Java 101 - Oriented Object Programming

Object Oriented Programming vs Procedural Programming?

Object oriented code can be quickly changed and since it's based on several classes one does not have to re-write the whole program but rather just change classes.

There are two kind of classes on Java.

Special Classes: They pass to the JVM in order to start and require a main method.

Regular Classes: Do not need a main method.

How can we run a class without a main method. We can instantiate it as an object in our code and then read/write to the instance variables and call the methods from other objects.

This is our main method:

public class ExternalClass
{
  public String name;
  public int age;    public void myMethod()
  {
  String status = "love"
  System.out.println("My name is " + name +
  "and I am" + age + "year's old");
  System.out.println("and I + status + "you");
  }
}

A quick note about scope. Variables have scope. Scope is the portion of the code that a variable exists within. Meaning the String “love” called status cannot be accessed outside of the method.

Now let's build our main class that will instantiate our ExternalClass.

public class MyClass
{
  public static void main ( String args [] )
  {
  //Instantiate ExternalClass
  ExternalClass myObj = new ExternalClass();
  //Access variables of ExternalClass
  myObj.name = "Gonzalo"
  myObj.age = 25;
  //Run the method
  myObj.myMethod();
  // Would output:
  // My name is Gonzalo and I am 25 year's old
  }
}

Congratulations!!!

You just wrote your first object oriented program.