Operators & Control Flow

Arithmetic, comparison and logical operators, if/else, switch expressions, and for/while loops in Java.

Operators

Java
int a = 10, b = 3;

System.out.println(a + b);   // 13  addition
System.out.println(a - b);   // 7   subtraction
System.out.println(a * b);   // 30  multiplication
System.out.println(a / b);   // 3   integer division (truncates!)
System.out.println(a % b);   // 1   remainder

System.out.println(a > b);   // true
System.out.println(a == b);  // false
System.out.println(a != b);  // true

boolean x = true, y = false;
System.out.println(x && y);  // false — logical AND
System.out.println(x || y);  // true  — logical OR
System.out.println(!x);      // false — logical NOT

Compound assignment shortcuts:

Java
int total = 0;
total += 5;   // total = total + 5
total -= 2;   // total = total - 2
total++;      // increment by 1
total--;      // decrement by 1

if / else

Java
int score = 82;

if (score >= 90) {
    System.out.println("Grade: A");
} else if (score >= 75) {
    System.out.println("Grade: B");
} else {
    System.out.println("Grade: C");
}

Switch expressions (modern Java)

Java 14+ supports the arrow-style switch expression, which is safer than the old fall-through switch statement — no break needed, and it can return a value directly:

Java
int day = 3;

String name = switch (day) {
    case 1, 7 -> "Weekend";
    case 2, 3, 4, 5, 6 -> "Weekday";
    default -> "Unknown";
};

System.out.println(name); // Weekday

Loops

Java
// for — when you know how many iterations
for (int i = 0; i < 5; i++) {
    System.out.println("i = " + i);
}

// while — condition checked before each iteration
int n = 5;
while (n > 0) {
    System.out.println(n);
    n--;
}

// do-while — body always runs at least once
int attempts = 0;
do {
    attempts++;
} while (attempts < 3);

// enhanced for-each — iterating a collection or array
int[] scores = {90, 85, 77};
for (int s : scores) {
    System.out.println(s);
}

break exits a loop entirely; continue skips to the next iteration.

Java
for (int i = 0; i < 10; i++) {
    if (i == 3) continue;  // skip 3
    if (i == 6) break;     // stop at 6
    System.out.println(i);
}
// prints: 0 1 2 4 5

Common mistakes

Using = (assignment) instead of == (comparison) inside an if — Java's compiler thankfully rejects this for non-boolean expressions.
Forgetting the old-style switch statement falls through without break — prefer the modern arrow syntax (->), which never falls through.
Off-by-one errors in for loops — double-check < vs <=.

Interview questions

Q: Why does the classic Java switch statement need break? Historically, switch falls through to the next case unless a break stops execution — inherited from C. The modern arrow-form switch (Java 14+) removes this footgun by never falling through.

Q: What's the difference between while and do-while? while checks the condition before running the loop body, so it may run zero times. do-while checks after, so the body always runs at least once.

Operators & Control Flow

Operators#

if / else#

Switch expressions (modern Java)#

Loops#

Common mistakes#

Interview questions#