Variables and Data Types in Java

What is a Variable?

A variable is like a box

It stores data that you can use and modify while the program runs.

edad

int

25

0x7FF...1234

nombre

String

"Ana"

0x7FF...5678

precio

double

19.99

0x7FF...9ABC

Technical Definition:

A variable is a space in memory reserved to store a value that can change while the program runs.

Name

Unique identifier to access the variable

edad, nombre, contador

Type

Determines what type of data it can store

int, String, double

Value

Data stored in the variable

25, "Ana", 19.99

Address

Location in the computer's memory

0x7FF...1234

Variable Declaration

1. Declaration

Reserve space in memory

int edad;

edad int ?

Variable declared but with no value assigned

2. Initialization

Assign initial value

int edad = 25;

edad int 25

Variable declared and initialized

3. Assignment

Change the stored value

edad = 26;

edad int 26

Updated value

Practice the Declaration

Variable Editor

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// Declara una variableint numero;

// Inicialízalanumero = 10;

// Modifica su valornumero = 20;

Memory State:

Output:

Run the code to see the output

Challenge:

Try declaring three variables: String nombre, double precio, boolean activo

Primitive Data Types

Integer Numeric Types

Decimal Numeric Types

Other Types

byte

8 bits

Description

Signed 8-bit integer. Ideal for saving memory when working with small values.

Range

-128

127

Minimum: -128 Maximum: 127 Possible values: 256

Usage Examples

byte edad = 25;

A person's age

byte temperatura = -5;

Temperature in degrees

byte contador = 100;

Small counter

Common Errors

byte numero = 200; // Error: fuera de rango

200 exceeds the maximum of 127 that a byte can store

Bit Calculator

Decimal value:

Sign: 0 (positive)

Value: 25

Primitive Type Comparison

Type	Size	Range	Default Value	Example	Typical Use
`byte`	8 bits	-128 to 127	0	`byte b = 100;`	Files, raw data
`short`	16 bits	-32,768 to 32,767	0	`short s = 1000;`	Memory saving
`int`	32 bits	-2^31 to 2^31-1	0	`int i = 100000;`	General integers
`long`	64 bits	-2^63 to 2^63-1	0L	`long l = 100000L;`	Large numbers
`float`	32 bits	±3.4E-38 to ±3.4E+38	0.0f	`float f = 3.14f;`	Single-precision decimals
`double`	64 bits	±1.7E-308 to ±1.7E+308	0.0d	`double d = 3.14159;`	Double-precision decimals
`char`	16 bits	'\u0000' to '\uffff'	'\u0000'	`char c = 'A';`	Unicode characters
`boolean`	1 bit*	true / false	false	`boolean b = true;`	Logical values

Type Conversion (Casting)

Implicit Conversion

Automatic, from lower to higher precision

byte

→

short

→

int

→

long

→

float

→

double

int entero = 100;
double decimal = entero; // 100.0 (implícito)

Explicit Conversion

Manual, from higher to lower precision (casting)

double precio = 19.99;
int precioEntero = (int) precio; // 19 (truncado)

Loss of precision when converting to smaller types

Casting Simulator

Original Value:

Original Type:

Convert to:

Original Value

127.8

double

Converted Value

127

int

Loss

0.8

decimals

Generated Code:

double valorOriginal = 127.8;
int valorConvertido = (int) valorOriginal;

Variables vs Constants

Variable

Its value can change during execution

int contador = 0;
contador = 1; // ✅ Permitidocontador = 2; // ✅ Permitido

When to use it?

Counters
Calculation results
User input
Changing state

VS

Constant (final)

Its value CANNOT change after being initialized

final double PI = 3.14159;
PI = 3.14; // ❌ ERROR: No se puede modificar

When to use it?

Mathematical values (π)
Fixed settings
Days of the week
Values that shouldn't change

Game: Identify the Constants

Drag each value to the correct category:

Math.PI

edadUsuario

DIAS_SEMANA

saldoCuenta

VELOCIDAD_LUZ

temperatura

Constants (final)

Variables

Score: 0/6

Variable Scope

Block Scope

Variables declared inside { }

public void metodo() {

                                            int x = 10;
                                        
System.out.println(x); // ✅ correcto
}
System.out.println(x); // ❌ Error

Only visible here

x = 10

Class Scope

Variables declared at class level

public class MiClase {

                                            int contador = 0;
                                        
public void metodo1() {
contador++; // ✅ correcto
}
public void metodo2() {
contador--; // ✅ correcto
}
}

Scope Explorer

public class ScopeDemo {
    // Variable de clase    static int claseVariable = 100;
    
    public static void main(String[] args) {
        // Variable local al método main        int mainVariable = 50;
        
        if (true) {
            // Variable de bloque if            int bloqueVariable = 25;
            System.out.println("Dentro del if:");
            System.out.println("claseVariable: " + claseVariable);
            System.out.println("mainVariable: " + mainVariable);
            System.out.println("bloqueVariable: " + bloqueVariable);
        }
        
        // bloqueVariable no es accesible aquí        // System.out.println(bloqueVariable); // ❌ Error
        
        System.out.println("Fuera del if:");
        System.out.println("claseVariable: " + claseVariable);
        System.out.println("mainVariable: " + mainVariable);
    }
}

Global Scope

claseVariable 100

Method main()

mainVariable 50

args []

Block if

bloqueVariable 25

1 claseVariable declared = 100

4 mainVariable declared = 50

7 bloqueVariable declared = 25

15 bloqueVariable no longer exists

Practical Exercises

Age Calculator

Declare variables to calculate age in different units:

Create a variable int anioNacimiento with your birth year
Create a variable int anioActual with the current year
Calculate the age in years and store it in int edadAnios
Calculate the age in months (approximately)
Calculate the age in days (approximately)

// Ejercicio 1: Calculadora de Edadpublic class CalculadoraEdad {
    public static void main(String[] args) {
        // Tu código aquí        
        
        
        
        // Display results        System.out.println("Edad en años: " + edadAnios);
        System.out.println("Edad en meses: " + edadMeses);
        System.out.println("Edad en días: " + edadDias);
    }
}

Mini-Challenge: Correct Declaration

Select the correct variable declarations:

int 1numero = 10;

double precio = 19.99;

String nombre = "Juan";

boolean esValido = "true";

char letra = 'AB';

Summary and Best Practices

Naming Rules

Use camelCase: miVariable
Starts with a letter or _
Don't use reserved words
Descriptive names
Avoid generic names

Common Errors

Not initializing variables
Using incorrect types
Forgetting the semicolon
Incorrect scope
Unnecessary casting

Professional Tips

Use final for constants
Choose the most appropriate type
Initialize when declaring
Document complex variables
Use self-explanatory names

Quick Reference Sheet

Basic Declaration:

tipo nombre;
tipo nombre = valor;

Primitive Types:

int, double, boolean, char
byte, short, long, float

Casting:

// Implícito (automático)int a = 10;
double b = a;

// Explícito (manual)double x = 9.99;
int y = (int) x;

Constants:

final double PI = 3.14159;
final int MAX_USUARIOS = 100;

Knowledge Test

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What is the size in bytes of the `int` type?

2 bytes 4 bytes 8 bytes 1 byte

Log In

Recover password

Create Account

What is a Variable?

A variable is like a box

Technical Definition:

Name

Type

Value

Address

Variable Declaration

1. Declaration

2. Initialization

3. Assignment

Practice the Declaration

Memory State:

Output:

Challenge:

Primitive Data Types

Integer Numeric Types

Decimal Numeric Types

Other Types

byte

Description

Range

Usage Examples

Common Errors

Bit Calculator

Primitive Type Comparison

Type Conversion (Casting)

Implicit Conversion

Explicit Conversion

Casting Simulator

Generated Code:

Variables vs Constants

Variable

When to use it?

Constant (final)

When to use it?

Game: Identify the Constants

Constants (final)

Variables

Variable Scope

Block Scope

Class Scope

Scope Explorer

Practical Exercises

Age Calculator

Mini-Challenge: Correct Declaration

Summary and Best Practices

Naming Rules

Common Errors

Professional Tips

Quick Reference Sheet

Basic Declaration:

Primitive Types:

Casting:

Constants:

Knowledge Test

What is the size in bytes of the int type?

What is the size in bytes of the `int` type?