Issues
Objects, Administrative Variables, and Types
Lecture 3
Due Today
Program 0
Due at 11pm Submit via Blackboard Try to submit well before 11pm You can submit multiple times Youll need time to figure out how to turn in your project!
Ongoing
Homework 0
No new assignments! (Except reading)
What are Objects?
Java is an Object-Oriented Language What the heck does that mean? What is an object?
Quoting the text: a fundamental entity in a Java program. The building blocks of your program
Example Objects
Program for bank accounting software Possible objects
One object for each bank account Object for each branch office Each customer represented by an object Each teller is an object Each ATM an object Headquarters is an object
Primitive Data
Common, fundamental values
Numbers: 1534 Characters: a Classes
Classes
Think of Data type for Objects Defines a set of values Defines possible operations on those values
More specialized values are represented as objects Values belong to a Data Type
Defines a set of values Defines possible operations on those values
Example: BankAccount class
Class describes data
Account number Balance
Operations
Withdrawal Deposit
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BankAccount Class
Many people have bank accounts Each bank account is an object
My bank account Your bank account Michael Jordans bank account
Encapsulation
Encapsulation means each object handles its own data Details are hidden from the outside Example:
Deposit to bank account You just drop off a check and see your balance Do you know what happens to that check?
All bank accounts type of information and services: they are of the same class.
Inheritance and BankAccounts
But are all bank accounts really the same? No, but similar! BankAccount Inheritance More in chap. 7 SavingsAccount CheckingAccount
IRAccount Four01KAccount
Classes: Chapter 4
Coverage of classes in Chapter 4 Key points
Primitive data is defined by a data type
a is a char
Objects are defined by a class
davesAccount is a BankAccount
Today:
Predefined objects Using objects
Using Objects
System.out.println(Go Heels!); Invoked the println method through the object System.out. The object represents an output device
By default, the monitor Predefined in standard Java programs println() is a service, or method Object
Calling a method
System.out.println(Go Heels!);
Parameter or Argument Dot Notation Method End of Line Syntax
Request service via dot notation
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Calling a method
In general:
object.service(argument list);
Back to System.out
Object that represents output Mapped to monitor by default
Could be disk, printer, etc.
Provides many methods for printing output
Recall: Methods = Services
Example:
myBankAccount.deposit(125.00);
Encapsulation
Hides details It just works
System.out Methods
Two important System.out methods
println(); print();
2 Minute Exercise
What output is produced by the following code fragment?
System.out.print(Go Heels!); System.out.println(Beat Dook!); System.out.println(I want); System.out.print( to get an A ); System.out.print(in Comp); System.out.println(); System.out.println(14.);
Small but important difference
println() prints the information then moves to the next line print() prints the information but does not advance to the next line
Strings
What parameters have we used so far?
Go Heels! Beat Dook! 14.
String Literals
Strings are fundamental, so String Literals are part of the Java language All quotations weve used so far are String Literals
Go Heels! Beat Dook! 14.
All character strings Represented as an object
Defined by class String Details on the class String in Section 2.6 (tomorrows lecture)
Delimited by quotation characters
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String Literals
String Literals cant span more than one line
Syntax Error! Example quote from Maurice Mascaranhas:
System.out.println(Profits are like breathing. You have to have them. But who would stay alive just to breathe?);
Multiple Print Statements
One answer is to use many print statements:
System.out.println(Profits are like); System.out.println( breathing. You ); System.out.pritnln(have to have ); System.out.println( them. But who ); System.out.println(would stay alive ); System.out.println(just to breathe?);
How does Java print out long string literals?
But this is tedious! Is there a better way?
String Concatenation
Appends two strings to form a single longer string Use the + operator between string literals
Ex: I love + Krispy Kreme
Fixing The Error
Use concatenation to handle long string literals Example:
System.out.println(Profits + are like breathing. You + have to have them. But + who would stay alive just + to breathe?);
Can also be done with numbers
Numbers get converted to strings automatically! Ex: I read + 20 + pages last night!
Concatenation Example
System.out.println(9 plus 2 is + 9 + 2);
Printing the Character
How do you print the character? Cant do this! System.out.println();
What output is produced? 9 plus 2 is 92 Why not? Escape Sequences are the answer!
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Escape Sequences
Represent special characters
Characters that cant be typed Characters that confuse the compiler Examples:
Backspace, tab, newline, carriage return, double quote, single quote, backslash
Printing the Character
Example Code Fragment:
System.out.println(As the last slide + said, \escape sequences are the + answer!\);
\b \t \n \ \ \\
Backspace Tab Newline
\r Carriage Return
Double Quote Single Quote Backslash
Example Output:
As the last slide said, escape sequences are the answer!
All start with a backslash \
2 Minute Exercise
Example Code Fragment: System.out.print(I + wish\n\tI were in + \Tahiti.\\nDo + you?\n); \b \t \n \ \ \\
Backspace Tab Newline
Variables
A variable refers a location in memory Variables used to store information From math:
Let x=10 What is the value of 2x? 20.
\r Carriage Return
Double Quote Single Quote Backslash
How can we use variables in Java?
Variable Example
public class CoffeeMaker { public static void main (String[] args) { int cups = 10; System.out.println(My coffee maker + makes + cups + cups.); }
Variable } Declaration
int cups = 10;
Type
(More on this in a few minutes)
Assignment Operator
End of Line Syntax
Identifier
Expression
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Variable Declaration Specification
Declaration Type final Declarator ,
Variable Declaration Specification
Declaration final Later Today Later Today Type Declarator ,
Declarator Identifier = Expression Array Initializer
Declarator Identifier =
Tomorrow Expression Array Initializer Chapter 6
Valid Declarations
int int int int int cups = 10; age; doves, count, jars; savings=2, count=5; candles=5, count;
Basic Declaration
Declaration reserves space in memory to hold a value No value is placed in that space int age;
1502 01001011 1503 00010101 1504 00000001 1505 00000010 1506 01010111 1507 00111100 1508 11101010
Basic Declaration
Value of age is left over from previous use of the memory
int age; DANGEROUS!
Always assign a value 1502 01001011 1503 00010101 1504 00000001 1505 00000010 1506 01010111 1507 00111100 1508 11101010
Assignment
How do you store a value in a variable?
The assignment statement
A variable can hold one value of the correct type Java is Strongly Typed
Must match type of variable and data being stored i.e. int variables can only store integers
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Assignment Statements
int numberOfStudents; // Value undefined numberOfStudents = 10; System.out.println(numberOfStudents); numberOfStudents = 4; System.out.println(numberOfStudents); numberOfStudents = 13; System.out.println(numberOfStudents);
Assignment Statement Specification
Assignment Identifier = Expression ;
Examples:
Strongly Typed: Both sides must be ints
pizzaSlices = 8; index = index + 1; sumOf2 = number1 + number2;
Cover Expressions Tomorrow
Constants
Some values are constant for the entire program Helpful to assign a name Examples:
In math, PI = 3.14159 Classroom has max occupancy, maybe 100 people
Constant Declarations
public class Classroom { public static void main (String[] args) { static int MAX_OCCUPANCY = 10; int occupany = 5; System.out.println(occupany + out of + MAX_OCCUPANCY + spots are used.); } }
Use the reserved word final to declare a constant
Declaration Type final Declarator ,
More Constants
Why use constants?
Provides symbolic name for a value (i.e. PI) Prevents accidental changes to value Makes changes easy: change needed in one place Declaration final
Types
Type Declarator ,
Constant Conventions
Declare constants using all capitals Examples
MAX_OCCUPANCY PI BUCKET_CAPACITY
Variables are of a specific type Primitive Data Types
int char
Classes
String BankAccount
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Primitive Data Types
Primitive data types are simple, low-level types of data
Integers
1,343 or -5
Integers
Four different integer types Each uses different number of bits From Chapter One: n bits = 2n possible items Type Storage
byte short int long
Floating-points
3.141 or -99.3
Min Value -128
Max Value 127 32,767 2,147,483,648
9,223,372,036,854,775,807
8 bits
Characters
a or Q
16 bits -32,768 32 bits -2,147,483,648 64 bits
-9,223,372,036,854,775,808
Booleans
either true and false
Roughly 10 Quintillion!
More On Integers
Use the best type to match needs
If type is too large, space is wasted If type is too small, information is lost
Floating Points
Stores numbers in two parts
Significand (aka Mantissa) Exponent Like math: 3.4E+38
A integer literal is a specific integer value
1345 -1
Two different floating point types Type Storage
float double
Java treats all integer literals as ints
Exception: specifically ask for a literal of type long 1246L No byte or short literals
Min Value
with 7 significant digits with 15 significant digits
Max Value About +3.4E+38
with 7 significant digits with 15 significant digits
32 bits About -3.4E+38
64 bits About -1.7E+308 About +1.7E+308
More On Floating Points
Floating Points have limited precision
Cant exactly represent PI!
Characters
Character literal delimited by single quotes
a or Z or 9 Recall: String literal uses double quotes
Use the best type to match needs
If type is too large, space is wasted If type is too small, loss of precision
There are floating point literals
3.14159 -1.0
Java treats literals as doubles
Exception: specifically ask for a literal of type float 3.14159F
Data type is char 9 is a char, while 9 is an int A char can store any symbol in the Unicode character set, Appendix C in your book.
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More on Characters
Unicode uses 16 bits per character Some symbols included in Unicode set:
Uppercase Letters
A, B, C true false
Booleans
Booleans variables can hold one of two values
Lowercase Letters
a, b, C
Punctuation
., {, ]
Digits
0, 1, 9
Booleans are like a light switch: on/off Just 1 bit needed, but uses 8! Why? The reserved words true and false are Boolean literals Example:
boolean finishedLecture = true;
Primitive Data Types
byte (8) short (16) int (32) long (64) char (16)
Using Multiple Variables
We now can:
Declare variables Assign values to variables
float (32) double (64) boolean (8)
Next step:
Combine multiple variables into expressions
What are Expressions
Expressions are composed of operands and operators
Operands are variables or objects Cover operators in detail tomorrow
Sample Expressions
// Calculating multiples pitcherSize = numOfCups*CUP_SIZE; // Summing up numbers total = subtotal1 + subtotal2; // Computing an average avg = total / count; // The Pythagorean Theorem aSquared = (b*b) + (c*c);
Example: +, -, \
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Sample Code Fragment
final int GRADE_FOR_A = 90; float test1=86, test2=90, test3=93; float pointTotal, avg; int testCount=3; // Compute the average pointTotal = test1+test2+test3; avg = pointTotal / testCount; // Display the results System.out.println(After + testCount + tests, + the test average is + avg); System.out.println(Note that an A requires an + average of at least + GRADE_FOR_A);
Sample Code Fragment
JBuilder Demo of the code fragment
For Next Class
Reading: Sections 2.5-2.9
More Expressions, Operators, and Methods
Program 0 is due today at 11pm Homework 0 is due tomorrow in class Program 1 will be handed out tomorrow!
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