2. HelloWorld.java import java.awt.*; public class HelloWorld extends
java.applet.Applet {
TextField t;
public void init()
t = new TextField(50);
t.setText(“Hello World!");
add(t); }

3. Java programs
Java programs are created as text files using a text editor (like emacs)
Save to disk with .java file extension
HelloWorld.java
The file contains characters (stored as bytes)
file can be printed, displayed on monitor, or edited
file cannot be directly executed (run) by the computer system
Java must first translate the program into bytecodes before it can be run

4. Bytecodes Java bytecode machine instruction for the Java processor
Java compiler javac translates the source program into bytecodes
Bytecode file has same name as the source program with a .class file extension:
HelloWorld.class
HelloWorld.java
javac
HelloWorld.class
Java compiler
source program
Java bytecodes

5. Java Virtual Machine (JVM)
Bytecode (class) file will contain exactly the same bytecodes no matter what computer system is used
Bytecode file is executed by a Java bytecode interpreter
processor specific executable program
Each type of computer system has its own Java interpreter that can run on that system
Any computer system can execute Java bytecode programs if it has a Java interpreter
Computers with Java interpreters are called Java Virtual Machines
a “computer” with a Java processor that can run Java bytecodes

6. Java applets
An applet is a Java bytecode program that runs on a Web browser
Most newer Web browsers have Java interpreters
Web pages on the Internet contain instructions that send Java bytecodes to your computer
Web browser runs the Java applet with its built-in interpreter

7. Data types Computer memory stores arbitrary bit patterns
Meaning of a bit pattern depends on its use
Pattern used for a particular string of bits is a data type
values are any kind of data a computer can process
all values are represented using some data type
Example: What does the following pattern of 16 bits represent?
No way to know without more information
If data type is short (a Java type) it represents 103

8. Java data types Primitive
types of data that are so fundamental ways to represent them are built into Java
Object
built-in or user-defined

9. byte short int long float double char boolean
Primitive data types
All primitive values belong to one of eight primitive types
byte short int long float double char boolean
Primitive data types use a fixed number of bytes
four of these types designate different sizes of bounded integers: byte, short, int, long
A programmer can not create new primitive data types
Any data type you invent will be a type of object
Most commonly used types in practice: int, boolean, and double

10. Java primitive data types
Primitive Type
Description
Range
byte
8-bit integer
-128 to 127
short
16-bit integer
to 32767
int
32-bit integer to
long
64-bit integer
-263 to 263-1
float
32-bit floating point
10-46 to 1038
double
64-bit floating point
to 10308
char
Unicode character
boolean
Boolean variable
false and true

11. Basic operators Operator Java Description Assignment =
assigns rhs to lhs
Arithmetic
+,-,*,/,%
addition, subtraction, multiplication, division, remainder
Unary
-,++,--
negative, auto increment, auto decrement
Equality
==, !=
equals to, not equals to
Relational
<,<=,>,>=
less than, less than or equals to, greater than, greater than or equals to
Logical
&&,||,!
AND, OR, NOT

12. type <variable-name> = <value>;
Variable declaration
Declaration type <variable-name>;
Declaration + initialization:
type <variable-name> = <value>;
Variable names
any combination of letters, numbers, and the underscore character
may not start with number
may not be reserved word
e.g. int, return, if, for, while
may not be same as a subroutine name
case-sensitive (num and Num are different)

13. Examples int x, y, z; int sum = 0; float f; double pi = 3.14;
char first = ‘T’, middle = ‘L’, last = ‘B’;
char first = ‘T’; char middle = ‘L’; char last = ‘B’;

14. Operator precedence Evaluate a + b * c
multiplication first? a + (b * c)
addition first? (a + b) * c
Java solves this problem by assigning priorities to operators (operator precedence)
operators with high priority are evaluated before operators with low priority
operators with equal priority are evaluated left to right
Operator priority
(highest to lowest)
( )
* / %
+ - =

15. When in doubt, use parentheses
a + b * c = a + (b * c)
because * has higher priority than +
To perform the + operation first we need to use parentheses
(a + b) * c
If in any doubt use extra parentheses to ensure the correct order of evaluation
parentheses are free!
cause no extra work for the computer
only make it easier for you to work out what is happening

16. Examples Java adheres to traditional order of operations
* and / have higher priority than + and –
int x = * 6; (x = 33)
int y = (3 + 5) * 6; (y = 48)
Parentheses are free, use them liberally
int z = ((3 + 5) * (6)); (z = 48)
Equal priority operations are evaluated left-to-right in the absence of parentheses
int w = 3 * 4 / 2 * 6; (w = 36)
int x = 3 * 4 / (2 * 6); (x = 1)
int y = 3 * * 6; (y = 24)
int z = 3 * (4 + 2) * 6; (z = 108)

17. Syntax and semantics Addition, subtraction: + and –, int and double
int x = 21+4; (x = 25)
double y = ; (y = 12.1)
Multiplication: *, int and double
int x = 21*4; (x = 84)
double y = 14.1*2.5; (y = 35.25)
Division: /, different for int and double
int x = 21/4; (x = 5)
double y = 21/4; (y = 5.0)
double y = 21/4.0; (y = 5.25)
Modulus: %, only for int
int x = 21%4; (x = 1)

18. Automatic type conversion
Mixed type expressions are converted to higher compatible types
If all operands are of type int then result is type int
If any operand is of type double then result is of type double
Cannot convert to a lower type
Conversion may result in loss of precision
Example:
Convert Fahrenheit to Celsius
double F=41.0;
double C=(F-32.0)*(5/9);
Question: What is the value of C? 5
0.0
9.0
5.0 9

19. More expressions int g = 12 + 2.5; int n = 1 – 2 * 3 – (4 + 5);
What is the value of g? 12 14
14.5
error
int n = 1 – 2 * 3 – (4 + 5);
What is the value of n? -4 -2 2 4
none of the above
int x = 8 * (7 – 6 + 5) % (4 + 3 / 2) – 1;
What is the value of x? -1 2 3
none of the above

20. Syntax errors
The following Java subroutine computes the inclusive sum between two integers. Find all the syntax errors.
int sumBetween( x, y ) {
int z = x;
Int sum = 1;
while( z <= y ){
sum = sum*z;
z++ }

21. Logic errors
The computer will do precisely what you say even though it may not be what you want
What is wrong with this code?
int sumBetween( int x, int y ) {
int z = x;
int sum = 1;
while( z <= y )
sum = sum*z;
z++; }

22. Java objects Java is an object-oriented programming language
use objects to define both the data type and the operations that can be applied to the data
Objects have attributes and functionality
attributes describe the state of the object
the functionality of an object is the set of actions the object can perform
In Java, we define an object’s attributes using variables and its functionality using methods

23. Real-world objects
Suppose we want to describe a car in terms of its attributes and functionality
Attributes:
int year; int mileage;
String make; String model;
boolean manual_transmission;
Methods:
void brake()
int getMileage()
boolean needsGas()
void shift(int gear)

24. Java classes Java objects are created using classes Encapsulation
combining elements to create a new entity
A class encapsulates the variables and methods that define an object
Instantiation
the act of creating an object
objects are called class instances
Java provides many predefined classes
You can also define your own classes

25. Java String class The String class represents character strings
String first = “Tammy”;
String last = “Bailey”;
Strings can be concatenated (added together) using the concatenation operator +
String fullname = first + “ ” + last;
Testing for equality:
first.equals(“Tammy”); /* returns true */
first.equals(“Amy”); /* returns false */

26. Instantiation Creating an object is called instantiation
the new operator is used with class name
Example: Create a TextField object
TextField t = new TextField();
Can create multiple instances of the same class
TextField t1 = new TextField(); TextField t2 = new TextField();
Exception
the new operator is not required when creating a String

27. Java TextField class
The TextField class allows the editing and display of a single line of text
TextField t = new TextField();
Methods
setText(String s)
set the text of the field to the string s
String getText()
get the text of the field and assign it to a variable of type string

28. Invoking an object’s methods
Once we create a text field, we can perform actions on it using its methods
The variables and methods of an object are accessed using the dot operator
TextField t = new TextField();
t.setText(“Hello”);
Syntax
object.verb(data);
Perform verb on object using data

29. Interactive objects
User interaction determines the behavior of the program
Program receives user input through mouse and keyboard and performs associated method or action
Text fields
edit and display single line of text
Buttons
can specify action to occur when button is clicked

30. Action listeners
If we want a button to know when it is clicked, we have to enable it to “listen” for user input
Use the button method addActionListener
Button b = new Button(“click!”);
b.addActionListener(this);
If we don’t invoke the addActionListener method on a button, nothing will happen when the button is clicked

31. Example We would like our applet to do the following:
get text from text field t1 and display it in text field t2 when button b is clicked
TextField t1 = new TextField();
TextField t2 = new TextField();
Button b = new Button(“copy text”);
b.addActionListener(this);

32. Actions
We specify actions to occur when a button is clicked in the actionPerformed method
public void actionPerformed(ActionEvent event) {
Object cause = event.getSource();
if(cause == b)
t2.setText(t1.getText()); }

33. Numeric input
Suppose we want an applet that allows the user to enter two integers and display the minimum
a text field contains a character string
If we want to perform numeric operations on the input from a text field, we have to convert the string to a numeric data type
numbers are primitive data types, not objects
Can convert using Java type wrappers

34. Type wrappers Convert primitive types into objects Primitive Type
Wrapper Type
byte
Byte
short
Short
int
Integer
long
Long
float
Float
double
Double
char
Character
boolean
Boolean

35. String conversion Convert String str to byte b
byte b = Byte.parseByte(str);
Convert String str to short s
short s = Short.parseShort(str);
Convert String str to int i
int i = Integer.parseInt(str);
Convert String str to long l
long l = Long.parseLong(str);
Convert String str to float f
float f = Float.parseFloat(str);
Convert String str to double d
double d = Double.parseDouble(str);

36. Action performed for integer input
public void actionPerformed(ActionEvent event) {
Object cause = event.getSource();
int x = Integer.parseInt(t1.getText());
int y = Integer.parseInt(t2.getText());
if(cause == b)
int min = minimum(x,y);
t3.setText(“The minimum is: ” + min); }