3.1 Selection: if, if-else, else-if Ladder

if (marks >= 90)      grade = 'A';
else if (marks >= 75) grade = 'B';
else if (marks >= 60) grade = 'C';
else                  grade = 'F';

The condition must be boolean — if (x = 5) is a compile error in Java (unlike C, where it is a silent bug). The dangling else binds to the nearest unmatched if, so always use braces.

3.2 switch — Classic vs Arrow Form

Classic switch works on int-compatible types (byte, short, char, int), String (since Java 7), enums, and wrapper types — not long, float, double, or boolean.

switch (day) {
    case 1:
    case 7:  System.out.println("Weekend"); break;
    case 2:  System.out.println("Monday");  break;   // forget break => fall-through!
    default: System.out.println("Weekday");
}

Fall-through is the number-one trap: without break, execution continues into the next case, even default.

Arrow switch (Java 14+) eliminates fall-through and can be an expression that yields a value:

String type = switch (day) {
    case 1, 7      -> "Weekend";
    case 2, 3, 4, 5, 6 -> "Weekday";
    default        -> {
        System.out.println("Invalid input");
        yield "Unknown";              // yield returns a value from a block
    }
};

3.3 Iteration: while, do-while, for, for-each

for (int i = 1; i <= 3; i++) System.out.print(i + " ");   // 1 2 3
int[] arr = {10, 20, 30};
for (int v : arr) System.out.print(v + " ");              // 10 20 30

for-each limitations (exam favourite): no index available, cannot modify the array slot through the loop variable, traverses forward only. All three sections of a classic for are optional — for(;;) is an infinite loop.

3.4 Jump Statements: break, continue, Labels

• break exits the innermost loop or switch.
• continue skips to the next iteration.
• A label lets you break/continue an outer loop — the only sanctioned "goto-like" jump in Java:

outer:
for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if (i * j == 4) break outer;   // leaves BOTH loops
        System.out.println(i + "," + j);
    }
}

Output:

1,1
1,2
1,3
2,1

3.5 Classic Loop Patterns (Write-a-Program Bank)

Reverse digits / palindrome number:

int n = 121, rev = 0, t = n;
while (t != 0) { rev = rev * 10 + t % 10; t /= 10; }
System.out.println(n == rev ? "Palindrome" : "Not palindrome");

Prime check (trial division up to √n):

boolean prime = n > 1;
for (int i = 2; i * i <= n; i++)
    if (n % i == 0) { prime = false; break; }

Pyramid pattern (nested loops — printed in the code snippet below), Fibonacci series, Armstrong numbers, and factorial round out the standard exam bank. Master the pattern skeleton: outer loop = rows, inner loop(s) = spaces then symbols.

🎯 Exam Focus

1. Compare while and do-while loops. Write a menu-driven program using do-while and switch.
2. What is fall-through in switch? How does the arrow (->) form of switch eliminate it? Which data types are NOT allowed in a switch expression?
3. Explain labelled break and continue with a nested-loop example and its exact output.
4. Write a program to check whether a number is prime, and another to print all Armstrong numbers between 1 and 1000.
5. Write a program to print a pyramid of stars of height n using nested loops.
6. Predict the output of a switch with missing break statements (fall-through trace).