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Learning Java so I can get at clojure [closed]

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I have a history of hating Java, having used it pretty regularly in the late 90's during the 'slow as balls' era. As such, I never really learned it well. From what I understand, Java is actually a pretty good language to use these days. I've been thinking about diving into it because of Jython and Clojure. That is to say, I'd like to program in Java and use inline Jython or Clojure where appropriate. But truthfully, I'll probably just be programming in Jython/jRuby and calling up clojure from there.
Which brings me to my question. I know both of these languages can be called from Java, but is that necessarily good practice? Should I even bother learning java if I just want to use Jython as the primary language? Seeing as how that's a large part of my motivations here, I'd like to know that I'm not terribly misguided before jumping in. I'm aware there is a very high risk for projects to become a kludge if done in multiple languages like this.
I'm still learning about the JVM and the like, so I apologize if this question is painfully obvious.

Jython can be viewed as a cross compiler from Python to the Java Virtual Machine. As such, to get the most out of Jython you'll obviously have to learn Python, and probably will need to learn Java.
You can skip some of the Java learning, but at the end of the day, Java and the JVM grew up together. That means that Java code tends to lend understanding of the JVM. It is possible to gain understanding of the JVM without Java, but that's not a path well travelled. Any Jython code that imports a Java library will immediately have you searching Java documentation, so if you avoid leaning Java you're going to learn it piecemeal anyway.
You will have to decide if a piecemeal approach or a formal approach is more appropriate for you and your situation. A lot of deciding which path to take is knowing how you learn best.
As far as the "slow as balls" period of the 90's, that's when I was learning Java. Personally, I feel it is better to describe it as "slow as balls if you did incredibly stupid things with Java". Now I think people have built up a sufficient skill set to avoid translating C directly into Java. That said, I do occasionally encounter the 2000+ line method, so perhaps I'm being a bit rosy in my projection. The entire JVM is laid out in such a manner that good object oriented code runs faster, and if you're constantly trying to go to "other" objects for all the data you need locally, you'll just stack thrash the JVM.
Regardless of opinions, the JVM is now the hot Java item. There has been "other language" support by one means or the other for over a decade now; however, the excitement around Domain Specific Languages seems to have sparked an interest in compilation technologies and the JVM. The other languages benefit from the JVM being an easy target to hit with built-in cross platform support, excellent performance, huge availability of libraries, and generally good documentation. Learning Java and the JVM will help you with a lot of the JVM supported languages, as many of them don't flesh out their library space in favour of hooking into a pure Java library.

I'd say it's worth knowing Java even if you plan on only using other JVM languages. I use JRuby and Scala, and have played around with Clojure. If you are building things to run on the JVM, knowing Java is a bit like knowing C when working natively–you don't have to know C, but if you do, you can write the bits that need speed in C and wrap them in a Ruby or Python library or whatnot.
It's worth knowing the basic principles of how Java works in terms of things like interfaces and annotations and how the classpath works because otherwise you are working with basically a leaky abstraction. What happens when your interop isn't very good? This is especially true if you are planning to do Clojure and Jython!
The other reason to know Java is simply because if you are using code in the Java ecosystem, you have to be able to read and write Java. You need to write a library? Yes, you can probably write it in Clojure, but if you want other JVM language users to be able to use it, you should probably have written it in good, idiomatic Java. Scala is close enough to Java for this purpose; Clojure or Ruby or Python, not so much. Just being able to read and comprehend Java programs is very important too.
The other great benefit is simply that you get more libraries and they are better tested. You need a double-ended queue? Check the Java Collections Framework. Good random number generation? java.security.SecureRandom. UIs? Well, Swing, AWT and SWT are... okay, bad example. Knowing the benefits and shortcomings of these only comes from doing some Java programming and learning the various ways not to suck at Java.

From a couple of years experience of using Clojure (plus many more years of Java...) here is my perspective:
You don't strictly need any Java experience to write Clojure code - Clojure is a full language in its own right and you can write perfectly capable programs without using any Java.
You will need to set up the JVM environment - the Java environment has some rules about where code gets loaded from (i.e. the "classpath") that need to be followed to get a working environment. Not a big deal, and most IDEs will do it for you, but it can be a hurdle for people completely new to the JVM world. I'd suggest careful following of the setup instructions for whichever IDE/toolset you choose.
There are some Java-related concepts that are helpful to understand - for example, Clojure harnesses Java exception handling features with (try ... (catch ...)) etc. so it's useful to be somewhat familiar with the Java approach to exception handling.
Ultimately you will probably want to use Java APIs - bacause a huge amount of the value of being on the JVM in the first place is in having access to the huge diversity of libraries and tools that are available in the Java ecosystem. You don't need to write any Java code to use Java APIs from Clojure, but you do need to know enough Java (method signatures, data types etc.) to be able to read the JavaDoc documentation of the APIs and convert this into an appropriate Clojure function call. Often, this is as simple as (.someJavaMethod someJavaObject param1 param2) but sometimes it can be more complex (e.g. when you need to instantiate a subclass of some Java class to pass as a parameter)
Java isn't a bad language to learn anyway - while I'll readily admit Java has some weak points (as do all languages!), it's still a great, simple, high performance, cross-platform, object-oriented language that has a lot of value. Even if you only do a few short tutorials and never write anything substantial in Java, I'd still recommend it for the learning experience.
I believe most of the above would also apply to Jython.

I can't speak for Jython, but if you want to really get to grips with clojure, you want to understand its trade-offs compared to Java, especially wrt memory/gc and the basics of Clojure/Java interop. You also need at least an abstract understanding of how the clojure collections are implemented unless you really don't care about performance - that's not to say that clojure is particularly inefficient, but more the opposite: the implementation of its immutable collections is fairly unique and tailored to clojure's stance on persistence and performance and it helps to understand the underlying details when you're trying to improve on performance issues.
For all of that, I don't think you actually need a lot of Java knowledge. Being able to read Java fairly well, a basic understanding of the concepts, and a knowledge of where to find the documentation is probably enough.
I think if you want to do a hybrid Clojure/Jython project the interoperation details are most crucial. That probably means you have to know in some detail how classes, interfaces, some of the standard library and (to a minimal extend) generics work in Java and how to deal with all of those in your chosen languages since the interoperation necessarily reduces to the more basic Java constructs. Some of this is tricky and can be confusing, and in clojure's case at least the documentation often refers back to Java concepts and documentation for obvious reasons, so you have to make sure you read both, closely.

I would definitely learn java and learn it well, not only because Clojure is built on top of the JVM but also to get anything done you will be calling Java libraries all the time, and you may even need to dip into Java occassionally.
On another note it would be expand your mind to understand Java's OO concepts and pain poaints too and this will enhance your undersatnding of Clojure too.

Above all, study the Java libraries. Part of the joy of using the JVM is having access to "it's already been done" libraries, as well as to parts of the core language that accomplish certain tasks with optimum performance on the JVM. In addition, some languages (e.g. Clojure) purposefully dip directly into Java and don't completely discourage it in your own code, so if you want to be able to read others' code Java basics are a must.
As for the rest of "learning Java" (design patterns, concurrency in Java, etc.), I wouldn't waste your time unless/until specific projects requirements demand it.

Why should a Java developer learn an additional JVM language? [closed]

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I am a java developer and I want to know,
what is the main benefit from learning a language such as Scala or Groovy?

You can get the same benefit from learning another JVM language as learning any new language. It increases your understanding of programming in general and more importantly, it adds another tool to your toolbox.
So the next time you have to solve a problem, you may reach for a nail gun instead of a hammer.
To be more specific, Groovy is a good language for mocking up code quickly, and Scala, while I've never used it, is suppose to great for writing concurrent applications due to it's functional approach. As others have mentioned, the JVM languages can interact with Java code. Which can be useful for adding onto legacy systems or for mocking up pieces of a application quickly.

Some good reasons that come to mind:
They have features that Java doesn't have and that you may find useful in certain circumstances.
They use different programming paradigms, different way of thinking.
Learning other languages opens your mind.
Learning languages makes you conscious about their respective strengths and weaknesses.
They are getting more and more attention and better JVM support (with Java 7).
Actually, maybe ask yourself the reverse question:
Why not learn an addiitional JVM language?

The other answers here have very good points, but there's one thing I'm missing.
A good coder rarely identifies himself as a 'Java developer', a 'Python developer' or any ' developer'. Learning another language (be it a JVM language or not) will make you understand there's a lot more in the world to learn.
If you're satisfied with only one language, it usually means you're oblivious to the problems it has, and that there are many tasks that are better suited for other languages.
This is why the Pragmatic Programmers encourage every programmer to learn a language a year.

The languages you've mentioned practice a different programming paradigm that could help you be more productive. They are also more fun to work with.
Languages such as Scala and Clojure run on the JVM and exhibit great performance in multi-core systems without imposing synchronization requirements.
And of course you are still able to use the full wealth of libraries that are available for Java.

Because then you'll know a new language, which means a broader skill set and another way of looking at problems. But because Groovy and Scala run on the JVM and you know Java, you can integrate existing libraries and code if you want or need to.

from http://groovy.codehaus.org/:
"Groovy is like a super version of Java. It can leverage Java's enterprise capabilities but also has cool productivity features like closures, builders and dynamic typing. If you are a developer, tester or script guru, you have to love Groovy."
So in many cases it does make sense to use Groovy over Java; For instance in Java unit tests!

Scripting language advantages inside jvm. Seamless interaction with compiled java code.

I know with Groovy, you can load scripts (from files) at runtime from your Java application. This allows me to customize the behavior of application actions at a client site without requiring me to recompile code. It's rather lovely.

Sorry more questions then answers..
What would you spend your time on if you are not learning a new language?
Why are you limiting yourself to JVM languages?
Would it be of more benefit to learn test driven development?
What about learning standard design patents?
What are you trying to achieve with your investment of time?
Learning a language is always good, however if you don’t learn them very well and use them you don’t get a great benefit, as you need to be able to “think in a language” so as to broaden your mind.
Using more than one language in a company leads to lots of additional long term costs so you may gain more by learning how to programme better in your main language, only you can decide.

Open source managed programming languages [closed]

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Are there any good, portable, open-source, high-level, statically-typed, imperative, object-oriented, garbage collected, safe languages/runtimes with reasonable performance besides Mono and Java? Mono is nice, but it is a Microsoft technology, and I'm kind of afraid of using it (I'm not sure how rational this fear is). The problem with Java is that it is just too simple (no unsigned data types, no structs, etc.).

Mono isn't a Microsoft technology. .NET is, but Mono isn't - Mono is an open-source implementation of the ECMA-334 and ECMA-335 international standards. Yes, they originated from Microsoft, but Mono itself is not Microsoft technology.
Not that Mono is a language, of course - C# is the language. But then languages aren't really open-source or not - implementations are.
It does sound like you've probably got an irrational fear of C# - and likewise of Java. There are certainly things I'd like to see in Java (and I significantly prefer C#), but it's still perfectly possible to write very significant applications in it.

I'd like to point out Vala, which is a language based on the syntax of C#, but which targets a C compiler (similar to Eiffel) and a lightweight, self-hosted runtime. It lacks garbace collection, though.

Eiffel fulfills all your requirements

On the CLR, there is Boo.
Boo is a new object oriented
statically typed programming language
for the Common Language Infrastructure
with a python inspired syntax and a
special focus on language and compiler
extensibility
The Boo Manifesto is a good starting point.

C++ with a garbage collector? Though you issues with Java are not all that much of a show stopper (unless you can detail why you need those things, on the face of it those arguments are usually not very sound, but sometimes they are).

Well, having no unsigned data type in java is rarely considered as a problem.
But what about Lisp - it was the language to introduce garbage collection, and probably the most powerful language at all.
Check the performance comparison by Peter Norvig (the google's research director). You might be surprised that Lisp beats even C++ in some areas and Java in all.
However Lisp seems not to be a main stream language - I'm wondering why's that?
Because it is kind of minimalistic - it probably could not be beaten in power by any current or future language to come.
(I guess what makes C# or Java so popular is not just the language but the libraries.)

How can you say that java is too simple?
Ok, one of the reasons for developing Java is to make it less complicated than C++ which has a 900+ page of specification. However, it's much more complicated than C.
About 10 years ago I attended a course by Les Hatton and he did research on making your C program safer to use. He argued that almost no-one could remember the 140 pages of C standard, so it would be exceptionally hard to understand something like C++.
Of course, your target may not require safety, but if you do it's something to remember.

scala is less verbose than java and it runs wherever java runs, as it compiles to java bytecode. twitter reimplemented some of their core infrastructure in scala.

There's always Python and Ruby

First of all, Mono is not Microsoft technology.
At second, there are only two serious Virtual Machines (VM) with mentioned features - .Net/Mono and Java. But, setting aside requirement of statically-typed language, you can draw an attention at newcoming Parrot Virtual Machine (Wikipedia article about Parrot).
And, of course, Ruby, Python Erlang, etc. are also crossplatform VM (for single dynamic language)

You may take a look on the D programming language.
But to be honest? I'd suggest to stay away of mono/C#. Why? Many reasons...
Also, for practical puroses take Java or C++.
Java is as good as C# even it is little bit different. C++ is as good as well because 99% of problems of GC are solved using reference counting so...

If you want to learn this in order to be able to be more suited for a job, I suggest you consider either .NET (if you want to go for a Microsoft platform) or Java (if you want to go for platform independence), and do a lot of coding. Both are very large platforms!
Anything else would be niche languages being used in only a few places, which can be very nice but is unlikely to land you a job anywhere but in those few places.

Google App Engine -- Java or Python? [closed]

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I noticed that this was asked in the following post. Has anyone used both and can conclusively tell me which is better? It seems like python might be a little easier to implement and faster?

By this time, both implementations are reasonably mature and solid, and their functionality is pretty similar (I don't think it's quite 100% equal yet, but, moving in that direction). So, if you already know only one of the two languages, I'd say "just go for it". If you already know both, you don't need my advice;-).
If you know neither, but are rarin' to try some of the non-Java languages that are (more or less;-) supported on the JVM side of GAE (I believe this includes Scala, server-side Javascript, JRuby, etc), then it's worth trying. If you know neither, and don't want to risk a "not-first-class-supported language", I'd definitely agree with your observation that Python's faster to learn (but of course I'm biased in favor of Python;-).

I have used both, and my conclusion is that biggest difference is in libraries. In theory both have good libraries to do almost everything but in practice many libraries don't work with app engine:
All Java libraries are pure Java and should work, but as app engine does not allow threads and not even all *java.** dependencies it is... lets say problematic.
Python on the other hand have way less libraries out there, and many are just wrappers for native C libraries, which don't work either. :(
As both versions have such bad support for libraries, I call it tie. For very simple "just to show some content" -web app you don't need any external libraries of course.
I use now Python because it rules. ;)

"Better" is always subjective. For GAE, the choice of which is better for you depends on what you're most familiar with. If you've got experience with Java, use that, and if you've got experience with Python, use that. I don't think there's any objective criteria that would point to one being "better" than the other.

Its not just the language that you need to consider here. Libraries, frameworks, and tools are also relevant factors.
If you have no background in either language or web development, I would suspect Python would be easier to get off the ground with. It has been around longer, there is likely more support and knowledge available. Doing something simple is generally simple to do and the Python framework is in my opinion more lightweight.
Despite this, I would rather use Java. However, my reasons are controversial and I don't want to troll :D.

Python was the language Google AppEngine was originally designed for, and it has been supported for longer than Java (which only got included comparatively recently). Additionally, Guido van Rossum, the creator of Python, works on the Python side of AppEngine. The same can not be said about Java's creator.
It seems that there is more documentation and articles about Python on AppEngine than Java, probably because Python has been used in AppEngine for longer.
My advise: Use Python.
But that doesn't mean Java is a bad choice, because it is not, I just think Python is a better choice in this instance.

Python ( subjectively speaking )

.py chosen here for all gae projects for reasons more readable, less lines code, higher level same time java has more ways to solve a given problem. Approx estimate same function with python is 1/10 the code compared to java.

i use python, becouse always can get the best of django in appengine

I use Groovy (Grails) with GAE. It has Java's library universe and you should have roughly the same LOC compared to Python. Yeah, its not 100% Java (if you want to), but very close.
The BIG difference is roughly the JavaVM speed, that's one of the reasons Groovy (Grails) has gained so much popularity - especially people like my who dislike Python .)

It maybe possible to use Jython in appengine so you could use both. One thing I would say about the python productivity vs Java productivity argument is Java has excellent IDE's to speed up development where as Python is really lacking in this area because of its dynamic nature. So even though I prefer to use Python as a language, I don't believe it gives quite the productivity boost compared to Java when using a new framework. Also I have as yet not been able to get the python version of google app engine to work in Netbeans which is a royal pain.

Java Developer looking for a 2nd'ary language to play with [closed]

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What is best language to learn next to Java?
Criteria for this secondary language are:
High potential of being the "next big thing". e.g. If the market for Java open positions hypothetically dies/dwindles, what is the next programming language that will have a bigger market for open positions? Another way to frame this question is: If I own a small company that implements solutions in Java, what is the other language that I should use?
Can produce web applications.
Can produce desktop applications.
Easy and fun to learn.
Wide range of available libraries and frameworks (free or open source) that enhance and speed up your solutions.

Python almost meets all of them, but I don't know about being "the next big thing", but hey, Google uses it, and I think its popularity is raising.
It's a scripting language, btw.
I use it for web applications (using django), and you can definitely create desktop applications with it (although I haven't done that myself).
It is easy and fun! (although this is quite subjective, but it's tons easier and "funner" than Java)

For employability: Any of the .Net languages, probably C#. Then you're well set for most potential customers.
For stretching yourself: something functional (F# to cover .Net too?), or something Lisp, or Smalltalk - was once the next big thing but it probably never will be again, but still a language that changed signficantly my approach to programming in other languages.

If you are expert in java, you will probably really enjoy groovy.
It is backwards compatible with java and has a lot of the nice features of ruby and similar dynamic languages. It also has what is shaping up to be a nice rails-a-like in grails, and a good object-relational mapping framework in GORM.
What is really nice about it is you can mix and match groovy classes and java classes. So if you find yourself forgetting the groovy syntax, you can just drop into Java. And of course you can get at any Java library from it. At the same time you can benefit from groovy's really concise idioms for things like Beans properties.
(By the way it meets all your criteria listed above, too)

Learn a language that will probably not be the Next Big Thing™. Learning a new language usually means learning new concepts (unless you learn one that's very similar to another one you already know).
Learning new concepts has great advantages, no matter which language you program in, so it's a pretty good thing to do.
If you limit yourself to a language that's going to be the Next Big Thing™, then chances are that you'll find a lot of familiar concepts and only relatively few new ones (the IT world is conservative like that).
Learn something that looks interesting to you. Not only will you be able to learn faster, if it's interesting, but you'll also have more fun doing so!

How about Scala or JavaScript? Functional paradigms will stretch your thinking further and will be of benefit whether they are "the next big thing", or not.

Go for a different paradigm : functionnal, lisp, haskell, ocaml, erlang, scala, etc.

If you don't have any religious issues with Microsoft, I would argue for C#.
The only downside I can see is that it's not cross-platform. Otherwise it fits your requirements as far as I can see

Your criteria fairly closely matches Python and Ruby, they both have a growing market base in Web development, and are fully capable of producing desktop applications as well. Whilst they are fairly closely matched, Python has a slightly larger library of re-usable code modules, whilst Ruby's strength leans more to it's well-established frameworks.
As a side-note, You should ensure that you don't stop at 2 languages. Start learning a second one, then pick up a third and even fourth as you progress. Learning new programming languages becomes a more trivial task as the general learning curve evens out.

I must second Gilles here; to learn a different paradigm is very rewarding, and functional programming is [not unlikely to be] the Next Thing.
Erlang is great both in flexibility, ease of use (once you grasp the basic ideas) and - the best of all - it it very well suited for the high parallelism of future and current hardware. It also covers all your five criteria, though it might be more of a "server" language than a "desktop" language.
If you are interested, I suggest the book Programming Erlang by Joe Armstrong.

Personally, I'd go with Ruby and focus on running your Ruby scripts under JRuby. this way you get the expressiveness of Ruby with the JVM running your code. I've seen Rails and Merb running under Glassfish. Might seem odd, but you get all the nice instrumentation of Glassfish with your Ruby code. Things like JMX, RMI, etc. work well with JRuby.

Squeak by Example