W7003 The Basics

Within these castle walls be forged Mavens of Computer Science ...

— Merlin, The Coder

First Steps

• Emacs and Java
• mkdir ~/AP-Practice
• Within this directory, mkdir Hello, World
• Create a new file, HelloWorld.java
  • Names matter - the public class name MUST match the file name
• Compile with javac
• An executable must have an entry point which is a public, static function named main, accepting a single argument, an array of strings.
• Execute with java

Short Cuts

• Compilation and Execution can be combined
• All of this can occur within emacs
• Emacs shortcut and up arrow
• Define keyboard macro with f3 and f4, execute with f4
  • CTRL-X, CTRL-S, ALT-SHIFT-&, Up-Arrow, ENTER

Semicolons

Semicolons are required at the end of each statement

Static Typing

• Java is statically typed
• Variables must be declared before they can be used, including the type
• Types define the possible values of the variable and the operations that may be performed upon it

Primitive Types

NOTE: All primitive types begin with a lowercase letter

• byte (8-bit)
• short (16-bit)
• int (32-bit)
• long (64-bit)
• boolean
• char (16-bit unicode)
• float
• double

Common Errors

int gpa;
gpa = 3.75;

float f = 10.5; 

byte b = -128;
b -= 1;

Implicit Casting

• Magnitude of numeric types is preserved (precision may be lost)

Explicit Casting

• Required when magnitude of numeric type may not be preserved

Division by Zero

• Compare and contrast integer vs floating point

New

The new keyword is not used for primitive types

Primitive numeric types

• Primitive numeric types overflow and underflow silently