Review: reference data types and objects

Reference data type:
All classes are reference data types
Example:
Circle is a reference data type
Reference variables:
Variables defined with a reference data type are reference variables.

Reference variable (always) stores an address of an object
Example:
Circle circle1; // circle1 is a reference variable // I.e.: circle1 stores an address of a Circle object

Review: how reference data types and objects are stored in memory

Reference variables and objects are stored as follows:

Circle is a class or a reference data type
circle1 is a reference variable (stores an address) circle1 references (points to) a Circle object

Accesing members of an object - the member access operator (.)

An object's member can refer to:

After an object has been instantiated, its data fields can be accessed and its methods can be invoked using the dot operator (.):

objectRefVar.dataField accesses a data field in the object objectRefVar.method(arguments) invokes a method on the object

Example: circle1.radius

The dot (.) operator is also known as the object member access operator

Accessing members of an object - Example

public static void main() { Circle circle1 = new Circle(); // Create a Circle object circle1 Circle circle2 = new Circle(2); // Create a Circle object circle2 circle1.radius = 10; // Access the radius in circle1 object circle2.radius = 99; // Access the radius in circle2 object double area1 = circle1.getArea(); // Invoke getArea() method on circle1 double area2 = circle2.getArea(); // Invoke getArea() method on circle2 }

DEMO: demo/10-classes/06-access-members

Terminology: instance method

The method getArea( ) is invoked as an operation on a specific instance of Circle objects.
The method getArea( ) is referred to as an instance method, because you invoke it only on a specific instance.

Reference type variables and primitive type variables are stored differently

All variables consist of memory cells that store a value (binary number)
However: how the number is used can be different !
A variable of a primitive type stores the value itself
I.e.: the number in used in computations
A variable of a reference type stores a reference (= memory address) of the location of the object (= properties of the object)
Schematically:

An important consequence of the difference ways of storing variables

Assigning to variables of primitive type will make a (real) copy:

The variables k and c are separate/independent copies of the value 5

Therefore: update using c will not change k:

public static void main(String[] args) { int k = 5; int c = k; c = 99; // Does not update k System.out.println(k); // Prints 5 }

DEMO: demo/10-classes/07-aliasing/Demo.java

An important consequence of the difference ways of storing variables

Assigning to variables of reference type will make a alias:

The variables circle1 and circle2 reference the same Circle object (i.e.: alias !)

Therefore: update using circle1.radius will also change circle2.radius:

public static void main(String[] args) { Circle circle1 = new Circle(5); Circle circle2 = circle1; // alias ! circle2.radius = 99; // Same as circle1.radius System.out.println( circle1.radius ); // Prints: 99 }

DEMO: demo/10-classes/07-aliasing/Demo.java

Why Java have reference typed variables and primitive typed variables

Variables of a primitive data type can only store 1 value but can be accessed quickly
Objects can have many data fields and can allow the programmer to represent complex things in the real world