Java mock class construction

Question

I am trying to understand what is exactly happening in this java unit test, as I managed to find that there is a possibility to pass values to Java class's constructor not inside the parentheses, but inside brackets after the classes parentheses, yet, don't seem to understand what is exactly happening right here in the attached unit test? My guess would be that it is creating a variable for that Subject class, but I don't need to create it inside my class's definition(declare a variable inside the class file/code)

        Subject s1 = fill(new Subject() {
            private static final long serialVersionUID = 1L;
        });

/* The class definition */
public class Subject {
    private int id;
    private String address;
    private String city;
    private String county;
    private String state;
    private String country;
    private String email;
    private String phone;
    private String web;
    private String zip;

// setters and getter for the declared variables here, also using default constructor

I tried googling but couldn't define what exactly I am trying to find, and the issue I am facing is that it is showing an error message of: The value of field new Subject(){}.serialVersionUID is not used <- as I understand that the serialVersionUID is somehow nested inside the Subject class, but not directly inside, will a getter be able to show it?

"... don't seem to understand what is exactly happening right here..." - It is an anonymous class, sub-classing Subject. — Turing85
– Turing85, Commented Oct 1, 2023 at 17:41
Does this answer your question? Java, anonymous inner class definition — Sören
– Sören, Commented Oct 1, 2023 at 17:43
The term you want to Google is "Java anonymous inner class". — David Conrad
– David Conrad, Commented Oct 2, 2023 at 2:54

Holloway · Accepted Answer · 2023-10-01 17:51:16Z

This is not really related to tests or mocking. What's happing is you're creating an anonymous subclass.

Subject s1 = fill(new Subject() {
    private static final long serialVersionUID = 1L;
});

is equivalent to

class NewSubject {
    private static final long  serialVersionUID = 1L;
}

// and then where the original call was used
Subject s1 = fill(new NewSubject());

Without seeing the surrounding code it's not easy to know why it's done this way, but I'm guessing the tests end up serialising and deserialising the class at some point. The serialVersionUID field is a special field used by the built in serialisation which means it would generally appear unused in user code.

Note that this only works because the serialisation looks for a serialVersionUID field on the specific class being serialised. You cannot use this method to override private fields used in the superclass as any direct use of them in the parent class will still use the original.

meriton · Accepted Answer · 2023-10-01 17:48:29Z

an error message of: The value of field new Subject(){}.serialVersionUID is not used

That's a warning message, not an error message. It is emitted by the compiler because the field is declared, but not used by any code you have written, and the compiler suspects that it may be unused and therefore a programming mistake.

However, the compiler is not omniscient, and one thing it can not take into account is access at runtime through the Reflection API. And it is possible such is happening here, because a field called serialVersionUID has a special role in Java Serialization.

That is, if Java Serialization is used with this class, the field may be useful, and the warning spurious, in which case you can use @SuppressWarnings to silence the compiler.

On the other hand, if this class is not used with Java Serialization, the field is indeed useless, and can be safely deleted.

as I understand that the serialVersionUID is somehow nested inside the Subject class, but not directly inside, will a getter be able to show it?

The syntax

new Subject { 
    private static final long serialVersionUID = 1L;
}

declares an anonymous inner class that extends Subject, and instantiates it.

Collectives™ on Stack Overflow

Java mock class construction

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related