Variable Declaration in Java

Java requires explicit variable declaration before use. The syntax follows:

<data_type> <variable_name> [= initial_value];

Multiple variables can share a type declaration:

int count, index, total;          // Three uninitialized integers
double value = 3.14;             // Initialized floating-point
boolean isValid = true;           // Boolean initialization
String greeting = "Hello World";  // String declaration
char initial = 'A';               // Character assignment

Variable Categories

Java supports three variable scopes:

1. Class variables (static fields)
2. Instance variables (non-static fields)
3. Local variables (method-scoped)

Local Variables

• Declared within methods, constructors, or blocks
• Created upon scope entry and destroyed upon exit
• No access modifiers allowed
• Require explicit initialization before use

Valid Local Variable Example

public class Calculation {
  public void compute() {
    int result = 0;       // Initialized local variable
    result = 5 * 10;
    System.out.println("Result: " + result);
  }
  
  public static void main(String[] args) {
    Calculation calc = new Calculation();
    calc.compute();      // Output: Result: 50
  }
}

Uninitialized Variable Error

public class ErrorExample {
  public void process() {
    int uninitialized;   // Never initialized
    uninitialized += 10; // Compilation error
  }
}

Error: variable uninitialized might not have been initialized

Instance Variables

• Declared in classes outside methods
• Initialized when objects are created
• Default values: 0 (numeric), false (boolean), null (objects)
• Accessible throughout class methods

Instance Variable Implementation

public class Account {
  public String holderName;    // Public instance variable
  private double balance;      // Private instance variable

  public Account(String name) {
    holderName = name;         // Constructor initialization
  }

  public void deposit(double amount) {
    balance = amount;          // Method modifies instance variable
  }

  public void display() {
    System.out.println("Account Holder: " + holderName);
    System.out.println("Balance: $" + balance);
  }

  public static void main(String[] args) {
    Account acct = new Account("Alice");
    acct.deposit(1500.75);
    acct.display();
  }
}

Account Holder: Alice
Balance: $1500.75

Class Variables (Static Fields)

• Declared with static modifier
• Single copy shared across all instances
• Constants use public static final naming convention
• Accessed via ClassName.variableName

Static Variable Implementation

public class Organization {
  private static int employeeCount;        // Private static variable
  public static final String BRANCH = "HQ"; // Public constant

  public static void main(String[] args) {
    employeeCount = 125;
    System.out.println(BRANCH + " staff count: " + employeeCount);
  }
}

HQ staff count: 125