Java Data Types and Variables

What are Variables?

Variables in Java are containers that store data values. Think of them as labeled boxes where you can put different types of information. Before using a variable, you must declare it by specifying its type and name.

Variable Declaration Syntax

dataType variableName = value;

Example:

int age = 25;
String name = "John";

Data Types in Java

Java has two main categories of data types:

1. Primitive Data Types

These are the basic data types built into Java. They store simple values and are not objects.

Numeric Types:

1. byte

• Size: 8 bits
• Range: -128 to 127
• Use: Small integer values
• Example: byte temperature = 23;

1. short

• Size: 16 bits
• Range: -32,768 to 32,767
• Use: Medium integer values
• Example: short population = 12000;

1. int

• Size: 32 bits
• Range: -2^31 to 2^31-1
• Use: Most common for whole numbers
• Example: int score = 95000;

1. long

• Size: 64 bits
• Range: -2^63 to 2^63-1
• Use: Very large whole numbers
• Example: long population = 7800000000L;

Floating-Point Types:

1. float

• Size: 32 bits
• Precision: 6-7 decimal digits
• Example: float price = 19.99f;

1. double

• Size: 64 bits
• Precision: 15-16 decimal digits
• Example: double pi = 3.14159265359;

Other Primitive Types:

1. char

• Size: 16 bits
• Stores: Single character/letter
• Example: char grade = 'A';

1. boolean

• Size: 1 bit
• Stores: true or false
• Example: boolean isActive = true;

2. Reference Data Types

These are more complex types that refer to objects.

1. String

• Stores text
• Example: String message = "Hello World";

1. Arrays

• Stores multiple values of the same type
• Example: int[] numbers = {1, 2, 3, 4, 5};

1. Classes

• User-defined types
• Example: Student student = new Student();

Variable Naming Rules

1. Must start with a letter, underscore (_), or dollar sign ($)
2. Can contain numbers but cannot start with them
3. Case-sensitive (age and Age are different variables)
4. Cannot use Java keywords
5. Should use camelCase convention (firstName, lastName)

Variable Scope

1. Instance Variables

• Declared inside class but outside methods
• Accessible throughout the class

   public class Example {
       int instanceVar = 10; // Instance variable
   }

2. Local Variables

• Declared inside methods
• Only accessible within that method

   public void method() {
       int localVar = 20; // Local variable
   }

3. Static Variables

• Shared across all instances of a class
• Declared with ‘static’ keyword

   public class Example {
       static int staticVar = 30; // Static variable
   }

Best Practices

1. Always initialize variables before using them
2. Use meaningful variable names
3. Choose appropriate data types based on your needs
4. Use constants (final keyword) for values that won’t change
5. Keep variable scope as narrow as possible

Common Examples

// Basic variable declarations
int age = 25;
double salary = 50000.50;
String name = "John Doe";
boolean isEmployed = true;

// Array declaration
int[] scores = new int[5];

// Constant declaration
final double PI = 3.14159;

// Multiple variables of same type
int x = 1, y = 2, z = 3;

These fundamentals form the basis of Java programming. Understanding data types and variables is crucial for writing efficient and error-free code.