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Object-Oriented Programming Concept Examples

Let's recap. Everything can be described as an object. Every object has characteristics or states. The relationships are the structure of the object. has-a relationship can be described in Java code as member fields. is-a relationship can be described in Java keyword extends. The is-a relationship has an inheritance feature (like "extends" or "implements") and has-a relationship has an encapsulation feature (like private or protected modifier used before each member field or method).

Let's look at some examples. Translate the following perfect sentences into Java code.

1. Pizza is a food. A Pizza has ingredients, a cost, and a price.

   public class Pizza extends Food {//is-a relationship int ingredients;//has-a relationship double cost; double price; .... }

2. A car is a vehicle. A car has a make, model, year, and a motor.

   public class Car extends Vehicle {//is-a relationship String make; //has-a relationship int model; Date year; String motor; .... }

3. A dog is an Animal that has a name, color, weight, and sex.

   public class Dog extends Animal {//is-a relationship String name;//has-a relationship Color clr; double weight; boolean sex; .... }

4. Iowa is a state. A state has a budget, a department, a division and a state code.

   public class Iowa extends State {//is-a relationship double budget;//has-a relationship String department; String division; int stateCode; .... }

   A reader's comments:

   It would be better to make Iowa an instance of State: 
    public class State {
         String stateName;
         double budget;
         String department;
         String division;
         int stateCode;
   } 
   State iowa, california, newYork, texas, washinton;
   
   Or if State must be a super class, to make a subclass:
    
   public class UrbanState extends State
   
   public class RuralState extends State
   
   public class InteriorState extends State
   
   public class BorderState extends State
   

5. A savings account is a Bank account. A bank account has a bank transit number, an account number, an account number, a name, an address, an account balance, and an account status.

   public class SavingsAccount extends BankAccount {//is-a relationship int transitNumber; //has-a relationship int accountNumber; String name; String address; double accountBalance; String accountStatus; .... }

If you can tell difference or relationships between the Java code and the perfect sentences, you are on the right track in an object-oriented design and programming.

One more example here: Translate the following sentence into professional terms.

The heart of object-oriented programming is that one object can be composed of, or built from other objects.

Answer: "composed of" means "has-a" relationship and can be described as member fields or object composition. "built from" means "is-a" relationship, and can be described in Java keywords "extends" or "implements".

Understanding "is-a" and "has-a" relationship is a key to open your door to the object-oriented programming.

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