Learn Java Programming

// Declaration and initialization
int age = 25;
double price = 19.99;
boolean isActive = true;
char grade = 'A';
String name = "Alice";

// Declare first, assign later
int count;
count = 10;

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

// Constants (cannot be reassigned)
final double PI = 3.14159;
final int MAX_USERS = 1000;
// PI = 3.14; // ERROR: cannot assign to final variable

// Widening (implicit) — smaller to larger type, no data loss
int myInt = 100;
double myDouble = myInt;  // 100.0 automatically

// Narrowing (explicit) — larger to smaller, may lose data
double pi = 3.14159;
int rounded = (int) pi;   // 3 (decimal part lost)

// String to number conversion
String numStr = "42";
int parsed = Integer.parseInt(numStr);      // 42
double parsedD = Double.parseDouble("3.14"); // 3.14

// Number to String
String s1 = String.valueOf(42);       // "42"
String s2 = Integer.toString(100);    // "100"
String s3 = "" + 42;                  // "42" (concatenation trick)

public class ScopeDemo {
    // Instance variable — belongs to the object
    private String instanceVar = "I belong to each object";

    // Static variable — shared across all instances
    private static int count = 0;

    public void method() {
        // Local variable — exists only within this method
        int localVar = 10;
        System.out.println(localVar);       // OK
        System.out.println(instanceVar);    // OK
        System.out.println(count);          // OK

        if (true) {
            int blockVar = 5;  // Only visible inside this if-block
        }
        // System.out.println(blockVar); // ERROR: not in scope
    }
}

// Local variable type inference (Java 10+)
var message = "Hello";          // inferred as String
var numbers = List.of(1, 2, 3); // inferred as List<Integer>
var map = new HashMap<String, Integer>(); // inferred as HashMap

// Useful when the type is obvious from the right side
var reader = new BufferedReader(new FileReader("data.txt"));

// Cannot use var for:
// - fields (instance/static variables)
// - method parameters
// - uninitialized variables
// var x; // ERROR: cannot infer type without initializer