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 = "Gonzalo"
  myObj.age = 25;
  //Run the method
  // Would output:
  // My name is Gonzalo and I am 25 year's old


You just wrote your first object oriented program.