Data Types & I/O

Primitive types

• Eight primitives: byte, short, int, long, float, double, char, boolean. They are not objects.
• Integer literals are int by default; use L for long. Floating literals are double by default; use f for float.
• char is an unsigned 16-bit UTF-16 code unit, not a full Unicode code point (supplementary characters need two chars).

Reference types and local vs field defaults

• Classes, arrays, interfaces, and enums are references. Uninitialized *fields* are null (or zero/false for primitives).
• Local variables have **no** default — you must assign before use or javac fails.
• var (Java 10+) infers a local type from the initializer: var name = "Ada"; is String. You cannot write var x; with no initializer.

Console output and Scanner input

• System.out.print / println / printf write to standard output.
• Scanner in java.util reads tokens from System.in. Close it only if you own the stream; closing System.in is often a mistake in small demos.

import java.util.Scanner;

public class IoDemo {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Age: ");
        int age = sc.nextInt();
        sc.nextLine(); // consume leftover newline
        System.out.print("Name: ");
        String name = sc.nextLine();
        System.out.printf("%s is %d years old%n", name, age);
        sc.close();
    }
}

Widening vs narrowing conversion

• Widening (int → long → float → double) is implicit. Narrowing (double → int) needs a cast and truncates toward zero.
• Integer overflow wraps silently (Integer.MAX_VALUE + 1 is Integer.MIN_VALUE).

Scanner pitfalls and numeric precision

• nextInt() does not consume the line ending. Mixing nextInt() and nextLine() is a classic bug — call nextLine() to clear the rest of the line.
• next() reads one token (whitespace-delimited); nextLine() reads the rest of the line.
• float/double are binary floating point — 0.1 + 0.2 is not exactly 0.3. Use BigDecimal for money.
• Never use == to compare doubles for equality in real code; compare with an epsilon or Double.compare.

Primitives, cast, and formatted output

public class TypesDemo {
    public static void main(String[] args) {
        int whole = 10;
        double widened = whole;      // implicit
        int truncated = (int) 9.99;  // 9
        long big = 3_000_000_000L;
        System.out.println(widened);
        System.out.println(truncated);
        System.out.println(big);
    }
}

Read two numbers and print the sum

import java.util.Scanner;

public class SumInput {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        System.out.println("sum = " + (a + b));
        sc.close();
    }
}