == vs. equals() for Strings explained
What happens when you have
String s1 = new String("Hello");
String s2 = new String("Hello");
boolean strEqual = (s1.equals(s2));
strEqual is true! Why?
String’s .equals() method checks if all the characters in the two Strings are the same

26. Defining an equals Method
Every class has a default .equals() method if it is not explicitly written
Does not necessarily do what you want
You decide what it means for two objects of a specific class type to be considered equal
Perhaps books are equal if the names and page numbers are equal
Perhaps only if the names are equal
Put this logic inside .equals() method

27. Writing the .equals() Method
public class Book { private String name; private int page; //… public boolean equals(Book book) { return (this.name.equals(book.name) && this.page == book.page); }

28. Parameters of a Primitive Type
public void increaseNum(int num) {
num++; }
public void foo() {
int x = 5;
increaseNum(x);
System.out.println(x);
What is the output? 5

29. Parameters of a Class Type
public void changeBook(Book book) {
book = new Book("Biology"); }
public void foo() {
Book jacksBook = new Book("Java");
changeBook(jacksBook);
System.out.println(jacksBook.getName());
What is the output?
Java

30. Parameters of a Class Type
public void changeBook(Book book) {
book.setName("Biology"); }
public void foo() {
Book jacksBook = new Book("Java");
changeBook(jacksBook);
System.out.println(jacksBook.getName());
What is the output?
Biology 30

31. Wednesday
Constructors