1. Introduction to Object-Oriented Concepts in Java
Overview of Java
Introduction to Object-Oriented Concepts in Java

2. Outline How Java works The JVM Java Program Structure
Overview of classes

3. How Java Works
e.g. Bank.java
javac
e.g. Bank.class
by JIT compiler

4. The Compiler and the Java Virtual Machine
Most compilers translate source code into executable files containing machine code.
The Java compiler translates a Java source file into a .class file that contains byte code instructions.
Byte code instructions are the machine language of the Java Virtual Machine (JVM) and cannot be directly executed by the CPU.
3) JVM interprets and executes these class files on the target platform.
Uses JIT (Just In Time) translator to compile parts of bytecode into native code.
This native code is directly executed by platform

5. The Compiler and the Java Virtual Machine
Byte code files end with the .class file extension.
The JVM is a program that emulates a micro- processor.
The JVM executes instructions as they are read.
JVM is often called an interpreter.
Java is often referred to as an interpreted language.

6. Java Basics The Java compiler (javac) is a command line utility.
The command to compile a program is:
Javac filename.java
E.g. Javac Payroll.java
The .java file extension is required.
The command to execute a program is:
Java filename
Where filename.class file was produced by javac

7. The JVM and Portability
With most programming languages, portability is achieved by compiling a program for each CPU it will run on.
Java provides an JVM for each platform so that programmers do not have to recompile for different platforms.

8. The JVM and Portability
Byte code (.class)
Java Virtual
Machine for Windows
Java Virtual
Machine for Unix
Java Virtual
Machine for Linux
Java Virtual
Machine for Macintosh

9. Java Applications and Applets
Java programs can be of two types:
Applications
Stand-alone programs that run without the aid of a web browser.
Relaxed security model since the user runs the program locally.
Applets
Small applications that require the use of a Java enabled web browser to run.
Enhanced security model since the user merely goes to a web page and the applet runs itself.

10. Java Program Structure
Package statement
import statement
Class declaration 1
Class declaration 2 …
public class declaration …
public static void main(String[] args) …
Class declaration n

11. The Java Virtual Machine & main()
The main method is the first method that is executed when a class is loaded into memory and executed by the JVM.
If another class loaded has a main method, the second main method will be ignored.

12. Class with one method //This is my first Java program
Comment
//This is my first Java program
public class HelloWorld { }
Class header
public static void main (String[] args) {
System.out.println(“Hello World!”); }
The main method
Class
body
public static void main (String[] args) { }
// comments about the method
method header
method body 12

13. Syntax of Class Definition
[ClassModifiers] class ClassName
[extends SuperClass]
[implements Interface1, Interface2, …] {
ClassMemberDeclarations }
ClassModifiers = public/abstract/final
ClassModifiers is never private except for inner classes. 13

14. A Java Program may contain
A Simple Java Program
Name of class
Reserved word
Class accessibility
Main program
Note the syntax for the method header
Class HelloWorld
A Java Program may contain
many classes
public class HelloWorld {
public static void main (String[] args)
System.out.println(“Hello World!”); }
Class
Bank Account
Class BillPayment
Notation for using methods: class.method or package.class.method is how to refer to a public method (with some exceptions).
The HelloWorld Class
Class OnlineShopping 14

15. Class with many methods
public class Account {
String clientName;
char accType = `S’;
int accNumber;
double balance=0;
public static void deposit(double value){
if(value > 0)
balance = balance + value;
} //end of method
public static void withdraw(double value){
if(balance > value)
balance = balance - value
}//end of method
public static double getBalance(){
return balance;
public static String getName(){
return clientName;
public static void setName(String theName){
clientName = theName;
}//end of method
public static int getAccNumber(){
return accNumber;
public static void setAccNumber(int n){
accNumber = n;
}//end of class
public static void main (String[] args){
Account acc = new Account();
acc.deposit(200);
acc.deposit(400.95);
acc.withdraw (100); } 15

16. Identifiers Identifiers are programmer- defined names for: Classes
TestPrint
Variables
number, age, myState
Methods
main, print, locateVehicle
public class TestPrint {
public static void main(String[] args) {
int myState = “North Carolina”;
System.out.print("I live in “ + myState); }

17. Identifiers: Programmer Defined Names
Java programs are written in Unicode
Identifiers must follow certain rules:
An identifier may only contain:
Characters in the alphabets of all languages in Unicode
letters a–z or A–Z
the digits 0–9,
underscores (_), or
the dollar sign ($)
The first character may not be a digit.
Java is case sensitive.
Total, total, and TOTAL are different identifiers
Identifiers cannot include spaces.
A reserved word cannot be used as an identifier

18. Java Reserved Keywords
abstract
boolean
break
byte
case
catch
char
class
const
continue
default do
double
else
extends
final
finally
float
for
goto if
implements
import
instanceof
int
interface
long
native
new
package
private
protected
public
return
short
static
strictfp
super
switch
synchronized
this
throw
throws
transient
try
void
volatile
while
Reserved Words have a predefined meaning in the Java language, e.g., if, while.
A reserved word cannot be used in any other way

19. Java Naming Conventions
By convention, programmers use different case styles for different types of identifiers, such as
title case for class names - Lincoln
upper case for constants – MAXIMUM
lower case followed by title case for method names – myHomeState
A general rule of thumb about naming variables and classes are that, with some exceptions, their names tend to be nouns or noun phrases. 19

20. Other Rules for Creating Names
Variable names should be descriptive.
Descriptive names allow the code to be more readable; therefore, the code is more maintainable.
Which of the following is more descriptive?
double tr = ;
double salesTaxRate = ;
Java programs should be self-documenting.

21. Data Types in Java
A data type denotes a set of a certain collection of data. For example, numbers as against strings.
Java supports two broad categories of data types.
Primitive types
Reference types

22. Primitive Data Types
Primitive data types are built into the Java language and are not derived from classes.
There are 8 Java primitive data types.
Data Type
Name/Java Keyword
Size
Min Value
Max Value
Integer / Whole Number
byte
1 byte – 8 bits
-27
27 -1
short
2 bytes – 16 bits
-215
215 -1
int
4 bytes – 32 bits
-231
231 -1
long
8 bytes – 64 bits
-263
263 -1
Real /floating Point Number
float
~ -2126
~ 2127
double
~ 21022
~ 21023
Character
char
Can represent any Unicode character
True/False
boolean -

23. Reference Types There are three forms of reference types Class type
public class ClassName{
//code for class }
Interface type
public interface InterfaceName{
//method signatures
Array type
int[] numbers;//array declaration
numbers = new int[20]; //allocate memory for the array

24. Variable Declaration modifier = public, private, protected
[modifier] Type variableName1 [=InitialValue1] [, variableName2
[=InitialValue2]] [, variableName3 [=InitialValue3]] … ;
modifier = public, private, protected
protected int total;
public IAccount intf1, intf2;
public Account acc1, acc2;
private double average = 100, sum =44.88;
public char ch1, ch2=‘3’, ch3;
private boolean itemFound;

25. Numeric & String Literals
Literals are actual values.
Numeric literals
124
44.76
String literals
"To be or not to be"
"Love is lovely"
"1"
"X"

26. Character Literals Character literals
ASCII characters can be written directly in single quotes.
'1'
'X‘
In hexadecimal code \u is followed by four hexadecimal digits
'\u0040‘ – the character
In octal code \ is followed by three octal digits
'\044' – the character ‘$’

27. String Escape Sequences
What if we wanted to print a double quote character?
The following line would confuse the compiler because it would interpret the second quote as the end of the string
System.out.println ("I said "Hello" to you.");
An escape sequence is a series of characters that represents a special character
An escape sequence begins with a backslash character (\), which indicates that the character(s) that follow should be treated in a special way
System.out.println ("I said \"Hello\" to you.");

28. Java Escape Sequence Escape Sequence \b \t \n \r \" \' \\ Meaning
backspace
tab
newline
carriage return
double quote
single quote
backslash

29. Local, Global and Class Variables A variable is a location in memory where a value can be stored and changed.
Global/ Instance variable
Declared in the class
Local variable
Declared within the body of a method.
Class variable
Global variable declared as static

30. Primitive vs Reference Variables
References
32 bit pointers
Hold addresses not data values
Memory allocated from garbage-collected heap
No need to de-allocate memory.
Equality of references?
Garbage Collection
Unused/unreacheable memory (garbage) automatically dealocated.
Comes at a cost to performance.

31. One Dimensional Arrays
Declaration
int[] array;
Creation
Using the new operator
Size of array must be specified
All elements are initialized to default values
Integers 0, floating point 0.0, char \u000, Boolean false, Reference null
E.g. int numbers = new int[20];
Using the one-dimensional array initializer
{e0, e1, e2, …, en-1}
E.g. int[] numbers = {1,2,3,4,5,6,7};

32. One Dimensional Arrays
Indexing
Using for loop
Slicing
Using Arrays.copyOfRange(array, start, stopPlusOne)

33. Multi-Dimensional Arrays
Is treated as an array of arrays
A one dimensional array where each element is an array.
Declaration
String[][] names; // 2 dimensions
double[][][] values;//3 dimensions
Creation
Using the new operator
arrayName = new Type[n1] [n1] [n2] … [nk] // k dimensions
All elements are initialized to default values
E.g. int[][] numbers = new int[10][20;
Using the one-dimensional array initializer
{e0, e1, e2, …, en-1}
E.g. int[][] numbers = {1,2,3,4,5,6,7};
Indexing

34. Arithmetic Operators General Assignment operators Increment/Decrement
+ addition
- subtraction
* Multiplication
/ division
% remainder/modulus
Assignment operators
+=, -=, *=, /=, %=, <<=, >>=, &=, ^=, |=
Increment/Decrement
++, --

35. Bitwise Operators ~x x & y x | y x ^ y Complement of x
bitwise and of x and y
x | y
bitwise inclusive or of x and y
x ^ y
bitwise exclusive or of x and y

36. Shift Operators x << k = x*2k x >> k = x/2k
Shifts the bits in x k places to the left
x >> k = x/2k
Shifts the bits in x, k places to the right, filling in with the highest bit (the sign bit) on the left-hand side.
x >>> k
Shifts the bits in x k places to the right, filling in with 0 bits on the left-hand side.

37. Logical Operators
Translating logic into Java
AND &&
OR ||
XOR ^
NOT !

38. Relational Operators == !=
<
>
<=
>=

39. Selection Statements
if statements

40. Loop Statements
if statements

41. The break & continue Statements
if statements

42. The End Qu es ti ons? ______________________ Devon M. Simmonds
Computer Science Department
University of North Carolina Wilmington
_____________________________________________________________

43. Example of a Java Class public class Account implements IAccount {
private String accName;
private int accNumber;
private double balance;
public boolean deposit(double amount){
if(amount > 0){
this.balance += amount;
return true; }
return false;
public boolean withdraw(double amount){
if(this.balance >= amount){
this.balance -= amount;
}//end of class
Example of a Java Class

44. How Java Works
See interpreted-or-compiled-programming-language.html
interpreted-What-is-the-difference-What-is-the-JIT-compiler