Java Operators and Expressions

Operators are the symbols that do work on values: add, compare, combine conditions. You’ll use them in every line of logic you write.

Arithmetic operators#

Operator	Meaning	Example	Result
+	add	7 + 3	10
-	subtract	7 - 3	4
*	multiply	7 * 3	21
/	divide	7 / 3	2 (int division)
%	remainder (modulus)	7 % 3	1

% is incredibly useful: n % 2 == 0 checks for even numbers, n % 10 gives the last digit.

public class Digits {
    public static void main(String[] args) {
        int n = 4729;
        System.out.println("Last digit: " + n % 10);        // 9
        System.out.println("Without last: " + n / 10);      // 472
        System.out.println("Is even? " + (n % 2 == 0));     // false
    }
}

Assignment and compound assignment#

int score = 10;
score += 5;   // score = score + 5  → 15
score -= 3;   // 12
score *= 2;   // 24
score /= 5;   // 4
score %= 3;   // 1

Increment and decrement#

int a = 5;
int b = a++;   // b = 5, then a becomes 6  (post-increment)
int c = ++a;   // a becomes 7, then c = 7  (pre-increment)
System.out.println(a + " " + b + " " + c);   // 7 5 7

Post (a++) uses the old value then increments; pre (++a) increments first. Interviewers love asking this. In real code, use a++ on its own line to keep things clear.

Comparison operators#

All return a boolean:

Operator	Meaning
==	equal to
!=	not equal
> <	greater / less than
>= <=	greater or equal / less or equal

Logical operators#

Operator	Meaning	True when
&&	AND	both sides are true
||	OR	at least one side is true
!	NOT	flips true/false

int age = 20;
boolean hasId = true;
boolean canEnter = age >= 18 && hasId;      // true
boolean discount = age < 12 || age > 60;    // false
System.out.println(canEnter + " " + discount + " " + !canEnter);

Short-circuit evaluation#

&& stops as soon as the left side is false; || stops as soon as the left side is true. This protects you from errors:

String name = null;
if (name != null && name.length() > 3) {   // safe: length() never runs when name is null
    System.out.println("Long name");
}

The ternary operator#

A one-line if/else that returns a value:

int marks = 72;
String result = marks >= 40 ? "Pass" : "Fail";
System.out.println(result);   // Pass

Format: condition ? valueIfTrue : valueIfFalse. Great for simple choices; use a normal if for anything complex.

Operator precedence#

Like maths, some operators go first:

1. () brackets
2. ++ -- ! (unary)
3. * / %
4. + -
5. < > <= >=
6. == !=
7. &&
8. ||
9. ?:
10. = += …

System.out.println(2 + 3 * 4);       // 14, not 20
System.out.println((2 + 3) * 4);     // 20
System.out.println("Sum: " + 2 + 3); // Sum: 23 (text joining goes left to right!)
System.out.println("Sum: " + (2 + 3)); // Sum: 5

When in doubt, add brackets. They make code clearer for humans too.

The Math class#

public class MathDemo {
    public static void main(String[] args) {
        System.out.println(Math.max(12, 30));      // 30
        System.out.println(Math.min(12, 30));      // 12
        System.out.println(Math.abs(-7));          // 7
        System.out.println(Math.pow(2, 10));       // 1024.0
        System.out.println(Math.sqrt(144));        // 12.0
        System.out.println(Math.round(4.5));       // 5
        System.out.println(Math.ceil(4.1));        // 5.0
        System.out.println(Math.floor(4.9));       // 4.0
        int dice = (int) (Math.random() * 6) + 1;  // 1 to 6
        System.out.println(dice >= 1 && dice <= 6);  // true
    }
}

Common mistakes#

• if (x = 5) → assignment, not comparison. Java gives a compile error for ints (good!). Use ==.
• Expecting 5 / 2 to be 2.5. Use 5 / 2.0 or cast one side to double.
• Mixing & with &&: single & doesn’t short-circuit.
• "Total: " + a + b joins text instead of adding. Wrap the sum in brackets.

Exercise#

1. Given seconds = 3725, print it as 1 h 2 min 5 s using / and %.
2. Write an expression that is true when a year is a leap year: divisible by 4 and not by 100, unless divisible by 400.
3. Use the ternary operator to print “Adult” or “Minor”.
4. Predict the output: int x = 3; int y = x++ + ++x; What are x and y?

Hint for 2

(year % 4 == 0 && year % 100 != 0) || year % 400 == 0

Hint for 4

x++ gives 3 (x becomes 4), ++x makes x 5 and gives 5. So y = 8, x = 5.

Interview corner#

• **i++ vs ++i?** Post-increment returns the old value; pre-increment returns the new one.
• **&& vs &?** && short-circuits; & always evaluates both sides (and also works as bitwise AND on ints).
• **What is "5" + 3 + 2?** "532". And 3 + 2 + "5"? "55". Evaluation is left to right.

Summary#

• Arithmetic: + - * / %; watch for integer division.
• Comparisons return booleans; logical &&, ||, ! combine them with short-circuiting.
• Ternary ?: picks between two values; brackets beat memorising precedence.
• Math has max, min, abs, pow, sqrt, round, random and more.

Next: Strings, the most used class in Java.