Evolving the Java Language

We're starting to think about potential language features for Dolphin (7.0). So I've been reflecting a little on the principles that James so brilliantly infused into the original Java language and which we've tried to preserve as it evolves.

The big core principle is around making programs easy to understand. Java is focused on being a powerful but simple language, easy to read, with a consistent clear meaning. It is more important that Java programs be easy to read than to write.

That may sound trivial, but it isn't.

Lessons from C++

Looking back at C++ there were a whole set of factors at work that made reading source code difficult:

• First, the C++ language itself became very complex. Many new ideas were added incrementally and unfortunately the seams show.

• Second, the C++ language consciously chose to emphasize "power" and "flexibility".

  That sounds nice initially, but unfortunately it also means that there is very little you can rely on and almost any simple program statement can have weird side effects. In C++ the statement "a = b;" must be approached with caution.

• Finally, the macro pre-processor reinforced the "power" aspect, but again at risks to comprehension.

Many project teams working with C++ adopted conventions to create simpler subsets of C++. "We'll use this, but won't use that." This helped. However, it lead to the problem of language fragmentation, where different teams adopted different language subsets and then got cognitive dissonance when they read one another's source.

The Java approach

The Java language took a very different approach. Java tries to be an unobtrusive language. As a developer, you should be able to focus on what your application code is doing and on how it interacts with libraries. Code should do what it seems to do - people shouldn't need to worry about clever language side effects or about what "=" means this week.

This focus on clarity and on readability has affected Java in many ways. It has led to a focus on keeping the language simple and clear. It has led to relatively conservative uses of syntax and minimized the places where user defined code can modify base semantics.

As another part of this principle, there is also a strong focus on having "one standard language" which means the same thing everywhere. It should be possible for a Java developer to start reading a chunk of new Java source code and to be able to rely on a consistent set of language semantics. I shouldn't have to worry about different vendors implementing language features in different ways. And I really shouldn't have to read your compiler flags in order to understand what your source code will do. So part of our investment in the Java language includes a strong focus on both a very precise specification (currently driven by Sun's esteemed Computational Theologist, Gilad Bracha) and also an extensive language compatibility test suite.

Language updates in Tiger

Now principles are good, but so are pragmatics. If we stuck too rigorously to our principles, we'd probably never dare make any changes to the language. It is natural for things to evolve, and we want to be able to make developers more productive, within the spirit of the language. But we are also very conscious that adding individually useful features may slowly undermine the deep values of the language.

A lot of thought went into the Tiger language features. We reviewed scores of potential language changes before settling on the small set that made it into Tiger. There is always a risk in any change, but I think the Tiger features will help developers without disrupting their ability to understand Java code.

• Generics are clearly a major change which add important new type systems ideas. But they provide a deep benefit in improved source readability (you can see what specific types are being used, you don't have to guess) and in improved type checking, which is a core Java virtue.

• I'm a big fan of both the extended for loop semantics and the auto-boxing and unboxing features. At first glance they run the risk of heading into the C++ quagmire of user defined side effects. But in reality, they are both tightly bounded and run little risk of surprising developers. And they do remove some common drudgery.

• JSR-175 (Metadata Annotations) is one of the biggest opportunities and one of the biggest risks in Tiger. It adds a great deal of expressive power. It defines explicit language support for a style of declarative programming which was previously addressed through scattered ad-hoc mechanisms (such as JavaBeans method naming conventions or marker interfaces or EJB deployment descriptors). By promoting this declarative programming style into the language we hope to improve overall readability. We've also tried to define clear rules for what annotations can and can not do, to minimize the risks of weird side effects.

For Dolphin, as for Tiger, a lot of people both inside and outside of Sun will be involved in reviewing any language change proposals. The detailed designs will be handled through the JCP. But my suspicion is that we'll probably continue to stay very conservative on changes, as each little change also carries its little wad of additional complexity. We're unlikely to add a macro preprocessor (sorry) or any general form of operator overloading, or full-blown AOP, or any other mechanism for redefining and obscuring core semantics. But we will look for new language ideas that help developers with common problems. For example, one area I'm personally interested in is some kind of "friends" import mechanism to make large multi-package projects easier to manage.

One platform, many languages

One last thought: the Java language is only one language for the Java Platform. It has one particular set of values, which seem to be particularly good for creating large, maintainable programs. But different developers want different things at different times. It's OK to have multiple languages, each of which is well designed for a specific kind of programming. When I'm doing a quick Sunday morning hack, sometimes I want something more scripty, and I don't want to have to track all those checked exceptions. So I'd like to see more use of alternate languages on top of the Java platform, with different goals and styles. For example, I was really pleased to see the "Groovy" language brought into the JCP. It is designed as a new Java platform language, but with its own goals and design center that are very different from the classic Java language. Happiness!

My personal suspicion is that the Java language is good for at least another 20 to 30 years, if we are careful in managing its evolution. But I also expect that over time new research ideas will emerge and slowly gain acceptance and at some point we'll want a new synthesis that will create a great new simple language. And of course we'll want to make sure that sparkling new language runs on the Java Platform!

                                                                             - Graham