E187: Synchronized Call on Boxed Class

This warning is emitted when you call the synchronized method on a boxed primitive value (like Int, Boolean, Double, etc.).

Calling synchronized on a boxed primitive is suspicious because:

The boxed object may be a different instance each time due to boxing
It's unlikely to provide the synchronization behavior you expect
Different boxed values of the same primitive may or may not share the same lock

Longer explanation:

You called the synchronized method on a boxed primitive. This might not be what you intended.

Example

scala

def example(): Unit =
  1.synchronized {
    println("hello")
  }

Error

scala

-- [E187] Potential Issue Warning: example.scala:2:4 ---------------------------
2 |  1.synchronized {
  |  ^^^^^^^^^^^^^^
  |  Suspicious synchronized call on boxed class
  |-----------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  | You called the synchronized method on a boxed primitive. This might not be what
  | you intended.
   -----------------------------------------------------------------------------

Solution

Use a dedicated lock object instead:

scala

object Lock

def example(): Unit =
  Lock.synchronized {
    println("hello")
  }

Or use this if inside a class:

scala

class Counter:
  private var count = 0
  def increment(): Int = this.synchronized {
    count += 1
    count
  }

<aside class="warning"> This reference page was created with LLM assistance - the description of the error code may not be accurate or cover all possible scenarios. </aside>