Module 21 · Types and memory · Drill

Types and memory drill

Scenario prompts that check whether types and memory is usable under pressure. Answer out loud, then compare.

4 prompts20 minNew

0 of 4 graded

Prompt 1
A Java method applyDiscount(Cart cart) mutates the cart's items and then sets cart = new Cart() before returning. The caller sees the discounted items but not the empty cart. Explain exactly why, then redesign Cart so this whole category of surprise cannot happen.
Say your answer out loud first. Thirty seconds is enough.
Prompt 2
A Java service's memory grows for days until it is killed. Heap dumps show millions of RequestContext objects referenced from a static ConcurrentHashMap keyed by request id, plus listener objects registered on an event bus by short-lived components. Explain why the garbage collector cannot help and fix both leaks.
Say your answer out loud first. Thirty seconds is enough.
Prompt 3
Design a generic Result type for a codebase that currently returns null or throws for failures. Show its type parameters, explain how Java's erasure limits the design compared with C# or Rust, and explain whether a Result of Dog should be usable where a Result of Animal is expected.
Say your answer out loud first. Thirty seconds is enough.
Prompt 4
A deduplication job keeps a HashSet of Customer objects and still produces duplicates. Customer overrides equals to compare normalised email but not hashCode; a later fix added hashCode over all fields; and the job lower-cases the email after inserting customers into the set. Explain every bug and the correct design.
Say your answer out loud first. Thirty seconds is enough.