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Asked: 2026-05-11T07:46:47+00:00

Most OO languages prefix their interface names with a capital I, why does Java

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Most OO languages prefix their interface names with a capital I, why does Java not do this? What was the rationale for not following this convention?

To demonstrate what I mean, if I wanted to have a User interface and a User implementation I’d have two choices in Java:

  1. Class = User, Interface = UserInterface
  2. Class = UserImpl, Interface = User

Where in most languages:

Class = User, Interface = IUser

Now, you might argue that you could always pick a most descriptive name for the user implementation and the problem goes away, but Java’s pushing a POJO approach to things and most IOC containers use DynamicProxies extensively. These two things together mean that you’ll have lots of interfaces with a single POJO implementation.

So, I guess my question boils down to: ‘Is it worth following the broader Interface naming convention especially in light of where Java Frameworks seem to be heading?’

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  1. I prefer not to use a prefix on interfaces:

    • The prefix hurts readability.

    • Using interfaces in clients is the standard best way to program, so interfaces names should be as short and pleasant as possible. Implementing classes should be uglier to discourage their use.

    • When changing from an abstract class to an interface a coding convention with prefix I implies renaming all the occurrences of the class — not good!

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