package java.lang;
@jdk.internal.ValueBased
public final class Double extends Number
   implements Comparable<Double>, Constable, ConstantDesc {
   //...
}

public class APITester {
   public static void main(String[] args) {
      Double d = 10.0;
      synchronized (d) {
         System.out.println(d);         
      } 
   }
}

$javac APITester.java

APITester.java:4: warning: [synchronization] attempt to synchronize on an instance of a value-based class
   synchronized (d) {
   ^
1 warning