Head First Java Chapter 03 — Know Your Variables

3 min readJun 8, 2022

Hello Readers,

This article is to highlight the main points include in the chapter.

Declaring a Variable

Java is dealing with type. As a result, you must declare the type of your variable in order for this type safety to work.
Variables are two types:
1. Primitive Variables
2. Object Reference Variables

When you declaring a variable you need to consider these 2 rules:
1. Variable must have a type
2. Variable must have a name

Primitive Variables

Primitive types are the most basic data types available within the Java language. There are 8: boolean, byte, char, short, int, long, float and double. These types serve as the building blocks of data manipulation in Java (Source: https://wikibooks.org).

You can put a large value such as long into a little value such as int without losing any information.
However, a little value can be assigned to a large variable.

Primitive Naming Convention.

Here is the rules for naming a variables.
1. It must start with a letter, underscore(_), or dollar sign ($). You can’t start a name with a number.
2. After the first character, you can use numbers as well, just don’t start it with a number.
3. It cannot be from the Java reserved words.

Object Reference Variables

There is actually no such thing as an object variable.
There’s only an object reference variable.
An Object reference variable holds bits that represent a way to access an object.
It doesn’t hold the object itself.
Object reference are like Remote Control for actual object.
You cannot do arithmetic on a reference variable.

The 3 steps of object declaration, creation and assignment

Declare a reference variable — Dog myDog = new Dog();
Create an Object — Dog myDog = new Dog();
Link the Object and the Reference — Dog myDog = new Dog();

Array

Declare an int array variable, An array variable is a remote control to an array object.

int[] nums

Create a new int array with a length of 3.

nums = new int[3]

Give each element in the array an int value.

nums[0] = 6;

nums[1] = 5;

nums[2] = 4;

Arrays are always objects, whether they’re declared to hold primitives or object reference.
Once you’ve declared an array, you can’t put anything in it except things that are of the declared type.

BULLET POINTS

Variables come in two flavors: primitive and reference.
Variables must always be declared with a name and a type.
A primitive variable value is the bits representing the value (5, ‘a’, true, 3.1416, etc.).
A reference variable value is the bits representing a way to get to an object on the heap.
A reference variable is like a remote control. Using the dot operator (.) on a reference variable is like pressing a button on the remote control to access a method or instance variable.
A reference variable has a value of null when it is not referencing any object.
An array is always an object, even if the array is declared to hold primitives. There is no such thing as a primitive array, only an array that holds primitives.