Java

Overview

Java programs consist of class declarations containing method and data member declarations, and comments to explain them.
Each class is contained in a separate file.

Class Declaration Syntax

class <classname> {

<class member declarations>

}

Names can only contain letters, digits, underscores, or dollar signs.
Names are case sensitive.
First letter must be an upper case character.
Where names/identifiers have multiple words, the first letter of all words after the first will be upper case (don't use underscores for spaces).

Java programs composed of one or more classes: predefined or written by programmer
Class members are either methods or data members.

Import Statement Syntax

For importing a specific class:

import <packagename> . <classname> ;

For importing all classes in a package:

import <packagename> . * ;

Predefined classes are stored in packages.
Packages can be nested.
2 ways of using predefined classes:
- Using the fully qualified name to point to the class location using dot notation.
  - E.g. <packagename> . <classname>
- Or, using an import statement, which establishes a reference to the class(es) so you only need to use the class name in the rest of the program.

Java is compiled into platform-independent bytecode.
The Java Virtual Machine (JVM) interprets bytecode into machine code that a computer can execute.
The JVM is available for many different operating system.

This means that one program, compiled into bytecode, will run on any system with the JVM regardless of operating system.

only primitive values are truly stack resident
objects/arrays are always created by calling new
- allocated on the heap and accessed by reference variables
variables are copied by value
- occurs when assigning, initialising, or during parameter passing
reference variables also copied by value
- the referenced object is not copied, just the reference to it
- both variables then refer to the same object