OOP: Lesson 2 - Vocabulary

Hello and welcome to a new part of the famous, loved and anticipated series about object-oriented programming. Today we will be talking about objects, methods, functions and get a basic understanding what happens when we use them.

I was not sure if I should start with this because we still lack some basic understanding what variables are, but I think we will be doing fine. Let us take a look at the code we use today.

EDIT: I just see that GitHub does not include line numbers I will have to add a way to highlight code by myself for hopelesscom. Just click on the link to GitHub to see line numbers

Looks pretty simple, doesn't it? What do we have in this code?

We could also create two files, one for each class. When we are writing larger programs and classes this would be the preferred way to do it. But since we only got some lines of code this works fine.

What is a class and an object

You can think of a class as a template for a number of things. Let us look at a human, for example. A human has hair, well most of them, and a hair color. The fact that humans have hair is a fact, the hair color varies from being to being.

Those "things" are called objects (humans). When you create an object from a class you can also say instance (please remember that we are currently not 100% accurate with this).

We create an object from the class in line 28. Each object created from one class has the same variables and methods but they can contain different values.

class and instance variables

We will be talking about variables in another post. But to understand the concept of classes and objects I will give you a basic idea about the different types. This is not the final explanation and will sound a lot different later on but for now it is okay.

In line 2 and 3 we declare class variables. When you create an object and set a variable the variable is different from the class variable. From this point you call it object or instance variable.

Back to humans: That they have_hair (because they all do) would be an class variable, the hair color an object variable since it is different for each object. In a later post we will clarify how to call it and what the real difference is. For now we will stick to this explanation.

methods and functions

They are basically the same thing. A piece of code, if coded well, doing one thing that can be called multiple times. So you write your code one time and execute it - maybe - many times.

In Java everything is part of an object. Every function is called method. Why? When a function is part of an object it is called method. This is the basic difference. That was the hard part,... ;)

To execute a block of code aka method you have to call the method. We do this in line 30 for example. The way you do it is always the same: object.method(arguments). If the method should return something you have to tell Java where the return value should be stored. You need a variable of the type matching the return value. The variable that should hold the return value stand on the left side of the = and the call on the right side. Look at the lines 20 to 26.

public and private

A method is either public or private. If it is public you can always call it from everywhere.

If it is private you can only call it from the object itself. If we would have a private method in the class IchObjekt you could only call it from one of the methods in IchObjekt not from lesson2.

return values

If a method is returning nothing it is void. All methods in our IchObjekt class return nothing so they are all of the type void.

In line 20 we have brauchEineZahl which is supposed to return a number - an integer - so the method needs to know that it should return something from the type int. If you tell a method it should return something you absolutely have to do it.


Sometimes you want to call methods without creating an object from a class. If you have a class that is doing basic math stuff there is no need to have objects of this class. You just created them to put some of your code in another files so it is reusable and looks better.

For those methods you use the modifier static. Our main method is always static because we do not create an object before calling it. The same goes for the brauchEineZahl. There is no object so the method has to be static.


When you create a new object there is always a method called constructor executed. No matter if you wrote one or not. In our example our constructor starts at line 5. It always has the same name as the class.


In line 5 we tell our constructor that it will get an argument when it is called. If you would create and object without giving the constructor an argument - see line 28 - Java would complain like a little child that "it wants it so badly and refuses to do anything before it gets one".

You can specify any number of arguments and they can be from any type you want. Just make sure you follow the argument order when calling the function. If the function expects an integer and a string and you call it supplying a string and an integer it will start complaining again.

As you can see in line 28 and 32 it does not matter if you pass the content from a variable or just add the value directly, which is kind of pointless in a real program, the only thing that matters is that the type is right. The variables also do not need to have the same name.

How to build a method

First we should think about our method and what it is supposed to do.

When we know all these things we can use what we just learned. You always build a method the same way.

keywords name ( arguments )

Keywords are public/private, static, return type. Always follow this order. Do not change it. Seriously, I mean it - changing the order is like typing Google into Google - you will break the internet. (Cookies for everyone who knows that this is a quote from IT Crowd)

You need a name for your method - you and Java should both know how to call this block of code.

Arguments are sometimes needed. Check the types and the order.

Remember to always put your code in { } and remember that you need a return statement when you used something else than void.

Quick summary

Classes describe a number of variables and functions for objects you build of them.

You can use methods to run the same code again and again or to change or extend objects beyond variables. Methods need keywords to know what they are actually supposed to do.

Java has many things in common with little, spoiled kids.


As usual a short translation of everything that is important.

>> posted on Jan. 24, 2012, midnight in java, newbies