Summary
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Add the 'final' modifier to java.io.ObjectStreamClass.

Problem
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ObjectStreamClass instances are created only from within the serialization mechanism, using either its private constructor or its package-private constructor. There are no subclasses of ObjectStreamClass.

Solution
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ObjectStreamClass should be made final in order to express the design intent that it has no subclasses.

Specification
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    --- a/src/java.base/share/classes/java/io/ObjectStreamClass.java
    +++ b/src/java.base/share/classes/java/io/ObjectStreamClass.java
    @@ -80,7 +80,7 @@ import static java.io.ObjectStreamField.*;
      *      <cite>Java Object Serialization Specification,</cite> Section 4, "Class Descriptors"</a>
      * @since   1.1
      */
    -public class ObjectStreamClass implements Serializable {
    +public final class ObjectStreamClass implements Serializable {