How would you build a thread-safe cache with computeIfAbsent, and what is the trap?

computeIfAbsent is the right tool for a memoising cache: exactly one thread computes, everyone else waits and gets the same value, with no double computation and no check-then-act race.

The trap is what runs under the bin lock. If the mapping function updates the same map, you can get an IllegalStateException ("recursive update") or, on Java 8, a thread that spins forever inside the call (JDK-8062841). If it is slow (an I/O call, a remote lookup), every other writer that hashes to the same bin is blocked for its whole duration, and if two threads compute entries that recursively depend on each other, they deadlock.

For expensive values the classic alternative is storing a FutureTask or CompletableFuture: insert the future atomically, then compute outside the lock.