Java Variables
Declare, Initialize, and Use

Variables in Java

In Java, a variable is a named container used to store data that can be changed during the execution of a program. Think of it as a labeled box where you can keep values—whether that's numbers, text, or more complex objects.

Why Are Variables Important?

Variables make your program flexible and dynamic. They allow you to write code that can adapt based on inputs, conditions, or changing data. Without variables, programming would be static and manual.

How to Declare a Variable in Java

Declaring a variable means telling Java what type of data the variable will hold and giving it a name. The basic syntax is:

datatype variableName;

Example:

int age;
String name;
double price;

How to Initialize a Variable

Initialization means assigning a value to the declared variable. You can do this at the time of declaration or later in your code.

1. Declare and Then Initialize

int age;
age = 25;

2. Declare and Initialize Together

String name = "Alice";

Variable Types in Java

Java is a statically-typed language, which means every variable must have a data type. Here are some commonly used types:

Data Type	Description
int	Stores integers (whole numbers), e.g., 1, 100
double	Stores decimal numbers, e.g., 3.14
char	Stores a single character, e.g., 'A'
boolean	Stores true or false
String	Stores sequences of characters, e.g., "Hello"

Java Variable Naming Rules

Here’s what you need to know to name your variables properly, along with examples for better understanding:

1. Names can contain letters, digits, underscores, and dollar signs

   // Valid variable names
   int total_Amount = 100;
   double price2 = 49.99;
   String user$name = "John";

2. Names must begin with a letter, not a digit

   // Correct
   int count1 = 5;
   
   // Incorrect (will cause a compile-time error)
   int 1count = 5; // ❌ Invalid: starts with a digit

3. Names are case-sensitive (value and Value are different)

   int value = 10;
   int Value = 20;
   
   System.out.println(value); // Outputs: 10
   System.out.println(Value); // Outputs: 20

4. Names should not be Java reserved keywords

   // Invalid variable names (these are reserved by Java)
   int class = 1;   // ❌ Error
   boolean public = true; // ❌ Error
   
   // Correct alternative names
   int classNumber = 1;
   boolean isPublic = true;

Example: Declaring and Using Variables

public class VariableDemo {
    public static void main(String[] args) {
        int age = 30;
        String name = "John";
        boolean isAdult = true;
        
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("Is Adult? " + isAdult);
    }
}

Name: John
Age: 30
Is Adult? true

Explanation of the Output

Here, the program stores a person’s name, age, and adulthood status in variables. When the program runs, it prints each of those values using System.out.println. Java concatenates strings using the + operator.

Checkpoints for Beginners

• Always declare a variable with its correct type. A mistake like int name = "Alice"; will throw a compile-time error.
• Uninitialized variables cannot be used. For instance, trying to print a variable before assigning a value will result in a compile error.
• Be mindful of types during assignments. For example, you can't directly assign a double to an int without casting.

Best Practices

• Use meaningful variable names like totalAmount, isAvailable, rather than x or data.
• Stick to camelCase naming (e.g., userAge, itemPrice) for variables.
• Initialize variables when possible to avoid accidental use of undefined values.