Difference between revisions of "W7003 The Basics"

Revision as of 22:32, 27 November 2019

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

— Merlin, The Coder

First Steps[edit]

Prepare a Directory[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

Semicolons are required at the end of each statement

Static Typing[edit]

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[edit]

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[edit]

int gpa;
gpa = 3.75;

float f = 10.5; 

byte b = -128;
b -= 1;

Implicit Casting[edit]

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

Explicit Casting[edit]

Required when magnitude of numeric type may not be preserved

Division by Zero[edit]

Compare and contrast integer vs floating point

New[edit]

The new keyword is not used for primitive types

Primitive numeric types[edit]

Primitive numeric types overflow and underflow silently

@@ Line 1: / Line 1: @@
 == First Steps ==
-* Emacs and Java
+=== Prepare a Directory ===
-* mkdir ~/AP-Practice
+{{ConsoleLines|
-* Within this directory, mkdir Hello, World
+{{Lime|john-williams@codermerlin}}:{{Cyan|~/}}$ cd<br/>
-* Create a new file, HelloWorld.java
+{{Lime|john-williams@codermerlin}}:{{Cyan|~/}}$ mkdir AP-Practice<br/>
-** Names matter - the public class name MUST match the file name
+{{Lime|john-williams@codermerlin}}:{{Cyan|~/}}$ cd AP-Practice<br/>
-* Compile with '''javac'''
+{{Lime|john-williams@codermerlin}}:{{Cyan|~/AP-Practice}}$ mkdir HelloWorld<br/>
-* An executable must have an entry point which is a '''public''', '''static''' function named ''main'', accepting a single argument, an array of strings.
+{{Lime|john-williams@codermerlin}}:{{Cyan|~/AP-Practice}}$ cd HelloWorld<br/>
-* Execute with '''java'''
+{{Lime|john-williams@codermerlin}}:{{Cyan|~/AP-Practice/HelloWorld}}$ <br/>
+}}
+=== Create a New File ===
+{{ConsoleLines|
+{{Lime|john-williams@codermerlin}}:{{Cyan|~/AP-Practice/HelloWorld}}$ emacs HelloWorld.java<br/>
+}}
+Enter the following text:
+<syntaxhighlight lang="java">
+public class HelloWorld {
+    public static void main(String[] args) {
+        System.out.println("Hello, World!");
+    }
+}
+</syntaxhighlight>
+{{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.
+{{ConsoleLines|
+{{Lime|john-williams@codermerlin}}:{{Cyan|~/AP-Practice/HelloWorld}}$ javac HelloWorld.java<br/>
+}}
+=== 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.
+{{ConsoleLines|
+{{Lime|john-williams@codermerlin}}:{{Cyan|~/AP-Practice/HelloWorld}}$ java HelloWorld<br/>
+}}
 == Short Cuts ==