Review: Reference data types and objects

Reference data type:
Note: we have just learned how to create classes used to define variables !

Reference variables:

Variables defined with a reference data type is known as a reference variable.
Reference variable can reference objects of the corresponding data type

Reference data types and objects Illustrated

Reference variables and objects are stored as follows:

Circle is a reference data type circle1 is a reference variable circle1 references (points to) a Circle object

We create variables to store the properties of a new object when we create the object

The behavior of an object (= program instructions) is stored when Java compiles the class definition

Accesing members of an object

An object's member can refer to:

After an object is created, its data can be accessed and its methods can be invoked using the dot operator (.):

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

The dot (.) operator is also known as the object member access operator
Example: circle1.radius

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/03-classes/06-access-members

Terminology: instance method

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

Reference typed variables and primitive typed variables are stored differently

All variables consist of memory cells that can store a value (binary number)
A variable of a primitive type stores the value itself
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 way of storing variables

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

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 }

An important consequence of the difference way of storing variables

Assigning to variables of reference type will make a alias:

The variables circle1 and circle2 reference the same Circle object

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

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

DEMO: demo/03-classes/07

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