Variables & Types

public class Variables {
    public static void main(String[] args) {
        // Primitive types (stored by value)
        byte   b = 127;              // -128 to 127
        short  s = 32767;
        int    i = 2_147_483_647;    // underscores for readability (Java 7+)
        long   l = 9_999_999_999L;   // L suffix required
        float  f = 3.14f;            // f suffix required
        double d = 3.14159265358979;
        boolean flag = true;
        char   c = 'A';              // 16-bit Unicode

        // var — type inference (Java 10+)
        var name = "Alice";          // String
        var count = 42;              // int
        var pi = 3.14;               // double

        // String — special class (immutable, reference type)
        String first = "Hello";
        String second = "World";
        String combined = first + ", " + second + "!";

        // String methods
        System.out.println(combined.length());         // 13
        System.out.println(combined.toUpperCase());    // HELLO, WORLD!
        System.out.println(combined.contains("World")); // true
        System.out.println(combined.replace("World", "Java")); // Hello, Java!
        System.out.println(combined.substring(7));     // World!
        System.out.println(combined.trim());           // removes whitespace

        // String comparison — use .equals(), NOT ==
        String a = "hello";
        String b2 = "hello";
        System.out.println(a.equals(b2));     // true (correct)
        // System.out.println(a == b2);       // might be true (accident) or false

        System.out.println(combined);
    }
}

public class TypeConversion {
    public static void main(String[] args) {
        // Widening conversion (automatic)
        int i = 42;
        double d = i;   // int -> double, automatic

        // Narrowing conversion (explicit cast required)
        double pi = 3.14159;
        int truncated = (int) pi;  // 3 — decimal part lost!

        // String conversions
        int num = 42;
        String s = Integer.toString(num);    // "42"
        String s2 = String.valueOf(num);     // "42"
        String s3 = "" + num;               // "42" (concatenation trick)
        int back = Integer.parseInt("42");   // 42 (throws NumberFormatException if invalid)

        double d2 = Double.parseDouble("3.14");
        boolean flag = Boolean.parseBoolean("true");  // true

        // Wrapper classes — boxed versions of primitives
        Integer boxed = 42;     // auto-boxing
        int unboxed = boxed;    // auto-unboxing

        // Useful constants
        System.out.println(Integer.MAX_VALUE);   // 2147483647
        System.out.println(Integer.MIN_VALUE);   // -2147483648
        System.out.println(Double.MAX_VALUE);    // 1.7976931348623157E308

        System.out.println(s + " " + back);
    }
}