According to the documentation,

A program may produce unpredictable results if it attempts to distinguish two references to equal values of a value-based class, whether directly via reference equality or indirectly via an appeal to synchronization…​

This is because value-based classes are intended to be wrappers for value types, which will be primitive-like collections of data (similar to structs in other languages) that will come in future versions of Java.

Instances of a value-based class …​

This means that you can’t be sure you’re the only one trying to lock on any given instance of a value-based class, opening your code up to contention and deadlock issues.

Under Java 8 breaking this rule may not actually break your code, but there are no guarantees of the behavior beyond that.

This rule raises an issue when a known value-based class is used for synchronization. That includes all the classes in the java.time package except Clock; the date classes for alternate calendars, HijrahDate, JapaneseDate, MinguoDate, ThaiBuddhistDate; and the optional classes: Optional, OptionalDouble, OptionalLong, OptionalInt.

Note that this rule is automatically disabled when the project’s sonar.java.source is lower than 8.

Noncompliant Code Example

Optional<Foo> fOpt = doSomething();
synchronized (fOpt) {  // Noncompliant
  // ...
}

See