Data Formats in Java

Java Data Formats and other Basics

Primitive Java Data Types

Everything that directly maps onto hardware.

boolean: true/false (1 bit)
char: Unicode characters as numbers 0 to 2¹⁶-1 (16 bit)
short: -2¹⁵ to 2¹⁵-1 (16 bit)
int: -2³¹ to 2³¹-1 (32 bit)
long: -2⁶³ to 2⁶³-1 (64 bit)
float: -2^-149 to (2-2^-23)*2¹²⁷ (32 bit)
double: -2^-1074 to (2-2^-52)*2¹⁰²³ (64 bit)

Non-Primitive Data Types

Combines simple types to more complex but easier to use stuff.

Strings: Any kind of text
Arrays: Table of a data type accessible by an index
Class: Blueprints for objects
Double, Integer, Float, Long, Character, Byte, Boolean, Short: Classes of the primitives
Object: A "living" class
File: A handle to an OS file
Streams, channels: Stuff for I/O
...: Much much more...

Something special

Special types and states

void: When we do not care about the type
null: When something is not defined yet, null is not 0 aka null is not Zero. 0/zero is a state, while null is the state of nothing.

Classes

Something that sounds a little more like Java.

A class defines properties, states, and logic or activities
It is the blueprint/template/manual to build an object
Fields are properties
Logic or activities are methods
A special method is the constructor

public class Invoice
{
    private int subtotal;
    private int tax;

    public Invoice(int amount)
    {
        this.subtotal = amount;
        this.tax = 20;
    }

    private int getTaxAmount()
    {
        return this.subtotal * (tax / 100);
    }

    public int getTotal()
    {
        return this.subtotal + getTaxAmount();
    }

    public int getSubTotal()
    {
        return this.subtotal;
    }
}

Objects

Turn your class templates into real things.

An object or instance is a materialized class definition
Objects of the same type are independent
Data space is reserved with every new object
Methods are not copied, because they cannot be changed
Data identical objects are not identical objects

public void anyExample()
{
    final Invoice invoice1 = new Invoice(12);
    final Invoice invoice2 = new Invoice(42);
    final Invoice invoice3 = new Invoice(12);

    int total = invoice1.getTotal() + invoice2.getTotal() + invoice3.getTotal();
}

Mutable and Immutable

Change is not always good

Mutable

You can change objects data after creation
Can have side effects
Has positive and negative memory effects

public class Invoice
{
    private int subtotal;

    public Invoice(int amount)
    {
        this.subtotal = amount;
    }

    public int getSubTotal()
    {
        return this.subtotal;
    }

    public void setSubTotal(int newSubTotal)
    {
        this.subtotal = newSubTotal;
    }
}

Immutable

You cannot change data anymore
Easier to understand but also side effects
Functional programming mantra

public class Invoice
{
    private final int subtotal;

    public Invoice(int amount)
    {
        this.subtotal = amount;
    }

    public int getSubTotal()
    {
        return this.subtotal;
    }
}