Control Flow I — if, else, switch

শর্ত — if, else, switch (Java 14+ expression)

Read: ~25 min Beginner 5 practice problems Live code runner

1. Branches — Making Decisions

Programs decide. if/else lets the program take different paths based on a boolean condition. switch dispatches on a single value against many cases. Modern Java (14+) turned switch into a first-class expression — so it returns a value and eliminates fall-through bugs.

প্রোগ্রাম সিদ্ধান্ত নেয় — কোন শর্তে কী করবে। if/else boolean-এর ভিত্তিতে path বাছে। switch একটি মানকে অনেকগুলো case-এর সাথে মেলায়। Java 14+ থেকে switch expression হয়ে গেছে — value return করে এবং পুরনো fall-through সমস্যা দূর করে।

2. if / else if / else

Main.java
class Main {
    public static void main(String[] args) {
        double cgpa = 3.72;
        String honor;

        if (cgpa >= 3.90)       honor = "Summa Cum Laude";
        else if (cgpa >= 3.70)  honor = "Magna Cum Laude";
        else if (cgpa >= 3.50)  honor = "Cum Laude";
        else if (cgpa >= 2.00)  honor = "Pass";
        else                      honor = "Fail";

        System.out.println("CGPA " + cgpa + " → " + honor);
    }
}
if শর্ত সত্যি হলে block চলে। শর্ত false হলে পরের else if, তারপর else। শুধু প্রথম match-ই চলবে। Condition parens-এ boolean expression দিতে হবে (Java-তে if (x) — x অবশ্যই boolean)।

3. The Ternary Operator ? :

Main.java
class Main {
    public static void main(String[] args) {
        int temp = 33;
        String advice = (temp >= 30) ? "Hot — drink water" : "Nice and cool";
        System.out.println(advice);

        int n = -5;
        int abs = n < 0 ? -n : n;
        System.out.println("|n| = " + abs);
    }
}
condition ? a : b — condition সত্যি হলে a, না হলে b। সরল এক-লাইনের পছন্দের জন্য দুর্দান্ত; nested ternary এড়িয়ে চলুন, পড়তে কঠিন হয়।

4. Classic switch — and the Fall-Through Trap

The old C-style switch falls through from one case to the next unless you add break. A forgotten break is a classic bug.

Main.java
class Main {
    public static void main(String[] args) {
        int day = 3;
        switch (day) {
            case 1: System.out.println("Saturday"); break;
            case 2: System.out.println("Sunday");   break;
            case 3: System.out.println("Monday");   break;
            case 4: System.out.println("Tuesday");  break;
            default: System.out.println("Other");
        }
    }
}
পুরনো switch-এ প্রতিটি case-এর শেষে break না দিলে পরের case-গুলোও execute হয়ে যাবে (fall-through)। ভুলে গেলে গোপন bug জন্মায়। Java 14+ এর নতুন switch এই সমস্যা সমাধান করে।

5. Modern switch Expression (Java 14+)

The arrow form case X -> value; has no fall-through, can group labels with commas, and can return a value straight into a variable.

Main.java
class Main {
    public static void main(String[] args) {
        int day = 6;
        String kind = switch (day) {
            case 1, 2, 3, 4, 5 -> "Weekday";
            case 6, 7             -> "Weekend";
            default                -> "Invalid";
        };
        System.out.println(day + " → " + kind);

        // When you need several statements, use a block with 'yield'
        int score = 72;
        String grade = switch (score / 10) {
            case 10, 9 -> "A";
            case 8     -> "B";
            case 7     -> "C";
            case 6     -> "D";
            default  -> {
                System.out.println("Computing fail grade...");
                yield "F";
            }
        };
        System.out.println("grade = " + grade);
    }
}
নতুন switch expression (->) — fall-through নেই, একাধিক label একসাথে (case 1,2,3 ->), এবং সরাসরি value return করে। একাধিক statement লাগলে block-এর ভেতর yield ব্যবহার করুন।

6. Pattern Matching in switch (Java 21)

Starting in Java 21, switch can match on types — a powerful way to handle heterogeneous inputs.

Main.java
class Main {
    static String describe(Object o) {
        return switch (o) {
            case Integer i when i < 0 -> "negative int " + i;
            case Integer i                -> "int " + i;
            case String s                  -> "string of length " + s.length();
            case null                      -> "nothing";
            default                        -> "something else";
        };
    }
    public static void main(String[] args) {
        System.out.println(describe(42));
        System.out.println(describe(-3));
        System.out.println(describe("Dhaka"));
        System.out.println(describe(null));
    }
}
Java 21-এ switch type-এর উপর match করতে পারে (pattern matching)। case Integer i when i < 0 -> ... মানে Integer type এবং শর্ত দুই-ই মানানসই হলে। null-ও এখন case হিসেবে সরাসরি লেখা যায়।

7. Common Bugs in Branches

❌ Mistakes

  • if (x = 5) — assignment by mistake. Java refuses non-boolean; in other languages this silently compiles.
  • if (x == null && x.foo()) — wrong order; must be != null.
  • Forgetting break in classic switch.
  • Comparing strings with == instead of .equals().
  • Overly nested if — refactor to switch or early return.

✅ Good habits

  • Prefer the modern switch expression.
  • Use early returns to flatten nested ifs.
  • Put null checks first with &&.
  • Always include a default.
  • Keep ternary simple — one condition per expression.

8. Vocabulary

TermMeaningবাংলায়
BranchA code path taken when a condition matches.শর্ত অনুযায়ী path।
Ternarycond ? a : b — 3-operand conditional expression.এক-লাইনের if/else।
Fall-throughClassic switch cases execute in sequence without break.পুরনো switch-এ break না দিলে পরের case চলে।
Switch expressionJava 14+ arrow-form switch that returns a value.Value return করা আধুনিক switch।
yieldReturn a value from a block inside a switch expression.switch expression-এর block থেকে value return।
Pattern matchMatching case by type (Java 21).type অনুযায়ী case match।

9. Practice Problems

  1. Given an age, print Child (<13), Teen (13-19), Adult (20-59), Senior (60+).
    age দিয়ে Child, Teen, Adult, Senior print করুন।
    ✨ Show Answer (উত্তর দেখুন)
    Main.java
    class Main {
    public static void main(String[] args) {
        int[] ages = { 7, 15, 28, 70 };
        for (int a : ages) {
            String label;
            if (a < 13)      label = "Child";
            else if (a < 20) label = "Teen";
            else if (a < 60) label = "Adult";
            else              label = "Senior";
            System.out.println(a + " → " + label);
        }
    }
}
  2. Using a modern switch expression, map a day number (1-7) to a Bangla day name.
    switch expression দিয়ে 1-7 → বাংলা বার-এর নাম।
    ✨ Show Answer (উত্তর দেখুন)
    Main.java
    class Main {
    public static void main(String[] args) {
        for (int d = 1; d <= 7; d++) {
            String bn = switch (d) {
                case 1 -> "শনিবার";
                case 2 -> "রবিবার";
                case 3 -> "সোমবার";
                case 4 -> "মঙ্গলবার";
                case 5 -> "বুধবার";
                case 6 -> "বৃহস্পতিবার";
                case 7 -> "শুক্রবার";
                default -> "?";
            };
            System.out.println(d + " → " + bn);
        }
    }
}
  3. Rewrite an if/else that returns "even" or "odd" as a single ternary expression.
    "even/odd" ternary দিয়ে লিখুন।
    ✨ Show Answer (উত্তর দেখুন)
    Main.java
    class Main {
    public static void main(String[] args) {
        for (int n : new int[]{1,2,3,4}) {
            System.out.println(n + " : " + (n % 2 == 0 ? "even" : "odd"));
        }
    }
}
  4. Explain what "fall-through" means and give one case where it is intentional.
    fall-through কী এবং কখন এটি ইচ্ছাকৃতভাবে কাজে আসে?
    ✨ Show Answer (উত্তর দেখুন)

    Answer: In a classic switch, if you omit break, execution continues into the next case — that is fall-through. It is occasionally intentional when several adjacent cases share the same body (e.g., case 1: case 2: case 3: handleSmall(); break;). In modern switch expressions with arrows, fall-through is impossible; you group labels with a comma instead (case 1, 2, 3 -> handleSmall();).

  5. Write a helper that maps an HTTP status code (200, 301, 404, 500, anything else) to a description using a switch expression.
    HTTP status code → description — switch expression দিয়ে।
    ✨ Show Answer (উত্তর দেখুন)
    Main.java
    class Main {
    static String describe(int code) {
        return switch (code) {
            case 200 -> "OK";
            case 301 -> "Moved Permanently";
            case 404 -> "Not Found";
            case 500 -> "Server Error";
            default -> "Unknown (" + code + ")";
        };
    }
    public static void main(String[] args) {
        for (int c : new int[]{200, 301, 404, 500, 418})
            System.out.println(c + " → " + describe(c));
    }
}

Summary — Module 09

if / else if / else branches on booleans. The ternary ? : is a handy one-liner. Classic switch falls through unless you add break. The modern switch expression (Java 14+) removes fall-through, allows label groups with commas, returns a value, and supports pattern matching on types from Java 21 — prefer it in new code.

if/else, ternary এবং পুরনো switch-এর fall-through — সব জানুন, কিন্তু নতুন কোডে Java 14+ এর arrow-switch ব্যবহার করুন। Java 21-এ type-ভিত্তিক pattern match।

Next Module → Loops — for, while, do-while, and labeled break.

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