Variables and Data Types in Java

public class PrimitiveTypes {
    public static void main(String[] args) {
        byte studentCount = 45;
        short yearOfAdmission = 2024;
        int annualFee = 150000;
        long aadharNumber = 123456789012L;  // Note: L suffix for long

        float cgpa = 8.75f;                 // Note: f suffix for float
        double preciseGpa = 8.756789;

        char grade = 'A';
        boolean isPlaced = true;

        System.out.println("=== Student Profile ===");
        System.out.println("Students in class (byte): " + studentCount);
        System.out.println("Year of admission (short): " + yearOfAdmission);
        System.out.println("Annual fee (int): " + annualFee);
        System.out.println("Aadhar number (long): " + aadharNumber);
        System.out.println("CGPA (float): " + cgpa);
        System.out.println("Precise GPA (double): " + preciseGpa);
        System.out.println("Grade (char): " + grade);
        System.out.println("Placed (boolean): " + isPlaced);
    }
}

public class TypeCasting {
    public static void main(String[] args) {
        // Widening (implicit) - no data loss
        int marks = 85;
        double marksAsDouble = marks;  // int -> double automatically
        System.out.println("Widening: int " + marks + " -> double " + marksAsDouble);

        byte b = 42;
        int bAsInt = b;  // byte -> int automatically
        System.out.println("Widening: byte " + b + " -> int " + bAsInt);

        // Narrowing (explicit) - possible data loss
        double gpa = 8.97;
        int gpaAsInt = (int) gpa;  // Truncates, does NOT round!
        System.out.println("\nNarrowing: double " + gpa + " -> int " + gpaAsInt);

        int bigNumber = 300;
        byte bigAsByte = (byte) bigNumber;  // Overflow!
        System.out.println("Narrowing: int " + bigNumber + " -> byte " + bigAsByte);

        // char and int casting
        char ch = 'A';
        int charValue = ch;  // Widening: char -> int
        System.out.println("\n'A' as int: " + charValue);

        int num = 66;
        char numAsChar = (char) num;  // Narrowing: int -> char
        System.out.println("66 as char: " + numAsChar);
    }
}

public class ReferenceTypes {
    public static void main(String[] args) {
        // String - the most used reference type
        String name = "Vikram Sharma";
        String college = "NIT Warangal";

        System.out.println("Name: " + name);
        System.out.println("Name length: " + name.length());
        System.out.println("Uppercase: " + name.toUpperCase());

        // String is immutable - methods return NEW strings
        String greeting = "hello";
        String upper = greeting.toUpperCase();
        System.out.println("\nOriginal: " + greeting);     // Still "hello"
        System.out.println("Uppercase: " + upper);          // "HELLO"

        // null reference
        String address = null;
        System.out.println("\nAddress is null: " + (address == null));
        // System.out.println(address.length()); // This would throw NullPointerException!

        // Arrays - another reference type
        int[] marks = {85, 92, 78, 95, 88};
        System.out.println("\nFirst mark: " + marks[0]);
        System.out.println("Array length: " + marks.length);
    }
}

public class ConstantsAndVar {
    // Class-level constants
    static final double PI = 3.14159265358979;
    static final int MAX_MARKS = 100;
    static final String UNIVERSITY = "Anna University";

    public static void main(String[] args) {
        // final prevents reassignment
        final int PASSING_MARKS = 35;
        System.out.println("Passing marks: " + PASSING_MARKS);
        // PASSING_MARKS = 40; // ERROR: cannot assign a value to final variable

        // var keyword (Java 10+) - type inferred by compiler
        var studentName = "Neha";     // Compiler infers String
        var age = 19;                  // Compiler infers int
        var gpa = 8.5;                 // Compiler infers double
        var isTopRanker = true;        // Compiler infers boolean

        System.out.println("\nStudent: " + studentName);
        System.out.println("Age: " + age);
        System.out.println("GPA: " + gpa);
        System.out.println("Top ranker: " + isTopRanker);

        System.out.println("\nPI: " + PI);
        System.out.println("Max marks: " + MAX_MARKS);
        System.out.println("University: " + UNIVERSITY);
    }
}

public class WrapperDemo {
    public static void main(String[] args) {
        // Autoboxing: primitive -> wrapper
        Integer marks = 85;           // int 85 -> Integer object
        Double percentage = 87.5;     // double -> Double
        Character grade = 'A';        // char -> Character
        Boolean passed = true;        // boolean -> Boolean

        System.out.println("Marks: " + marks);
        System.out.println("Percentage: " + percentage);
        System.out.println("Grade: " + grade);
        System.out.println("Passed: " + passed);

        // Unboxing: wrapper -> primitive
        int rawMarks = marks;         // Integer -> int
        double rawPercentage = percentage;  // Double -> double
        System.out.println("\nRaw marks (int): " + rawMarks);

        // Useful wrapper methods
        System.out.println("\n--- Utility Methods ---");
        System.out.println("Parse string to int: " + Integer.parseInt("42"));
        System.out.println("Parse string to double: " + Double.parseDouble("3.14"));
        System.out.println("Int MAX: " + Integer.MAX_VALUE);
        System.out.println("Int MIN: " + Integer.MIN_VALUE);
        System.out.println("Double MAX: " + Double.MAX_VALUE);

        // Convert int to binary/hex/octal strings
        System.out.println("\n42 in binary: " + Integer.toBinaryString(42));
        System.out.println("42 in hex: " + Integer.toHexString(42));
        System.out.println("42 in octal: " + Integer.toOctalString(42));
    }
}

public class TypeRanges {
    public static void main(String[] args) {
        System.out.println("=== Primitive Type Ranges ===");
        System.out.println("byte:    " + Byte.MIN_VALUE + " to " + Byte.MAX_VALUE);
        System.out.println("short:   " + Short.MIN_VALUE + " to " + Short.MAX_VALUE);
        System.out.println("int:     " + Integer.MIN_VALUE + " to " + Integer.MAX_VALUE);
        System.out.println("long:    " + Long.MIN_VALUE + " to " + Long.MAX_VALUE);
        System.out.println("float:   " + Float.MIN_VALUE + " to " + Float.MAX_VALUE);
        System.out.println("double:  " + Double.MIN_VALUE + " to " + Double.MAX_VALUE);

        // Integer overflow demonstration
        int maxInt = Integer.MAX_VALUE;
        System.out.println("\n--- Integer Overflow ---");
        System.out.println("Max int: " + maxInt);
        System.out.println("Max int + 1: " + (maxInt + 1));  // Wraps to MIN_VALUE!

        byte maxByte = 127;
        System.out.println("\nMax byte: " + maxByte);
        byte overflowed = (byte)(maxByte + 1);
        System.out.println("Max byte + 1: " + overflowed);    // Wraps to -128!

        // Float precision loss
        System.out.println("\n--- Precision ---");
        float f = 1.23456789f;
        double d = 1.23456789;
        System.out.println("float:  " + f);   // Loses precision after ~7 digits
        System.out.println("double: " + d);   // Precise to ~15 digits
    }
}