According to the documentation,

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


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Instances of a value-based class ... 

* do not have accessible constructors, but are instead instantiated through factory methods which make no commitment as to the identity of returned instances;
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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

* https://docs.oracle.com/javase/8/docs/api/java/lang/doc-files/ValueBased.html[Value-based classes]

