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Interview Tasks — URL Shortener

Sequential user stories, one at a time. Budget 30 minutes.

Task 1: Shorten and Expand (8 min)

Story: shorten(longUrl) returns a short code. expand(code) returns the original URL, or empty when the code is unknown.

Acceptance Criteria:

  • Null or blank input is rejected
  • Invalid URLs are rejected; "valid" is agreed with the interviewer and written down (the solution uses: absolute http or https with a host)
  • Two different URLs never share a code
  • expand of an unknown code is Optional.empty()

Task 2: Idempotent (4 min)

Story: Shortening the same URL twice returns the same code.

Acceptance Criteria:

  • Same input, same code
  • Surrounding whitespace does not create a second code

Task 3: Pluggable Code Generation (10 min)

Story: Support a counter-based base62 strategy and a random 8-character strategy. Codes must stay unique either way.

Acceptance Criteria:

  • CodeGenerator interface, two implementations
  • Base62 encodes like an odometer: 0, ..., Z, 10; 62^7 - 1 is ZZZZZZZ
  • A random collision is retried, never overwrites an existing mapping

Hints:

  • Ask: "Do codes need to be unguessable?" That decides counter vs random

Task 4: Many Threads (8 min)

Story: 20 threads shorten the same URL at the same time. Exactly one code is created.

Acceptance Criteria:

  • A test proves exactly one generator call and one distinct code across 20 threads
  • Many threads shortening many URLs stay consistent (expand(shorten(u)) == u)

Hints:

  • ConcurrentHashMap.computeIfAbsent is the whole answer. Being able to say why (get then put is two operations with a gap; computeIfAbsent is one atomic one) is the point
  • Claim the code in the reverse map inside the same function so it can never lag

Questions to Ask Afterwards

  • Why base62? How many URLs fit in 7 characters?
  • What happens to the counter strategy on two servers? (Id ranges, or a database sequence)
  • Where does this live in production and what is the cache?

Tips for Interviewers

  • Watch for HashMap in task 4. If it survives, ask what happens under two threads.