Java Basics Part I

1
Java BasicsPart I

• Thursday, January 9

2
Red Tape
3
Readings

• Stop at Weiss Section 2.4
• Some might find 2.5 confusing
• Cover later
• Read
• Weiss Ch. 1
• Weiss Sections 2.1 2.4 by Th 1/17

4
Java Applications vs. Applets

• Very similar except Applet runs in browser
• Well write applications.
• Initially console apps

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A Simple Program

• /
• A very simple Java program.
• /
• public class Class1
• public static void main (String args)
• System.out.println("Hello Java")

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Classes
public class Class1

• Everything in Java is part of a class.
• A class is a blueprint for objects.
• A program contains at least one class.

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The Main Method
public static void main (String args)

• The main( ) method (in a class) is where the java
  program begins.
• You must define one.

Except in an Applet
8
Comments

• Any text after // is ignored
• Text between / and / is ignored.

// This is a comment public static void main()
/ This is too /
9
Identifier Names

• Must begin with letter, underscore ( _ ) or
  dollar ( ).
• Example 2a is not legal
• Cant contain operators (, -, etc.)
• Java uses Unicode, so a letter has a broader
  definition. For example, ? is legal.
• Questionable usage, though.

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Case

• Case matters!
• VariableName ? variablename

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Reserved Words

• You cant use reserved words as identifiers.
• Examples if, else, while, static class.
• Note that
• Class ? class

12
Declaring Variables

• Pretty standard
• int radius
• char letter
• Modifiers later

13
Variables

• Unlike C, no global variables
• Must all be in a class.

14
Initializing Variables

• Again, very standard
• int i 0
• Variable initialization makes program easier to
  read and understand

15
Numeric Data Types

• byte
• short
• int
• long
• float
• double

8 bits 16 bits 32 bits 64 bits 32 bits 64 bits
-128 to 127 -32,768 to 32,767 -2 billion to 2
billion -263 to 263 - 1 6 digits, exp 10-46 to
1038 15 digits, exp 10-324 to 10308
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Constants

• static final double PI 3.14159
• Why constants?

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Naming guidelines

• Class
• ClientData
• Variables and methods
• radius
• readDouble( ),
• Constants
• PI

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Modifiers

• public visible to any class
• private only visible in this class
• static defines a class method or data field.
• final cant be modified
• More on these when we talk about classes

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Assignment Statement

• Assignment operator is , not (or lt-)
• i 7
• Can get arbitrarily confusing
• i j k l p
• Variable must be on the left
• 1 i not correct.

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Expressions

• Conventional
• x 3 y
• Operators like C and C
• - /
• No operations on arrays

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Shortcut operators

• Like C
• --
• Example
• x 3
• y x
• What are values of x and y?
• May also be prefix, like x

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Prefix and Postfix

• Example
• x 3
• y x
• z x
• What are values of x, y and z?

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Precedence

• Prefix and -
• Operators , /, and
• Operators and -
• Postfix and --
• Use ( ) to group when result is obscure!

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Type conversion

• Java automatically converts less accurate types
  to more accurate ones in an expression
• byte i 100
• long l i 3 4
• double f i 3.1 1 / 2
• Will not automatically convert the other way

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Explicit Type Conversion

• double f 10.3
• int i (int ) f
• Note that number got truncated.
• Results can be surprising
• int i 257
• byte s (byte )i
• Whats the result?

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Character Data Type

• Used to represent a single character
• char letter A
• char number 5
• Characters enclosed in single quotation marks.
• They are Unicode.
• Some special characters
• Newline \n Carriage return \r
• Backspace \b Tab \t

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Program 0

• Not to be handed in (unless you want help).
• Target date Thursday, January 17
• Task
• Read in a series of positive and negative
  integers from the console.
• Entry stops when user types a zero.
• Print sum, as well as number of positive and
  negative integers entered.

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What Youll Need

• Basic console I/O
• Output
• system.out.println( )
• Input next slide.

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Input

• Ive provided a simple Class called Keyboard
• Same interface as in COMP14.
• int Keyboard.readInt()
• int Keyboard.readDouble()
• Demonstration project Keyboard Example
• Reads in only single numbers on a line.
• Bonus
• void Keyboard.pause()

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Resources

• http//java.sun.com
• Documentation
• Code
• Online Java tutorial
• http//java.sun.com/docs/books/tutorial/?frontpa
  ge-spotlight

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Recitation Tomorrow

• How to use Visual J
• demonstration
• Introduction to Java
• follow-up to what we talked about today

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Next Tuesday

• Binary numbers
• More Java