W7003 The Basics

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

— Merlin, The Coder

First Steps

Prepare a Directory

john-williams@codermerlin:~/$ cd

john-williams@codermerlin:~/$ mkdir AP-Practice

john-williams@codermerlin:~/$ cd AP-Practice

john-williams@codermerlin:~/AP-Practice$ mkdir HelloWorld

john-williams@codermerlin:~/AP-Practice$ cd HelloWorld

john-williams@codermerlin:~/AP-Practice/HelloWorld$

Create a New File

john-williams@codermerlin:~/AP-Practice/HelloWorld$ emacs HelloWorld.java

Enter the following text:

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Helpful Hint

• Names matter - The file name MUST match the name of the public class
• An executable must have an entry point which is a public, static function named main, accepting a single argument, an array of strings.

Compilation

A Java program can be compiled with the javac command. Note that the argument to the command is the file to be compiled, including the ".java" suffix. This will produce a file with the ".class" suffix.

john-williams@codermerlin:~/AP-Practice/HelloWorld$ javac HelloWorld.java

Execution

A Java program may be executed with the java command. Note that the argument to the command is the file to be executed, excluding the ".class" suffix.

john-williams@codermerlin:~/AP-Practice/HelloWorld$ java HelloWorld

Short Cuts

We can combine these two commands in bash as follows:

• 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