Objectives

Students will be able to:

  • understand that property and behaviour definitions can be encapsulated in a class file
  • understand that we can instantiate objects by calling a class file's constructor method
  • write a program to communicate with properties and methods encapsulated in an instantiated object(s)

Advantages

Improved security

Using data-hiding / Use of private variables

So that other classes cannot (accidentally) access/change the data (directly) / So other classes have to use accessor methods to access the data / So programs can be made read-only

Ease of maintenance

Programs will be easier to update

Because changes to the encapsulated class should not impact on other classes

Code reuse

Methods are stored in a class.

These methods can be used by mutiple instances of the class ie code can be reused, reducing the amount of code needed in a program.

Ease of understanding

Programs will be easier to follow

Because all data and methods are in the same place

Ease of testing

Because the class can be fully tested in isolation

And limited code will contain fewer mistakes than the full program

Faster development (of whole program)

Different classes can be worked on by teams

Each class is effectively independent of others (although this is not always true - see dependencies)

ENCAPSULATION

So, we can classify digital objects by designing class files!

We can determine the properties (attributes/variables) associated with the class.

We can also define behaviours (methods) that can access (get) and mutate (set) the values of the properties.

In this example, we are defining what a digital Chicken is like. Some Chickens lay eggs, some don't.

public class Chicken{

private boolean eggLayer; //eg true or false

private String name; //eg Titan

private double weightInGrams; //eg 120.5


//constructor method

public Chicken(boolean eggLayer, String name, double weight){

this.eggLayer = eggLayer;

this.name = name;

this.weightInGrams = weight;

}


//accessor methods

public boolean getEggLayer(){

return this.eggLayer;

}

public String getName(){

return this.name;

}

public double getWeight(){

return this.weightInGrams;

}


//mutator methods

public void setEggLayer(boolean layerStatus){

this.eggLayer = layerStatus;

}

public void setWeight(double newWeight){

this.weight = newWeight;

}

}

Encapsulation

So, we can see that everything we need to know about Chicken objects is defined in the Chicken class.

The class includes both:

  • properties/attributes eg name, weightInGrams
  • behaviours/methods eg getWeight(), setWeight()

Encapsulation is when properties and methods are stored in a single class file.

Data hiding is also part of encapsulation - data can only be accessed and changed via methods

Exercises

Attempt the Dragon Project in the Exercises tab. Keep in mind the concepts covered so far:

When designing a class, identify the attributes/properties that each instance of that class will have eg name, score.

Use data hiding! This will only allow access to class attributes via secure methods...

Design a constructor method which will usually initialise class attributes.

Design accessor methods to allow other classes access to the attributes of the class you are designing.

Design mutator methods to allow other classes an opportunity to edit/modify data stored in the attributes of the class you are designing.

Test your design by creating instances of the class in a Main program!

Exercises

Tags

idesyntax file class instantiate constructor type propertymethod