TopolearnMost bugs in beginner code come from integer division, precedence or comparing the wrong way — this lesson heads them off.
The arithmetic operators + - * / % behave as expected with one big surprise: when both operands are integers, / performs integer division and throws away the fraction. 7 / 2 is 3, while 7 / 2.0 is 3.5. % gives the remainder (7 % 2 == 1), handy for "every n-th" logic.
Mixed expressions are widened automatically to the larger type (int + double → double). Going the other way needs an explicit cast, which can lose information: (int) 3.99 is 3 (it truncates, it does not round).
Shortcuts: x += 5 means x = x + 5 (and casts back to the type of x). i++ and ++i both add one; they differ only in the value the expression yields, so avoid using them inside larger expressions.
Comparison operators == != < <= > >= return a boolean. For primitives == compares values. For objects it compares references, which is why strings need .equals (next lesson).
The logical operators && and || short-circuit: the right side is evaluated only if needed. That makes guards like s != null && s.isEmpty() safe. ! negates.
The ternary operator chooses between two values: String label = n == 1 ? "item" : "items";.
Precedence follows math (* before +), and comparison binds tighter than &&, which binds tighter than ||. When in doubt, add parentheses — readers will thank you.
Example
public class Ops {
public static void main(String[] args) {
System.out.println(7 / 2); // 3 (integer division)
System.out.println(7 / 2.0); // 3.5
System.out.println(7 % 3); // 1
System.out.println((int) 3.99); // 3 (cast truncates)
System.out.println(Math.round(3.5)); // 4
int score = 10;
score += 5; // 15
String s = null;
boolean empty = s != null && s.isEmpty(); // short-circuit: no NPE
String label = score >= 15 ? "pass" : "fail";
System.out.println(score + " " + empty + " " + label); // 15 false pass
}
}Remember
- int / int truncates — make one operand a double for a fractional result
- Casting a double to int truncates toward zero; use
Math.roundto round &&and||short-circuit, so they can guard against null
Watch out for
- Computing an average as
sum / countwith two ints and silently losing the fraction.