Reference types include class types, interface types, and array types.

When the constructor is called, an object is created on the heap and a

reference is assigned to the variable. When a variable of an object gets

out of scope, the reference is broken and when there are no references

left, the object gets marked as garbage. The garbage collector then

collects and destroys it some time afterwards.

A reference variable is null when it does not reference any object.

Arrays in Java are created at runtime, just like class instances. Array

length is defined at creation and cannot be changed.

int[] numbers = new int[5];

numbers[0] = 2;

numbers[1] = 5;

int x = numbers[0];

// Long syntax

int[] numbers = new int[] {20, 1, 42, 15, 34};

// Short syntax

int[] numbers2 = {20, 1, 42, 15, 34};

In Java, multi-dimensional arrays are represented as arrays of arrays.

Technically, they are represented by arrays of references to other

arrays.

int[][] numbers = new int[3][3];

numbers[1][2] = 2;

int[][] numbers2 = {{2, 3, 2}, {1, 2, 6}, {2, 4, 5}};

Due to the nature of the multi-dimensional arrays, sub-arrays can vary

in length, so multi-dimensional arrays are not bound to be rectangular

unlike C:

only

numbers[0] = new int[3];

numbers[1] = new int[2];

Classes are fundamentals of an object-oriented language such as Java.

They contain members that store and manipulate data. Classes are divided

into top-level and nested. Nested classes are classes placed inside

another class that may access the private members of the enclosing

class. Nested classes include member classes (which may be defined with

the static modifier for simple nesting or without it for inner classes),

local classes and anonymous classes.

| Top-level class | class Foo { // Class members } |

|---|---|

| Inner class | class Foo { // Top-level class class Ba… |

| Nested class | class Foo { // Top-level class static c… |

| Local class | class Foo { void bar () { class Foobar… |

| Anonymous class | class Foo { void bar () { new Object ()… |

Non-static members of a class define the types of the instance variables

and methods, which are related to the objects created from that class.