Methods, Object Oriented Programming(OOP)

CS 171000 – Lecture 1
Algorithms and
Object Oriented Programming
Reading: Chapter 1.1-1.2
Anca Doloc-Mihu
August 29th, 2016
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Contents
Algorithms
Data
types
Methods
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What is an Algorithm?
• An algorithm is a method for solving a problem expressed
as a sequence of steps that is suitable for execution by a
computer (machine).
• Can be expressed in
–natural languages
–Flowcharts
–Pseudocode
–programming languages
3
What is an Algorithm?
Example of algorithm: determine if a number n is a prime number
pseudocode:
k = 2;
As long as k < n do {
1. Divide n by k
2. If n is divisible by k, then return NO
3. Otherwise, increase k by 1 }
return YES
Java
//wrong!
int k = 2;
while ( k++< n ) {
if ( n%k == 0) return false; }
return true;
//correct
int k = 1;
while ( ++k< n ) {
if ( n%k == 0) return false;}
return true;
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What is a data structure
•A data structure is a way for organizing and accessing data
(example data on next slide)
•Example data structures
–Arrays
–Trees, Graphs
•We will learn
–Fundamental data structures and
their operations
–How to implement some of them
–How to evaluate them and decide
when to use what
–How to use Java’s provided data
structures
Tree
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A day on the internet
•294 billion emails are sent
•2 million blog posts
•172 million different people visit facebook
–532 million statuses are being updated
–250 million photos are uploaded
•Twitter: 40 million
•LinkedIn: 22 million
•22 million hours of TV shows and movies are watched on
Netflix
•864,000 hours of video are uploaded to YouTube
•18.7 million hours of music is streamed on Pandora
•35 million apps are downloaded
•2 million search queries per minute on Google
•….
By Li Xiong
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Algorithms and data structures
 Algorithm
+ Data Structure = Program
–An algorithm must use some data structure to store its
information
–An algorithm manipulates the data in the data structures in
various ways
 To
write a program
–Design the data structures to store the information
–Design the algorithm that uses the information to
solve the problem
–Implement the algorithm
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Algorithms and data structures
“ I will, in fact, claim that the
difference between a bad
programmer and a good one is
whether he considers his code or his
data structures more important. Bad
programmers worry about the code.
Good programmers worry about
data structures and their
relationships. ”
— Linus Torvalds (creator of Linux)
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Good Algorithms and Data Structures
 Good
algorithms and data structures are keys to
write a good program for solving a problem
Eg: social network (maintaining and searching many profiles)
 Need
ways to measure “goodness” of data
structures and algorithms
 Algorithm analysis
–Runtime analysis, Big-O notation
 Other
goodness metrics: space usage, power
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Data Types
 Primitive
types
–Integers, with arithmetic operations: byte, short,
int, long
–Real numbers, with arithmetic operations: float,
double
–Booleans, with {true, false} values and logical
operations: boolean
–Characters: char
 Reference
types
–Class types, array types
–Special null types
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Variables
A
variable is a name for a location in memory
used to hold a data value.
–Name (identifier), type, and value
 Using
a variable
–Declaring a variable – type and name
int i;
 Instructs the compiler to reserve a portion of main memory
to hold a particular type of value referred by a particular
name
–Assign a value to a variable
–Use a variable in an expression
i = 1;
i+ = 1; i++;
•A variable cannot be used if it is not declared or initialized
•The left hand side of the assignment operator is always a variable and
the right hand side is an expression
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Primitive Data Types vs. Object data types
 Variables
of primitive data types store the actual
value
 Variables of object types store the reference to the
object
• If it does not reference any object, it holds a special value: null
Circle c;
c = new Circle(1);
double r = c.getRadius();
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Comparison Operators
Operator
Name
<
<=
>
>=
==
!=
less than
less than or equal to
greater than
greater than or equal to
equal to
not equal to
Comparing objects
==
compares references
– Check whether an object variable contains a null reference
equals()
method compares contents
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Questions???
Nearpod.com/student
Enter code:
Answer questions!
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Conditionals
if (booleanExpression) {
statement(s);
}
if (booleanExpression) {
statement(s)-for-the-true-case;
}
else {
statement(s)-for-the-false-case;
}
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World Without Loops is Painful…
System.out.println(“I will write good code!”);
System.out.println(“I will write good code!”);
System.out.println(“I will write good code!”);
System.out.println(“I will write good code!”);
System.out.println(“I will write good code!”);
System.out.println(“I will write good code!”);
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int count=0;
while (count < 100){
System.out.println(“I will write good code!”);
count++;
}
Loops: while
while (loop-continuation
-condition) {
// loop-body;
Statement(s);
}
count = 0;
Loop
Continuation
Condition?
false
(count < 100)?
true
Statement(s)
(loop body)
(A)
false
true
System.out.println("I’ll write good code!");
count++;
(B)
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Loops: do-while
int count=0;
do{
System.out.println(“I will write good code!”);
count++;
} while(count < 100);
do {
//loop body
Statement(s);
} while (loop-continuation-condition);
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Loops: for
for(int count=0; count < 100; count++ ){
System.out.println(“I will write good code!”);
}
for (initial-action; loop-condition; action-after-each-iteration)
{
// loop body;
Statement(s);
}
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Which loop to use?
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Pair grouping
Questions???
Each group/pair:
Nearpod.com/student
Enter code:
Answer questions!
21
Low-level programming: computer programs
manipulate primitive types such as numbers and
characters
Methods: Encapsulate routine computations to
black boxes
Object-oriented programming: Encapsulate data
fields and methods to black boxes
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Example:
Find the sum of integers from 1 to 10, from 20 to 30, and
from 35 to 45, respectively.
int sum = 0;
for (int i = 1; i <= 10; i++)
sum += i;
System.out.println("Sum from 1 to 10 is " + sum);
sum = 0;
for (int i = 20; i <= 30; i++)
sum += i;
System.out.println("Sum from 20 to 30 is " + sum);
sum = 0;
for (int i = 35; i <= 45; i++)
sum += i;
System.out.println("Sum from 35 to 45 is " + sum);
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Solution
public static int sum(int i1, int i2) {
int sum = 0;
for (int i = i1; i <= i2; i++)
sum += i;
return sum;
}
public static void main(String[] args) {
System.out.println("Sum from 1 to 10 is " + sum(1, 10));
System.out.println("Sum from 20 to 30 is " + sum(20, 30));
System.out.println("Sum from 35 to 45 is " + sum(35, 45));
}
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Defining Methods
A method is a collection of statements that are
grouped together to perform an operation.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
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Method Signature
Method signature is the combination of the method name and the
parameter list.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
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Formal Parameters
The variables defined in the method header are known as
formal parameters.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
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Actual Parameters
When a method is invoked, you pass a value to the parameter. This
value is referred to as actual parameter or argument.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
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Passing Parameters

When calling a method, the arguments must match the
parameters in order, number, and compatible type
public static void nPrintln(String message, int n) {
for (int i = 0; i < n; i++)
System.out.println(message);
}
public static void main(String[] args) {
nPrintln(“Hello!”, 3);
nPrintln(“So that’s how the methods work”, 10);
}

When invoking a method, the value of the argument is
passed to the parameter. The variable itself is not
affected. This is referred to as pass-by-value.
Mechanics of the Method-Calling Process
1. Evaluate the argument expressions
2. Copy
argument value into the corresponding
parameter, (allocated in a newly assigned region of
memory called a stack frame)
3. Execute body, using the new stack frame for local
variables.
4. On a return statement, compute the return value
and substitutes that value in place of the call.
5. Discard the stack frame for the method and returns
to the caller, continuing where it left off.
Sum Example: Call Stack
public static void main(String[] args) {
int i = 0;
…
// 1. evaluate arguments
System.out.println("sum(1, 10) is: " + sum(1, 10) ); // 1+2+...+10
System.out.println("sum(25, 30) is: " + sum(25, 30) ); //25+26+...+30
System.out.println("sum(40, 50) is: " + sum(40, 50) ); //40+41+...+50
}
public static int sum(int start, int end) { // 2. copy args, new SF
int sum = 0; // 3. execute the body
for (int i = start; i <= end; i++) {
sum += i;
}
return sum; //4 and 5. return the value, and discard stack frame
}
Overloading Methods
 Method
overloading: multiple methods can
have the same name but different parameter
lists
 Compiler
determines which method is used
based on the method signature (method name
and parameters)
– Early binding
Overloading Methods
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;
}
max(1, 3);
max(1.0, 3.0);
max(1.0, 3);
Overloading Methods
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;
}
max(1, 3);
max(1.0, 3.0);
max(1.0, 3);
Homework 1
 First
assignment is up and running!
Next time…
Arrays
(passing/returning arrays
to/from methods), Searching arrays
(linear search, binary search)
Abstract Data Types: Objects and
classes
Quiz
1 (bring laptop, check to
make sure you are in the class on
Canvas)
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