Default method 'toString' overrides a member of 'java.lang.Object'

An interface "inherits" methods from the Object class. I wrote about this here¹.

The quotes are appropriate, because Interfaces can only inherit from other Interfaces. An interface, when it does not have a super interface, declares all public non-final methods of the Object as members of the Interface (if not explicitly defined in the interface.

I recently attempted something like this TaxBracket²:

It doesn't work. In fact my IDE started complaining immediately with the following message.

Default method 'toString' overrides a member of 'java.lang.Object'

It is supposed to do that, so says the Java Language Spec³:

It is a compile-time error if a default method is override-equivalent with a non-private method of the class Object, because any class implementing the interface will inherit its own implementation of the method.

My colleague helpfully provided the alternative, which is to make an "elementString()" method in the Interface, which can be called in the toString() method of every Class that implements the Interface. (Further down in the JLS, this approach is almost word for word also given)

StackOverflow⁴ has a helpful link to the JDK mailinglist⁵ that explains it a lot more in depth.

One of the remarks that really struck a cord with me, is the fact that toString(), equals() and hashCode() are all basically related to the state of a Class, and do not belong in an Interface.

I have therefore decided to remove the offending default method.

References

[1] Do interfaces inherit from object class?

http://randomthoughtsonjavaprogramming.blogspot.nl/2017/07/do-interfaces-inherit-from-object-class.html

[2] Wikipedia - Tax bracket

https://en.wikipedia.org/wiki/Tax_bracket

[3] JLS 8 - 9.4.1.2. Requirements in Overriding

https://docs.oracle.com/javase/specs/jls/se8/html/jls-9.html#jls-9.4.3

[4] StackOverflow - Java8: Why is it forbidden to define a default method for a method from java.lang.Object?

https://stackoverflow.com/questions/24016962/java8-why-is-it-forbidden-to-define-a-default-method-for-a-method-from-java-lan

[5] Malinglist OpenJDK - Allow default methods to override Object's methods

http://mail.openjdk.java.net/pipermail/lambda-dev/2013-March/008435.html

Do interfaces inherit from Object class in java?

I was thinking this question suddenly at work...

Quick, Batman! To the StackOverflow¹!!!

The answer comes straight from the JSL².

References

[1] StackOverflow - Do interfaces inherit from Object class in java

https://stackoverflow.com/questions/6056124/do-interfaces-inherit-from-object-class-in-java

[2] JLS Java 8 - 9.2. Interface Members

http://docs.oracle.com/javase/specs/jls/se8/html/jls-9.html#jls-9.2

Java is pass-by-value!

Java is always pass by value, with no exceptions, ever.

From the JLS¹:

When the method or constructor is invoked (§15.12), the values of the actual argument expressions initialize newly created parameter variables, each of the declared type, before execution of the body of the method or constructor.

What this means is that the parameter is actually a copy of the value you provided. Hence, Java is always pass by value.

It is the same for the programming language C³.

It is the same for JavaScript⁴.

Confusion

The following quote from the JLS² causes a bit of confusion in the discussion:

An object is a class instance or an array.

The reference values (often just references) are pointers to these objects, and a special null reference, which refers to no object.

So, yes, Java does use pointers (to objects). However, these pointers are passed-by-value! This is why you can change the contents of an object but never switch the object for a different object.

References

[1] JLS SE 8 - 8.4.1. Formal Parameters

http://docs.oracle.com/javase/specs/jls/se8/html/jls-8.html#jls-8.4.1

[2] JLS SE 8 - 4.3.1. Objects

https://docs.oracle.com/javase/specs/jls/se8/html/jls-4.html#jls-4.3.1

[3] Dennis Kubes - Is C Pass by Value or Reference?

http://denniskubes.com/2012/08/20/is-c-pass-by-value-or-reference/

[4] What's The Pointy - JavaScript does not have "pass by reference"

http://whatsthepointy.blogspot.nl/2013/11/javascript-does-not-have-pass-by.html

JavaDude.com - Java is Pass-by-Value, Dammit!

http://javadude.com/articles/passbyvalue.htm

StackOverflow - Is Java “pass-by-reference” or “pass-by-value”?

http://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value