Computer Science 026a

1
Computer Science 026a
Distilled
2
Agenda

• Module 1 Introduction and Definitions
• Module 2 Algorithms
• Module 3 Primitive Data Types - Numeric
• Module 4 Primitive Data Types - Boolean
• Module 5 Iteration
• Module 6 Modularity
• Module 7 Object-Oriented Design
• Module 8 Equivalence
• Module 9 Strings
• Module 10 Arrays

3
Purpose of This Review

• Not a substitute for good, hard studying of your
  notes.
• 3 hours is very little time to go over all of the
  material in the course. Well be moving fast.
  Remember this is a review session.
• Feel free to ask questions as we cover the
  material.
• At the end, well open up the floor to more in
  depth questions, and revisit topics that are
  still unclear.

4
Computer Science 026a
Distilled
Module 1 Introduction
5
Definitions

• Computer Science
• The creation and investigation of a scientific
  foundation for the study of computing.
• Computing
• Computing is simply problem solving
  particularly problems that deal with data.
• Data
• Data is just another word for information.
  Computer programs process data to accomplish some
  task.
• In a computer, data is represented by binary
  digits (bits) 1s and 0s.
• Computer
• A device that executes and follows a set of
  instructions to carry out some computing
  activity.

6
Definitions

• Central Processing Unit (CPU)
• Also known as the processor
• Executes instructions.
• Main Memory (RAM)
• Internal storage that holds the programs
  (instructions and data) currently being executed
  by the CPU.
• Volatile information not retained when the
  computer is turned off.

7
Definitions

• Secondary Memory
• Diskettes, hard disks, CDs, DVDs, USB sticks
• Long-term (persistent) storage for programs and
  other information.
• Organized as files each of which has a file
  name and a folder or directorythat contains it.
• Input/Output (I/O) Devices
• Keyboard, mouse, screen, printer, webcam,
• Used for communicating information from the user
  to the computer and the computer to the user.

8
Definitions

• Computer program
• A sequence of statements which specifies how to
  accomplish a particular task in a specific
  programming language.
• Comprised of instructions that tell the computer
  what to do, and data that the computer uses to
  accomplish the task at hand.
• A computer will always do exactly what it is
  instructed to do by a program it does not think
  on its own!

9
Definitions

• Programming
• The process of creating, representing, and
  implementing tasks on a computer to solve a
  particular problem and produce a computer
  program.
• Requires a language that can be translated into
  machine language for the computer to execute.

10
Definitions

• Programming Language
• A special language that allows a computer program
  to be written.
• All programming languages can be classified under
  one of the following types
• Compiled A special program called a compiler
  translates the program into an executable
  program.
• Interpreted Each line of the program is compiled
  on the fly and executed as the program runs.
• Hybrid More on this later.

11
Definitions

• Source code
• A file (or files) containing the program written
  in a high-level language (e.g. Java, C, Pascal).
• Cannot be executed by the CPU (must be translated
  first).
• Executable (or binary) code
• A program that has been translated into machine
  language code.
• This translation occurs by running the source
  code through a compiler or assembler.
• Can be executed by the CPU.

12
Definitions

• Pseudo-code
• An outline of a program, written in English, that
  can be easily translated into a real computer
  program.
• Programming-language independent
• Any given piece of pseudo-code can then be
  translated into any number of different computer
  programming languages (Java, Visual Basic, C,
  C, etc.)
• Gets you to think about the logic of the problem
  before worrying about the finer details of
  programming.

13
Definitions

• Software
• Refers to computer programs. Types of software
• Operating Systems Control the hardware and
  software of a computer. Examples Windows XP,
  Linux, Mac OS.
• Application Software Word processors,
  spreadsheets, web browsers.
• Compilers Translates high-level language
  programs into a form (machine language) that can
  be executed by the CPU.

14
Definitions

• Computer network
• A group of interconnected computers.
• Internet
• A world-wide interconnection of computers.
• World Wide Web
• An application running on top of the Internet.
  The web is not the same as the Internet.
• Hosts web pages that can be viewed through a web
  browser (i.e. Internet Explorer, Firefox)

15
Types of Languages

• Machine language
• The set of instructions that the computers CPU
  can execute directly.
• Machine dependent. Cannot execute machine
  language for a PC on a Mac, and vice versa.
• Difficult for programmers to use. Imagine
  writing a program like this

000100111000010100100110101111001
16
Types of Languages

• Assembly language
• Mnemonic representation of machine language.
• Easier than machine language, but still
  difficult.
• Assembler Software that translates assembly
  language programs into machine language.
• One to one correspondence exists between assembly
  language instructions and machine language
  instructions.

LOAD R1, PRICELOAD R2, TAXADD R1, R2, R6STOR
R6, TOTAL
17
Types of Languages

• High-level language
• English-like language that makes programming
  easier.
• One to many correspondence exists between
  high-level language instructions and machine
  language instructions.
• Machine-independent. A program written in Java
  can be executed on a PC, and also a Mac.
• High-level language program segment example

if ((numberOfStudents) MAX_STUDENTS)
fullCourse true
18
von Neumann Architecture

• In old computers, programs were wired directly
  into the hardware.
• Todays computers are based on the von Neumann
  architecture
• Refers to a computer design model that uses a
  single storage structure (main memory) to hold
  both programs and data.
• This storage structure is main memory it
  contains programs written in machine language
  and the data for the programs.

19
von Neumann Architecture

• Employs a simple fetch-execute cycle
• do
• get next machine language instruction execute
  the instruction
• while there are no more instructions

20
The Java Language

• A high-level language
• Developed in the mid 1990s
• Object-oriented
• Portable
• Java is a hybrid language. It is compiled, but
  not directly into machine language. Instead, it
  is compiled into an intermediate language called
  Java bytecode.

21
The Java Language

• The Java interpreter (aka Java Virtual Machine)
  then translates and executes the bytecode on the
  computer, one bytecode instruction at a time.
• This makes Java very portable create one
  compiler, and then create an interpreter for each
  platform.
• Java bytecode is machine independent.

22
Computer Science 026a
Distilled
Module 2 Algorithms
23
What is an Algorithm

• An ordered set of effective, unambiguous
  instructions which, when executed, terminates.
• Ordered Steps are executed in a clear,
  sequential order (one after another).
• Effective Each step is possible (can be done)
• Unambiguous There is no uncertainty as to what
  to do, or how to perform the step
• Terminates The set of instructions will
  terminate in some finite amount of time.
• If you write a set of instructions that does not
  meet the above criteria, it is not an algorithm!

24
What is an Algorithm

• Once an algorithm is written, we can just blindly
  follow it we dont have to know how it works.
• A recipe is a good example of an algorithm. If
  youre baking bread, you dont have to know how
  the yeast works at a biological level. You just
  have to follow the steps of the recipe as they
  were given to you, and voilà! You have bread.

25
Algorithmic Constructs

• Data storage/retrieval (aka input/output)
• Information (data) is stored in variables so that
  it can be retrieved and used later.
• Pseudo-code keywords
• GET
• Obtain a value (e.g. from the user) and store it
  in main memory
• REPORT
• Display a value stored in main memory to the user
  
• LET
• Store a value in a location in main memory

26
Algorithmic Constructs

• Conditional execution
• Instructions may or may not be executed,
  depending on some condition at time of execution
  (run time).
• Pseudo-code keywords
• IF-THEN
• Decides whether or not some sequence of
  instructions will be executed
• IF-THEN-ELSE
• Selects one of two sequences of instructions to
  be executed

27
Algorithmic Constructs

• Repetition
• A sequence of instructions is executed 0, 1, or
  more times
• Also called a loop
• Pseudo-code keywords
• WHILE
• FOR
• DO-WHILE

28
Feeling Loopy Yet?

• There are 2 types of loops
• Pre-tested
• Body executed 0 or more times
• Two main forms
• FOR DO body statements
• WHILE
• END WHILE

29
Feeling Loopy Yet?

• Post-tested
• Loop body executed 1 or more times
• Continues to repeat the execution of the body statements as long as is true
• DO WHILE

30
Feeling Loopy Yet?

• Other ways to classify loops
• Deterministic (Definite) We know before the
  loop starts how many times it will execute.
• The for loop is deterministic.
• Non-Deterministic (Indefinite) We cannot know
  before the loop starts how many times it will
  execute. It just loops until its condition
  becomes false.
• The WHILE and DOWHILE loops are
  non-deterministic.

31
From Pseudo-code to Java

• Data storage/retrieval
• REPORT
• GET
• Consists of two Java statements a prompt
  statement and an input statement

System.out.println("The average of the numbers is
" (float) sum /
howmany)
System.out.print("Please enter number " i "
")aNumber Integer.parseInt(keyboard.readLine()
)
32
From Pseudo-code to Java

• Data storage/retrieval (contd)
• LET
• is the assignment operator it does not
  represent equality in the mathematical sense!

int howmany3
33
From Pseudo-code to Java

• Conditional execution
• IF-THEN-ELSE
• Note If more than one statement appears below
  the if or the else, they must be enclosed in
  braces.

if (jeffShutsUp true) System.out.println(
Hurray!) else if (jeffFaints true)
System.out.println(Haha!) else
System.out.println(Boo!)
34
From Pseudo-code to Java

• Repetition
• FOR howMany REPETITIONS
• Note If more than one statement appears below
  the for, they must be enclosed in braces.

for (int i 0 i System.out.println(Hello!)
35
From Pseudo-code to Java

• Repetition
• WHILE
• Note If more than one statement appears below
  the while, they must be enclosed in braces.

int i 0 while (i System.out.println(Hello!) i
36
From Pseudo-code to Java

• Repetition
• DOWHILE
• Note If more than one statement appears between
  the do and the while, they must be enclosed in
  braces.

int i 0 do System.out.println(Hello!)
i while (i
37
From Pseudo-code to Java

• Each Java file has to have the same name
  (case-sensitive) as the class it contains.
• This would have to be in a file named Test.java
• To compile this class, we use javac
• This produces the file Test.class our compiled
  bytecode. To run this program, we use java

public class Test . . .
javac Test.java
java Test
38
Stepwise Refinement

• The process of breaking a problem down into
  smaller sub-problems.
• Each sub-problem is implemented in a separate
  module.
• The main algorithm then uses each module to
  accomplish its task.
• We say that the main algorithm is at the highest
  level of abstraction, while modules are at lower
  levels.
• This all leads to the notion of modularity, which
  makes our code easier to modify, maintain, and
  reuse.

39
Computer Science 026a
Distilled
Module 3 Primitive Data Types - Numeric
40
Primitive Types

• Store simple data values. Primitives can only
  store 1 value at a time. If we assign a value to
  a primitive type, it overwrites (replaces) the
  value that was already there.
• Type of primitives
• Integers (int)
• , -2, -1, 0, 1, 2,
• Real numbers (float, double)
• , -2.12, -2.101, -1.999, 0.0, 1.5, 2.2789,
• Booleans (boolean)
• true or false

41
Primitive Types

• The values stored in primitives are called
  literals
• -1 is an integer literal
• 112.0 is a floating-point literal
• true is a boolean literal
• Each different primitive type is represented in
  memory in a different way. So, we have to
  declare the types of our variables so that
  compiler knows how to represent our variable in
  memory.

42
Primitive Types

• Declaring a variable Reserving a named space in
  main memory where a value will be stored, and
  explicitly specifying its data type.
• Syntax
• Example

int myInteger
43
Primitive Types

• int
• Used to store integral values
• Stored in 32 bits
• Can store numbers up to approx. 2 billion
• long
• Used to store larger integral values
• Stored in 64 bits
• byte, short
• Used to store smaller integral values
• Stored in 8 bits and 16 bits, respectively

44
Primitive Types

• float
• Used to store smaller floating point values
• Stored in 32 bits
• Has a precision of approximately 7 digits
• double
• Used to store larger floating point values
• Stored in 64 bits
• Has a precision of approximately 15 digits.
• Used more commonly in Java to avoid rounding
  errors

45
Java Arithmetic Operators

• Addition
• Subtraction -
• Multiplication
• Division /
• When performed on integers, returns an integer
• 9 / 2 4 (not 4.5!)
• When performed on doubles, returns a double
• 9.0 / 2.0 4.5
• When performed on a mix of both, returns a double
• 9.0 / 2 4.5
• Modulus
• Remainder of a division
• 12 7 5

46
Precedence Rules

• Java evaluates arithmetic expressions in the same
  order as we do normally in mathematics
• Parentheses
• Multiplication, division, modulus from left to
  right
• Addition, subtraction from left to right
• Examples
• 12 6 5 13
• 10 2 5 / 3 10
• 10 2 5 / 3.0 10.66

47
Named Constants

• Magic Numbers
• A number in source code that means something to
  the programmer, but looks like gibberish to other
  readers. Example
• The reader is left wondering what does the 28
  represent?
• Solution named constants!

if (i 28) System.out.println(Hello!)
48
Named Constants

• We can declare a named constant using the keyword
  final to make the code more readable
• Constants must be declared and assigned in the
  same line. After they are declared, they cannot
  be changed at a later time. They are constant.
• Convention is to use capital letters and separate
  words by underscores.

final int HELLO_MENU_OPTION 28 if (i
HELLO_MENU_OPTION) System.out.println(Hello!)

49
Assignment Rules

• An integer value can be assigned to
• An integral variable type
• A floating point variable type
• A floating point value can only be assigned to a
  floating-point variable type
• However, we can cheat and change a values type
  by typecasting it. For example
• i will now store the value 5

int i 5
double i 5
double i 5.0
int i (int)5.4
50
Computer Science 026a
Distilled
Module 4 Primitive Data Types - Boolean
51
Conditional Execution

• Recall that our if statements have the
  following form
• is nothing more than a Boolean
  expression (an expression that evaluates to true
  or false)
• If it is true, then statement is executed.
• If it is false, then statement is not executed.

if ( ) statement
52
Comparison Operators

• Boolean expressions compare values using
  relational operators

53
Logical Operators

• Boolean expressions can be combined using the
  logical operators
• Although not explicitly required, good style
  dictates that we should put parentheses around
  each individual condition, as well as around the
  entire compound condition.

54
Logical Operators

• Truth table for the logical operators

55
Logical Operator Precedence

• Java evaluates logic operators according to the
  following precedence
• Parentheses
• ! (Negation)
• (AND)
• (OR)
• Example
• A B !C
• evaluates as
• A (B !C)

56
Primitive Type boolean

• In Java, boolean is a primitive type that have
  can a value of either true or false.
• We can declare variables of type boolean and
  assign values to them
• Shortcut when using boolean variables in
  conditions, we can say
• This is equivalent to

boolean isThisOverYet false
if (isThisOverYet)
if (isThisOverYet true)
57
Nested If Statements

• if statements can be nested within other if
  statements
• Notice that we dont need braces in this case.
  This is because an entire if-else block is
  considered to be one statement.

if (age 16) if (hasLicense) System.out.pri
ntln(Turn on the car!) else System.out.pri
ntln(Sorry, go get a license!) else
System.out.println(Youre not even old enough!)
58
The Evil Dangling Else

• Consider the following code
• Problem Do we associate the else with the outer
  if statement, or the inner if statement? We call
  this the dangling else problem.

if (age 16) if (hasLicense) System.out.pri
ntln(Turn on the car!) else
System.out.println(Youre not even old enough!)
59
The Evil Dangling Else

• Solution Java implements the dangling else
  rule. This rule states that an else will be
  associated with the nearest if statement.
• So, in the previous example, the else will be
  associated with the inner if statement
• Result Not what we wanted!

if (age 16) if (hasLicense) System.out.pri
ntln(Turn on the car!) else System.out.pri
ntln(Youre not even old enough!)
60
The Evil Dangling Else

• Solution Use braces to explicitly tell Java the
  if statement with which the else should
  be associated.

if (age 16) if (hasLicense) System.out.p
rintln(Turn on the car!) else
System.out.println(Youre not even old enough!)
61
Switch Statement

• Sometimes, we have so many conditions to test
  that we have a long chain of if-else statements.
• We can use a switch statement to replace this
  chain

if (month 1) else if (month 2)
. . .
switch (month) case 1 case
3 break case 2 break
62
Switch Statement

• Case values must be integers
• We can put multiple cases above the same block of
  code.
• The default case is equivalent to an else
  statement. If no other cases match, then the
  code under the default case is executed.
• Important every case must end with a break!

63
Switch Statement - Example
switch (menuOption) case
1 System.out.println(Rock, Paper,
Scissors) break case 2 System.out.println(
One Two Penny Pickup) break case
3 System.out.println(Quit!) break
default System.out.println(Invalid choice,
please try again.) break
64
Computer Science 026a
Distilled
Module 5 Iteration
65
Control Structures

• Statements are executed in sequential order.
• Problem How to execute statements multiple
  times?
• Solution Use a loop!
• Java has 3 loop structures
• for (pre-tested, deterministic)
• while (pre-tested, non-deterministic)
• dowhile (post-tested, non-deterministic)

66
The for Loop

• Performed once before loop
  begins
• Checked before loop starts and after
  the end of each iteration. Loop starts/continues
  as long as it is true.
• Performed at the end of each
  iteration
• Statement performed each iteration.

for ( )

67
The for Loop

• Example Executing a statement 3 times.
• Lets examine the flow of execution

for (int i 0 i System.out.println(This is iteration i)
68
The for Loop
Declare a variable i and set it to 0
for (int i 0 i System.out.println(This is iteration i)
Output
i 0
69
The for Loop
Check is i less than 3?
for (int i 0 i System.out.println(This is iteration i)
Output
i 0
70
The for Loop
Yep, lets execute the loop!
for (int i 0 i System.out.println(This is iteration i)
Output
i 0
71
The for Loop
for (int i 0 i System.out.println(This is iteration i)
Output This is iteration 0
i 0
72
The for Loop
Increment i by 1
for (int i 0 i System.out.println(This is iteration i)
Output This is iteration 0
i 1
73
The for Loop
Check is i less than 3?
for (int i 0 i System.out.println(This is iteration i)
Output This is iteration 0
i 1
74
The for Loop
Yep, lets execute the loop!
for (int i 0 i System.out.println(This is iteration i)
Output This is iteration 0
i 1
75
The for Loop
for (int i 0 i System.out.println(This is iteration i)
Output This is iteration 0 This is iteration 1
i 1
76
The for Loop
Increment i by 1
for (int i 0 i System.out.println(This is iteration i)
Output This is iteration 0 This is iteration 1
i 2
77
The for Loop
Check is i less than 3?
for (int i 0 i System.out.println(This is iteration i)
Output This is iteration 0 This is iteration 1
i 2
78
The for Loop
Yep, lets execute the loop!
for (int i 0 i System.out.println(This is iteration i)
Output This is iteration 0 This is iteration 1
i 2
79
The for Loop
for (int i 0 i System.out.println(This is iteration i)
Output This is iteration 0 This is iteration
1 This is iteration 2
i 2
80
The for Loop
Increment i by 1
for (int i 0 i System.out.println(This is iteration i)
Output This is iteration 0 This is iteration
1 This is iteration 2
i 3
81
The for Loop
Check is i less than 3?
for (int i 0 i System.out.println(This is iteration i)
Output This is iteration 0 This is iteration
1 This is iteration 2
i 3
82
The for Loop
No sir, were moving on!
for (int i 0 i System.out.println(This is iteration i)
Output This is iteration 0 This is iteration
1 This is iteration 2
i 3
83
The while Loop

• Checked before loop starts and after
  the end of each iteration. Loop starts/continues
  as long as it is true.
• Statement performed each iteration.
• Note The body needs to contain a statement that
  will eventually make the condition false.
  Otherwise, we will have what is known as an
  infinite loop.

while ()
84
The while Loop

• Example Executing a statement 3 times.
• Lets examine the flow of execution

int i 0 while (i This is iteration i) i
85
The while Loop
int i 0 while (i This is iteration i) i
Output
i 0
86
The while Loop
Check Is i int i 0 while (i This is iteration i) i
Output
i 0
87
The while Loop
Yep, lets execute the loop!
int i 0 while (i This is iteration i) i
Output
i 0
88
The while Loop
int i 0 while (i This is iteration i) i
Output This is iteration 0
i 0
89
The while Loop
int i 0 while (i This is iteration i) i
Output This is iteration 0
i 1
90
The while Loop
Check Is i int i 0 while (i This is iteration i) i
Output This is iteration 0
i 1
91
The while Loop
Yep, lets execute the loop!
int i 0 while (i This is iteration i) i
Output This is iteration 0
i 1
92
The while Loop
int i 0 while (i This is iteration i) i
Output This is iteration 0 This is iteration 1
i 1
93
The while Loop
int i 0 while (i This is iteration i) i
Output This is iteration 0 This is iteration 1
i 2
94
The while Loop
Check Is i int i 0 while (i This is iteration i) i
Output This is iteration 0 This is iteration 1
i 2
95
The while Loop
Yep, lets execute the loop!
int i 0 while (i This is iteration i) i
Output This is iteration 0 This is iteration 1
i 2
96
The while Loop
int i 0 while (i This is iteration i) i
Output This is iteration 0 This is iteration
1 This is iteration 2
i 2
97
The while Loop
int i 0 while (i This is iteration i) i
Output This is iteration 0 This is iteration
1 This is iteration 2
i 3
98
The while Loop
Check Is i int i 0 while (i This is iteration i) i
Output This is iteration 0 This is iteration
1 This is iteration 2
i 3
99
The while Loop
No sir, were moving on!
int i 0 while (i This is iteration i) i
Output This is iteration 0 This is iteration
1 This is iteration 2
i 3
100
The dowhile Loop

• Checked after the loop has executed
  once, and after each subsequent iteration. Loop
  continues as long as it is true.
• Statement performed each iteration.
• Note The body needs to contain a statement that
  will eventually make the condition false.
  Otherwise, we will have what is known as an
  infinite loop.

do while ( )
101
The dowhile Loop

• Example Executing a statement 3 times.
• Lets examine the flow of execution

int i 0 do System.out.println(This is
iteration i) i while (i
102
The dowhile Loop
int i 0 do System.out.println(This is
iteration i) i while (i Output
i 0
103
The dowhile Loop
int i 0 do System.out.println(This is
iteration i) i while (i Output This is iteration 0
i 0
104
The dowhile Loop
int i 0 do System.out.println(This is
iteration i) i while (i Output This is iteration 0
i 1
105
The dowhile Loop
int i 0 do System.out.println(This is
iteration i) i while (i Check Is i Output This is iteration 0
i 1
106
The dowhile Loop
int i 0 do System.out.println(This is
iteration i) i while (i Yep, lets execute the loop!
Output This is iteration 0
i 1
107
The dowhile Loop
int i 0 do System.out.println(This is
iteration i) i while (i Output This is iteration 0 This is iteration 1
i 1
108
The dowhile Loop
int i 0 do System.out.println(This is
iteration i) i while (i Output This is iteration 0 This is iteration 1
i 2
109
The dowhile Loop
int i 0 do System.out.println(This is
iteration i) i while (i Check Is i Output This is iteration 0 This is iteration 1
i 2
110
The dowhile Loop
int i 0 do System.out.println(This is
iteration i) i while (i Yep, lets execute the loop!
Output This is iteration 0 This is iteration 1
i 2
111
The dowhile Loop
int i 0 do System.out.println(This is
iteration i) i while (i Output This is iteration 0 This is iteration
1 This is iteration 2
i 2
112
The dowhile Loop
int i 0 do System.out.println(This is
iteration i) i while (i Output This is iteration 0 This is iteration
1 This is iteration 2
i 3
113
The dowhile Loop
int i 0 do System.out.println(This is
iteration i) i while (i Check Is i Output This is iteration 0 This is iteration
1 This is iteration 2
i 3
114
The dowhile Loop
int i 0 do System.out.println(This is
iteration i) i while (i No sir, were moving on!
Output This is iteration 0 This is iteration
1 This is iteration 2
i 3
115
Computer Science 026a
Distilled
Module 6 Modularity
116
Benefits of Modularity

• By breaking up a problem into modules, we make it
  easier to modify and maintain
• Dont have to spend time wading through mountains
  of code just edit the module containing the code
  to be changed
• Allows for easier code reuse
• Modules can be designed generically so that they
  are applicable to many different programs. We
  can then just plug them in to new applications
  as needed
• Result Less time spent developing code!
• Corollary More time spent sleeping!

117
Modularity in Java

• In Java, there are 3 main types of modules
• Packages
• Contain related sets of classes
• Can be imported into a program and used,
  achieving code reuse
• Example
• Recall the line from your Java programs import
  java.io.
• java.io is a package containing many different
  classes (including BufferedReader)
• The asterisk means that we want to import all
  classes in the java.io package

118
Modularity in Java

• Classes
• A collection of related data (state) along with
  methods (behaviour) to act upon that data
• Variables of class types are called reference
  variables
• This is in contrast to variables of primitive
  types, which are called simple variables
• Example
• BufferedReader is a class you used in your
  assignments to retrieve input from the user.
• More on classes later

119
Modularity in Java

• Methods
• Contained within classes
• Implement the behaviour of a class that is, they
  perform actions (like calculating how much longer
  Im going to talk)
• Example
• The BufferedReader class has a method with which
  you are all familiar readLine()
• It waits for the user to enter data and press
  Enter. Upon doing so, it returns the String
  entered by the user.
• Another Example
• The main method is a method.
• It is a special method that serves as the
  starting point of every program.

120
Methods

• Two types of methods
• Static methods
• Defined by using the keyword static in the method
  definition
• Recall public static void main(String args)
• Called on a class not on an object
• Math.abs(-1)
• Here, were using the class name (Math) not the
  name of a variable.
• However, if were invoking (calling) static
  methods that exist in the same class, we dont
  need to specify the class name
• Recall from assignment 3 searchForCompany(compan
  y)
• This is a static method, but since were calling
  it from the same class, we dont need to say
  TestPopcorn.searchForCompany

121
Methods

• Two types of methods
• Instance methods
• Defined without the keyword static in the method
  definition
• public void setCompanyName(String newName)
• Called on an object not on a class
• More on these methods a bit later

// instantiate (create) a new Person
object Person me new Person(Jeff) // method
is called on variable me not on the Person
class me.setEyeColour(Green)
122
Defining a Static Method

• Syntax
• The type of the value returned by
  the class. Some possible types
• int, String, double, float
• void Class doesnt return anything
• An identifier associated with the
  method that will be used to call it.
• A list of variable types
  that the method expects callers to pass in, along
  with names that will be used for those variables
  in the method.

public static (
)
123
Defining a Static Method

• Example
• getAverage is the name we will use to call this
  method
• This method expects callers to pass in 3 integer
  variables
• It returns a value of type double the average
  of values of the 3 parameters
• How do we know what it returns?
• Is it because of the return statement?

public static double getAverage(int num1, int
num2, int num3) return (double)(num1 num2
num3) / 3
124
Invoking a Static Method

• Example
• We invoke a static method by using its name
  followed by parentheses. If the method takes
  parameters, we put the values were passing into
  the method in the parentheses.
• Actual parameters The values were passing in
• Formal parameters The value types expected by
  the method (the contract of the method)

public static double getAverage(int num1, int
num2, int num3) return (double)(num1 num2
num3) / 3 public static void main (String
args) double avg getAverage(1,2,3)
125
Formal vs. Actual Parameters

• Formal parameters
• The contract of the method
• The method definition says, Hey! It you want to
  call me, this is what youre going to have to
  give me!
• If you pass in variables of types that are not
  defined in the formal parameters of the method,
  you get a compile error.
• Actual parameters
• The actual values that you pass into the method.
  These could be literals (-1, 1.201, etc.),
  constants (MAX_AGE), or variables (myAge)

126
Formal vs. Actual Parameters

• Example

/ This method declares 3 integers as its formal
parameters. It will refer to them using the
identifiers num1, num2, and num3 in the
method body. / public static double
getAverage(int num1, int num2, int num3)
return (double)(num1 num2 num3) /
3 public static void main (String args)
// 1, 2, and 3 are the actual
parameters double avg getAverage(1,2,3) /
Compile error! Not only does getAverage not
allow Strings, but it requires 3 parameters!
/ double avg2 getAverage(1)
127
Passing Parameters

• Passing by value
• When we pass primitive variable types (int,
  float, double, boolean, etc.) into a method, we
  pass them by value.
• This means that a copy of the variables get
  passed not the variables themselves.

public static void doSomething(int myParameter)
myParameter 4 public static void main
(String args) int i 5 doSomething(i) //
What will i equal here?
128
Passing Parameters

• Passing by reference
• When we pass class variable types (Rectangle,
  Person, etc.) into a method, we pass them by
  reference.
• This means that the objects themselves get passed
  not copies of the objects.

public static void doSomething(Person p)
p.setName(Jeff) public static void main
(String args) Person me new
Person(Bob) // create a new Person named
Bob doSomething(me) // What will me equal
here?
129
Method Overloading

• Defining multiple methods with the same name in
  the same class.

public static int max(int num1, int num2)
if (num1 num2) return num1 else return
num2 public static double max (double num1,
double num2) if (num1 num2) return
num1 else return num2
130
Method Overloading

• Each overloaded method has to take different
  parameters. Either
• Different parameter types
• A different number of parameters
• Different parameter types and a different number
  of parameters.

public static int max(int num1, int num2)
public static double max (int num1, int num2)
public static void main(String args)
/ Java has no way to know which version of
max you want to call since both version
takes 2 integer parameters / double d
max(1,2)
131
Method Prototypes

• The prototype of a method is comprised of the
  following information
• Return type
• Method Name
• Number and types of parameters
• A few method prototypes
• A methods prototype is also called its signature

public static int max(int num1, int num2) public
static double max (int num1, int num2) public
static float max(float num1, float num2)
132
Computer Science 026a
Distilled
Module 7 Object-Oriented Design
133
Object-Oriented Design

• Java is considered an object-oriented programming
  language
• Everything we wish to model in the program can be
  represented by an object.
• Objects have
• A type (i.e. Person, Rectangle, )
• Properties (i.e. Hair colour, height, weight,
  ...)
• Behaviour (i.e. speak(), eat(), run(), walk(), )

134
Classes

• Class
• Formal definition Collection of data (state /
  properties) and methods (behaviour) that act upon
  that data.
• Informal definition A blueprint that defines a
  group of objects that have common attributes and
  behaviours.

135
Classes

• For example, think back to high school biology
  when you learned about biological classes
• The canine is a class of mammals that have common
  state and behaviour
• State 4 legs, 1 tail, fur
• Behaviour bark, run, howl, yelp, sleep, eat
• Each individual dog (instance of the canine
  class) will have these attributes and behaviours.
• However, they will be distinct objects, and
  will have different state values. One dog might
  have brown fur, while the other has white fur.

136
Classes vs. Objects

• Consider a blueprint of a house.
• We cannot open the blueprints door, or close the
  blueprints window.
• It is not a concrete house, but merely an
  abstract representation of a house.
• But we can create as many houses as well like
  based on the blueprint.
• Each house will share common attributes since the
  same blueprint was used.

The blueprint is analogous to a class. The
houses are instances (objects) of the class.
137
Types of Classes

• Pre-defined Classes
• The Java API
• Other classes written by other programmers /
  companies.
• Reduce development time since code is already
  written
• Three pre-defined classes we have seen in class
• Math
• Rectangle
• String
• User-defined Classes
• These are classes that you create yourself.
• In assignment 3, you created the PopcornCompany
  class. Because you wrote it yourself
  (hopefully!), we call it a user-defined class.

138
Defining a Class in Java

• For example, we might define a Person class
• We are then required to save this class in a file
  named Person.java.

public class
public class Person
139
Instance Variables

• Each object has its own attributes
• Fred is 175 cm tall, weighs 70 kg, and is 20
  years old.
• Jane is 150 cm tall, weighs 45 kg, and is 23
  years old.
• Attributes are represented by instance variables
  in a class.
• Instance variables are normal variables that are
  declared in a class, but outside of any method.
• Every object then has its own set of instance
  variables in which it stores its attributes
  (state).

140
Instance Variables

• Lets add name and age instance variables to our
  Person class
• Notice that our instance variables are declared
  inside the class body, but outside of any method
  body.
• We use the keyword private when declaring them to
  prevent any outside classes from
  accessing/modifying them directly.

public class Person private String name
private int age
141
Instance Methods

• Instance methods are used to model the behaviour
  of an object.
• They are called on objects not on classes
• Person.setName(Jeff) // bad! Person is a
  class
• myPerson.setName(Jeff) // good! myPerson is
  an object
• They are declared exactly the same way as static
  methods, except without the static keyword

public ( parameters )
142
Instance Methods

• We classify instance methods as one of two types
• Accessor Methods
• Retrieve (access) data from the objects instance
  variables
• Do not change the objects state
• Examples
• getName()
• getAge()
• Mutator Methods
• Change (mutate) data in the object.
• Generally do not return anything.
• Examples
• setName(String newName)
• setAge(int newAge)

143
Instance Methods

• Lets start by adding getName() and getAge()
  accessor methods to our Person class
• Notice the return types on our accessor methods
  they correspond with the types of the variables
  they access.
• There is no static keyword on instance methods.
• No data is being changed it is simply returned
  to the user.

public class Person private String name
private int age public String getName()
return name
public int getAge() return age //
end of class
144
Instance Methods

• Now we need a way to change the objects state.
  Well declare setName() and setAge() mutator
  methods
• Notice the parameter types on our mutator methods
  they correspond with the types of the variables
  they change.
• No data is being returned, so the return value is
  void.

public class Person private String name
private int age public void setName(String
name) this.name name
public void setAge(int age) this.age
age // accessor methods omitted //
end of class
145
Instance Methods

• Notice in our mutator methods that we used the
  keyword this.
• Because the parameter names are the same as the
  instance variable names, we have to use the
  keyword this to distinguish between the two.
• this.name refers to the instance variable name.

public class Person private String name
private int age public void setName(String
name) this.name name
public void setAge(int age) this.age
age // accessor methods omitted //
end of class
146
Constructors

• Now we have data and methods, but we also want a
  way to initialize our Person objects with some
  data upon creating them.
• To do this, we can declare one or more
  constructors
• Special methods that have the same name as the
  class
• No return value
• Can take 0 or more parameters, just like any
  other method
• Not called directly by the programmer Java
  calls the constructor when the object is created
  using the new keyword
• Parameters to constructors are normally the
  initial values for the object

147
Constructors

• A constructor that takes no parameters is called
  the default constructor. It generally
  initializes the object to some pre-defined set of
  default values.
• For example, a default constructor for our Person
  object might look something like this

public class Person private String name
private int age public Person()
this.name NO NAME this.age 0
148
Constructors

• The default constructor would be called if we
  created a new Person object using the following
  code
• What would the following output?
• Answer The string NO NAME
• Why? Because our default constructor was coded
  to initialize the Person objects name to NO
  NAME.

Person p new Person()
Person p new Person() System.out.println(p.getN
ame())
149
Constructors

• We usually overload constructors (specify
  multiple constructors, each taking different
  types and/or numbers of parameters).
• Lets create a more useful constructor that takes
  a name and age as parameters

public class Person private String name
private int age public Person(String name,
int age) this.name name this.age
age // mutators and accessors omitted
150
Constructors

• We can now create and initialize a new Person
  object all in our line
• What would the following output?
• Answer The string John

Person p new Person(John,12)
Person p new Person(John,12) System.out.print
ln(p.getName())
151
Computer Science 026a
Distilled
Module 8 Equivalence
152
Identity vs. State

• Lets see what insight Mary-Kate and Ashley Olsen
  can offer to help clarify the finer subtleties of
  identity and state in the context of
  object-oriented programming.

153
Identity vs. State
154
Identity vs. State
155
Identity vs. State
Crickets Chirping
Crickets Chirping
Crickets Chirping
Crickets Chirping
Crickets Chirping
156
Identity vs. State
I have sandy blonde hair!
157
Identity vs. State
Like, so do I!
158
Identity vs. State
I have blue eyes!
159
Identity vs. State
Whoa! Me too!
160
Identity vs. State
We have state equality!
We have state equality!
161
Identity vs. State
But Im not the same person as her!
162
Identity vs. State
Ewww! No way!
163
Identity vs. State
We dont have identity equality!
We dont have identity equality!
164
Identity vs. State

• Thanks, ladies. Well take it from here.
• Recall is the equality operator
• For primitive types, is unambiguous it just
  compares two variables to see if they store the
  same value.

int i 5 int j 5 if (i j)
System.out.println(Theyre equal!)
165
Identity vs. State

• For reference types, however, we have 2 different
  types of equality
• State The data contained in the reference types
  are the same.
• Mary-Kate and Ashley both had the same state
  (hair colour, eye colour). In other words, they
  had the same instance variables.
• Identity The reference types point to the same
  object.
• Mary-Kate and Ashley are two different objects.
  They do not point to the same Person object, so
  they do not have identity equality.

166
Identity vs. State

• Example of Identity Equality
• The first line creates a new Person object.
• The second line merely creates a new variable
  that points to the Person object created in the
  first line.
• These variables have identity equivalence.
• A change made in one (i.e. by calling a mutator
  method) will affect the other variable since they
  both point to the same object.

Person p new Person(John,12) Person q p
167
Identity vs. State

• When dealing with reference types, the equality
  operator () compares identity equality.
• Line 3 will evaluate to true since they point to
  the same object.

Person p new Person(John,12) Person q
p if (p q) System.out.println(Theyre
equal!)
168
Identity vs. State

• If we want to check state equality, we need to
  use the equals() method
• Line 3 will evaluate to true since they have the
  same state, even though they point to different
  objects in memory.

Person p new Person(John,12) Person q new
Person(John,12) if (p.equals(q))
System.out.println(Theyre equal!)
169
Computer Science 026a
Distilled
Module 9 Strings
170
What is a String?

• A string is simply a sequence of characters a
  combination of any of the following
• Letters
• Numbers
• Symbols
• Spaces
• In Java, a string literal is enclosed in quotes
  (but the quotes are not part of the string they
  merely delimit it!)
• Hello, World!
• When we use strings in Java, were actually using
  objects of the String class (defined in the Java
  API)

171
Using Strings

• Normally, we need to use the keyword new to
  create a new object
• The String class, however, is special. We can
  use string literals without having any object
• We can declare String variables without using the
  keyword new (we could technically create them
  using the new keyword if we wanted to)
• Despite these differences, String variables are
  still reference variables that is, they point to
  objects in memory

System.out.println(Hello, world!)
String myString Hello, world!
172
Using Strings

• If we want to clear a String variable, we can
  assign to it the empty string
• We could check a Strings length using the
  length() method
• We could convert all letters in a String to
  uppercase

String myStr
System.out.println( myStr.length() )
myStr myStr.toUpperCase()
173
Changing Strings

• Strings are said to be immutable. This means
  that they cannot be changed. Every method in the
  String class returns a new String object it
  does not modify the String on which the method
  was invoked.
• But sometimes we want to change the values of
  String variables!
• No problem