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Java Module 3

Control flow, methods and recursion interview questions

Interview questions on control flow, methods and recursion: warm-ups, then what each harder question tests and how a good answer starts.

18 questions: 6 warm-ups and 12 with model answers · Levels: Fresher to senior

Warm-up questions

Short questions with short answers, each answered in full.

  1. Fresher

    Why does if (stock = 5) not compile in Java, while if (paid = true) does?

    Answered in the lesson if, else and boolean logic

  2. Fresher

    What is the difference between a switch statement and a switch expression in Java?

    Answered in the lesson switch statements and switch expressions

  3. Fresher

    What can't the enhanced for loop do that a classic for loop can?

    Answered in the lesson Loops: while, do-while, for and for-each

  4. Fresher

    What is the difference between break and continue in a Java loop?

    Answered in the lesson break, continue and labelled loops

  5. Fresher

    Is Java pass-by-value or pass-by-reference?

    Answered in the lesson Methods, parameters, return and overloading

  6. Fresher

    What is a base case in recursion, and what happens if a recursive method has none?

    Answered in the lesson Recursion and the call stack

Questions with model answers

What each question tests, the first step of a good answer and the follow-ups to expect.

  1. Fresher to mid

    A courier charges by weight band: up to 500 g, up to 2 kg, up to 5 kg, up to 10 kg, and above 10 kg. A parcel of exactly 2 kg was charged the 5 kg rate. How do you find the bug, fix the else-if chain, and choose test values so that no boundary slips through again?

    What it tests
    Reading an else-if chain at its edges, choosing between < and <=, and designing boundary tests for bands.
    First step
    Write the bands as ranges with explicit edges, then run the chain by hand at 2,000 g: a test written as grams < 2000 sends exactly 2,000 g on to the next band. Fix the comparison, then list the values every edge needs: the edge itself and the value just past it.
    Follow-ups

    From the lesson if, else and boolean logic

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    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  2. Mid to senior

    A permission check reads if (!(!isAdmin && (!isOwner || isLocked))). Rewrite it so that a teammate can read it at a glance, prove that the rewrite means the same, and say what could still change behaviour if the three operands were method calls instead of variables.

    What it tests
    De Morgan's laws, double negation, exhaustive truth-table checks, and short-circuit order when operands have side effects.
    First step
    Remove the outer negation with De Morgan's law: !(!A && (!B || C)) becomes A || !(!B || C), which becomes A || (B && !C). Read it back as a rule, "admins, or owners of a document that is not locked", then compare all 8 combinations of the three inputs with the original.
    Follow-ups

    From the lesson if, else and boolean logic

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  3. Mid to senior

    An enum DeliveryMode is switched on in about 30 places across a service and two client libraries. Product adds a new constant, SAME_DAY. How do you make sure that every switch handles it, and what happens in a library that was compiled against the old enum and not rebuilt?

    What it tests
    Exhaustive switch expressions, why default hides new constants, statements against expressions, and what separately compiled code does at run time.
    First step
    Let the compiler find the switches. A switch expression over the enum with no default stops compiling when SAME_DAY appears, while a switch statement, or any switch with a default, compiles silently. So convert the statements to expressions, drop defaults that exist only "to be safe", rebuild everything and fix each error.
    Follow-ups

    From the lesson switch statements and switch expressions

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  4. Mid

    A pricing method is a 60-case switch on a product code string, and the business now wants to change prices every week without a release. Would you keep the switch, use an enum with a price field, or load a map from configuration? Justify the choice, including what happens with unknown and null codes.

    What it tests
    Separating data from code, compile-time checks against run-time flexibility, unknown and null keys, and how javac compiles a switch on strings.
    First step
    Separate what changes weekly from what changes with a release. The prices are data that change without a release, and a switch is compiled into the class, so the prices belong in a map loaded from configuration and checked at start-up. The codes and the rules around them stay in code.
    Follow-ups

    From the lesson switch statements and switch expressions

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  5. Fresher to mid

    for (double x = 0.0; x != 1.0; x += 0.1) { … } was meant to run 10 times, and it never ends. Explain why, fix it, and say how you would notice and locate a loop like this in a running service.

    What it tests
    Floating-point accumulation, loop termination conditions, integer counters, and finding a spinning thread from outside the process.
    First step
    0.1 has no exact binary value, so ten additions give 0.9999999999999999, not 1.0, and x != 1.0 stays true for ever. Count with an int instead, for (int i = 0; i < 10; i++), and compute x = i / 10.0 inside the loop.
    Follow-ups

    From the lesson Loops: while, do-while, for and for-each

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  6. Fresher to mid

    A report prints records in pages of 20. With 41 records the last record never appears, and with 40 records an empty last page appears. Write the page count and the loop bounds correctly, and choose the totals you would test.

    What it tests
    Ceiling division, half-open ranges, off-by-one errors at both ends of a loop, and choosing boundary test values.
    First step
    Count pages with ceiling division, (total + size - 1) / size, which gives 3 for 41 records and 2 for 40. Page p, counted from 0, covers the half-open range from p * size up to but not including Math.min(total, (p + 1) * size), so the loop test is i < end, never i <= end.
    Follow-ups

    From the lesson Loops: while, do-while, for and for-each

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  7. Mid to senior

    After a deploy, one worker thread of a service sits at 100 % of a CPU core and its queue stops draining. You suspect a loop that never ends. How do you confirm it, find the line, stop the damage and prevent this kind of bug?

    What it tests
    Thread dumps with jcmd or jstack, reading repeated samples, loop progress arguments, and tests with time limits.
    First step
    Take two or three thread dumps a few seconds apart with jcmd <pid> Thread.print and find the worker. If it is RUNNABLE in every dump, with the same top frame and a cpu= value that keeps climbing, it is spinning, and the line number in that frame points at the loop.
    Follow-ups

    From the lesson break, continue and labelled loops

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  8. Fresher to mid

    Reading a colleague's change, you find a method that searches a stadium plan (levels × rows × seats) for the first block of k adjacent empty seats, using three nested loops, two boolean flags and a labelled break. How would you restructure it, and what does the search cost in the worst case?

    What it tests
    Replacing flags and labels with methods that return, splitting a search into named steps, and counting the work of nested loops.
    First step
    Name the pieces. One method returns where the first run of k empty seats starts in a single row, or -1. A second loops over levels and rows and returns the first hit. Each return replaces a flag and the labelled break, and the run check can be tested on its own.
    Follow-ups

    From the lesson break, continue and labelled loops

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  9. Fresher to mid

    After List<Integer> ids = new ArrayList<>(List.of(10, 20, 30, 1)); ids.remove(1); what does ids hold, and why? How would you remove the value 1, and what does this teach about designing overloads?

    What it tests
    The phases of overload resolution (widening before boxing), the two remove methods of List, and overload design that avoids surprises.
    First step
    Two overloads apply: remove(int index) and remove(Object o). The argument 1 is an int, and the first phase of overload resolution, which allows no boxing, already finds remove(int). So the element at index 1, the value 20, is removed, and ids holds [10, 30, 1].
    Follow-ups

    From the lesson Methods, parameters, return and overloading

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 2 minutes

  10. Senior

    A logging method void debug(String format, Object... args) is called about 10 million times a minute, almost always with debug logging switched off, and a profiler shows a lot of garbage coming from those calls. Where does the garbage come from, and how would you remove it without changing the callers?

    What it tests
    What a varargs call allocates, boxing of primitive arguments, fixed-arity overloads, and when the JIT can or cannot remove the allocation.
    First step
    Each call builds a new Object[] for args before debug runs, and boxes every int or long argument into a wrapper object, all before the method can even check whether debug logging is on. Count it: two arguments mean one array and up to two boxes per call.
    Follow-ups

    From the lesson Methods, parameters, return and overloading

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  11. Mid to senior

    A recursive method walks the nested folders of an uploaded archive. It passes every test, but one customer's upload is nested 50,000 levels deep and the method throws StackOverflowError. What are your options, what does each one cost, and which would you ship?

    What it tests
    How stack depth grows with the input, stack size settings, an explicit stack, depth limits as input validation, and why catching the error is unsafe.
    First step
    Estimate first. On JDK 25 a trivial interpreted frame took about 112 bytes, and a real method's frame is larger, so 50,000 levels need several megabytes, more than the default 2 MiB thread stack on macOS on Apple silicon. Then ask whether such depth is a real use or an attack on the service.
    Follow-ups

    From the lesson Recursion and the call stack

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  12. Fresher to mid

    A plain recursive fib(40) takes about a fifth of a second on a laptop. Estimate how long fib(50) takes, explain why, and compare memoisation, a bottom-up table and a loop that keeps only two values. What changes once n passes 92?

    What it tests
    Exponential growth of calls, memoisation against tabulation, time and space of each version, and long overflow after fib(92).
    First step
    Count calls, not seconds. The plain version makes 2 × fib(n + 1) − 1 calls, 331,160,281 for n = 40, and each step of n multiplies that by about 1.618. Ten more steps multiply it by about 123, so fib(50) takes around half a minute.
    Follow-ups

    From the lesson Recursion and the call stack

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

All Java interview questions · The Java track