Arithmetic: Division, Remainder, Overflow
Integer division's silent truncation, % beyond even/odd, wrapping overflow and its cures.
Java's arithmetic operators: + - * / %, plus ++ and --. The surprises
live in division and remainder:
System.out.println(7 / 2); // 3 — integer division truncates!
System.out.println(7.0 / 2); // 3.5 — one double makes the math double
System.out.println(7 % 2); // 1 — remainder ("mod")
System.out.println(-7 % 2); // -1 — sign follows the dividend in Java
Integer division is the classic beginner bug: double avg = sum / count;
where both are ints gives a truncated int result before the assignment.
Fix by widening first: (double) sum / count.
% is not just for even/odd checks — it drives clock arithmetic
((hour + 3) % 24), cycling indexes, and "every Nth item" loops.
Overflow awareness. Primitives are fixed-size boxes:
int max = Integer.MAX_VALUE; // 2_147_483_647
System.out.println(max + 1); // -2_147_483_648 — wraps around, no error!
The math wraps silently. For counters that could exceed ~2 billion (or where
correctness matters more than speed), use long; Java 8+ also offers
Math.addExact(max, 1), which throws instead of wrapping.
Augmented assignment updates in place: score += 10 is score = score + 10;
also -=, *=, /=, %=. And ++/-- add or subtract one — i++ in a
loop header is the idiom you will see everywhere.
Next: comparing values and combining conditions.