W7003 The Basics

From Coder Merlin
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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


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!");
Hint.pngHelpful 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.


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


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]

We can combine these two commands in bash as follows:

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

We can perform the same actions in emacs using the Async Shell Command and typing in the same line as we used in bash. The Async Shell Command can be invoked in emacs as ALT-SHIFT-&. Then, type: javac HelloWorld.java && java HelloWorld ENTER.

In order to repeat the command in emacs, again enter the key combination ALT-SHIFT-& followed by ENTER.


While we can enter this command very quickly, we can do even better by defining a macro within emacs. Our macro will first save the file to preserve our changes, and then compile and execute the program. We beginning defining a macro with F3 and complete the definition with F4. We can then execute the macro with F4.

Thus, the complete sequence for defining the macro is:

We can now execute the macro by simply pressing F4.

Language Basics[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
  • Values do not always have to be assigned when a variable is declared, though it is best practice to do so. In some cases, a reasonable default value will be assigned by the compiler.

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


The new keyword is not used for primitive types

Primitive numeric types[edit]

  • Primitive numeric types overflow and underflow silently