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This topic is about Java 16 – Warning for Value-Based Classes.

Some classes, such as java.util.Optional and java.time.LocalDateTime, are value-based. Such Instances of a value-based class are final and immutable. Such classes have annotation @jdk.internal.ValueBased and Java 16 now generates compile time warnings in case such classes are synchronized using synchronized keyword. Wrapper classes are value based. For example, Double class is a value based.

Example

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

Consider the following example:

ApiTester.java

Example

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

Compile and Run the program

$javac APITester.java

Output

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

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