IllegalStateException ☕ Java

java.lang.IllegalStateException

A method was called when the object wasn’t in the right state for it.

Seen on: Java

Meaning

Examples: iterator.remove() twice, writing to a committed HTTP response, Optional.get misuse in some libraries, Spring context not refreshed. The message explains the state problem.

Common causes

  • Calling methods out of order
  • Response already committed (servlets)
  • Stream already operated upon or closed
  • Duplicate keys in Collectors.toMap

⚡ Quick fix

  1. Read the message for the expected state
  2. Don’t reuse streams
  3. Pass a merge function to Collectors.toMap

Detailed fix by platform

Java

  1. Collectors.toMap(User::getEmail, u -> u, (a, b) -> a)

How to diagnose

  1. Message — Which state is wrong?
  2. Order — Which call came before?

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