The Coder's Handbook   



What is a Constant?

Sometimes in a program you'll use a value that is consistent across the entire program and it never changes.  For instance, we might want to have a variable represent an approximation of pi.  Your code really shouldn't change pi, because it's a fixed value.

To do this, we can use the keyword final to designate that a value is a constant.  This means that the value cannot be changed anywhere else in the program.

final double PI = 3.14159265359;

When you use a constant, think of it as a sealed box.  Once we initialize it with a value, it cannot be changed.

Why not just use a literal?

A literal is a hardcoded value.  For example, we might just print out the number 5 or the letter 'a'.   When you have a very important literal that's in your program, it is often referred to as a magic number.  Magic numbers are bad.  Here are a few reasons why:

When given proper names, constants and variables can be considered self-documenting.  That means their names describe their purpose, making your code clearer without even needing comments.

Sorry - no magic allowed in Java. 

Why not just use a variable?

Let's consider a new example - a very large video game you are working on a large programming team.  There are thousands of fixed values about player abilities, classes, and weapons.  You might represent some of those values as constants:


final int FIREBALL_MIN_DAMAGE = 8;

final int FIREBALL_MAX_DAMAGE = 48;

final double CRITICAL_HIT_CHANCE = .05;

If these were these variables, there's nothing stopping anyone on the team from changing those values anywhere in the code.  Imagine, for a moment, they are variables.  

double criticalHitChance = .05;

What if Kyle the intern writes the following line of code, trying to detect if a user scored a critical hit.

double roll = Math.random();   // Random number between 0 and 1

if(criticalHitChance = roll)




Kyle forgot that we use the equality operator (==) to test equivalence rather than the assignment operator (=), and in his code above he accidentally changed the value of criticalHitChance to a random number.  Now everything's a mess across the whole program.  Way to go, Kyle.

Using constants prevents us from making errors.  Even when you're working on a problem alone, you will make mistakes.  All of us are Kyle sometimes.  So if a value isn't ever meant to change, use a constant.


Why All Caps? 

In the examples above all the constants are in all capital letters with words split by underscores.   We call this MACRO_CASE.  This is for the same reason we use camelCase to name variables.  It's simply a useful convention.  These are industry standards, and it helps make your code easier to read:

If your code is well written, a reader should never have to look at the declaration to understand what they're looking at.


Three Forms of Data

In Java, data can come in three forms:  literals, variables, and constants.





No memory

Cannot change






Can change


int number

char letter


Stored in memory

Cannot change


final int NUMBER

final char LETTER


Alex Lee - Basic Tutorial on Final Keyword

Coding With John - All uses of final (includes objects / classes)