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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.

Looking for strong/explicit-typed language without GIL [closed]

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Are there any languages which feature static type checking like in C++ with modern syntax like in Python, and does not have GIL?
I belive, Python 3 with ability to explicitly declare type of each variable would be 'almost there', but GIL makes me sad.
Java is nice, but I need something more 'embedable' without bulky JRE.
Update: Anything .NET-related or non-open source is a no-go.
Update2: I need explicit+strong typing to write safer code in the expense of development speed. GIL is important as the code is going to be quite computing extensive and will run on multicore servers, so it has to effectively use multiple CPU.
Update3: Target platform is Linux(Debian) on x86

Boo
Boo is an object oriented, statically
typed programming language that seeks
to make use of the Common Language
Infrastructure's support for Unicode,
internationalization and web
applications, while using a
Python-inspired syntax and a
special focus on language and compiler
extensibility. Some features of note
include type inference, generators,
multimethods, optional duck typing,
macros, true closures, currying, and
first-class functions. Boo has been
actively developed since 2003.
cython
Cython is a language that makes
writing C extensions for the Python
language as easy as Python itself.
Cython is based on the well-known
Pyrex, but supports more cutting edge
functionality and optimizations.
The Cython language is very close to
the Python language, but Cython
additionally supports calling C
functions and declaring C types on
variables and class attributes. This
allows the compiler to generate very
efficient C code from Cython code.

Anything in the ML family might work for you. Ocaml is a great place to start, but it does have a stop-the-world GC last I looked. Haskell is famous as a lab for innovative concurrency models. Python's comprehensions came from Haskell, where they'rr a convenient syntax for some very fundamental ideas. And Erlang is strongly dynamcally typed, fun to write in, and does concurrency better than anybody else.

Ada is a strongly-typed, compiled language with a modern, easy-to-read syntax and proven reliability for multicore computing. Ada was designed for use in large, critical, real-time systems where software MUST work at all costs.
"Ada supports run-time checks to protect against access to unallocated memory, buffer overflow errors, off-by-one errors, array access errors, and other detectable bugs. These checks can be disabled in the interest of runtime efficiency, but can often be compiled efficiently. It also includes facilities to help program verification. For these reasons, Ada is widely used in critical systems, where any anomaly might lead to very serious consequences, i.e., accidental death or injury. Examples of systems where Ada is used include avionics, weapon systems (including thermonuclear weapons), and spacecraft." (quote from Wikipedia article linked above).
Ada is freely available as part of GCC / GNAT and should be an easy "apt-get install" on Debian. You can also find up-to-date compilers and libraries (both community-supported GPL-licensed and commercially-supported packages) at http://libre.adacore.com/libre/
Ada can compile to Java bytecode for use in a JVM or compile to binary for bare-metal or embedded use.

I think GO would fit your requirements.
This is my personal feeling but go code looks very similar to python code.
It still has classic compile approach but google will develop some interpreter certainly.
From google site:
Go is a general-purpose language
designed with systems programming in
mind. It is strongly typed and
garbage-collected and has explicit
support for concurrent programming.
Programs are constructed from
packages, whose properties allow
efficient management of dependencies.
The existing implementations use a
traditional compile/link model to
generate executable binaries.

After reading your updated spec:
I need explicit+strong typing to write safer code in the expense of development speed. GIL is important as the code is going to be quite computing extensive and will run on multicore servers, so it has to effectively use multiple CPU
What exactly does "computing extensive" mean? What problem domain? What do others who work in this problem domain use? If you are serious with this specification, you can't do much other things than using C++ in connection with well-tested libraries for multithreading and numerical computing.
my $0.02
rbo

Is there a way to transfer/translate the code written in Java to other languages? [closed]

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Theoretically this seems possible to me. So can Any one confirm this to me, if it's possible? and if there is such a software that does this?(like Java to C++ or C#)
And in general would it be possible to transfer languages like Java to server-side programing language like PHP?

Translating the syntactical elements of one language and producing another is not trivial but it's not impossible. A good parser can build syntax trees in one language and then emit another. The difficulty of porting code outside the context of simple "Hello World" type applications is twofold:
The libraries of one language will probably differ (e.g. WinForms vs Swing)
Some language features will have to be catered for: (lambda expressions, anonymous methods, different inheritance implementations etc).

It is possible, but the major problem is that Java has a very large runtime library which needs to be made available in the target language in order to be able to do a fully automatic conversion.
For the special case of Java -> .NET, you can use J# from Microsoft to compile it into a .NET assembly which can then be used. Also ikvm.net allows for running a JVM inside .NET.
For PHP I do not believe such a solution exist. You MAY be able to use gcj to create a native library which can be linked in, but I do not believe it is a feasible soultion.
What functionality do you need in PHP?

Visual Studio ships with a Java to C# translator, and even tough it does a pretty decent job, there's still a lot to clean up afterwards.
In my experience you really have to ask yourself if it makes sense to translate code from one language to another. What is the gain? Will the translated code be maintainable? If the answers to these questions point in the wrong direction, translating is probably not the right approach.

Google Web Toolkit does conversion from Java to JavaScript:
http://code.google.com/webtoolkit/overview.html

to answer your question, yup, theoretically this is indeed possible and practically such technology is used every day :)
The interesting thing, in my opinion, is that the Java converters typically convert by taking the bytecode, not the source code. Then it's, say, bytecode-to-ObjectiveC source code. For some converters (at least one opensource one) it's bytecode-to-XML then XML-to-target-language.
For example, the Uniwar application for the iPhone, which has been acclaimed by all and made its way to the appStore's top ten, as been written in Java (JME) and automatically converted from the Java bytecode. Reaching the top ten, even for a few days, means that this is deployed on a lot of machines ;)
In the Real-World [TM], Cobol-Java and, weirdly, Java-Cobol are not unheard of.
For all this to work that said you need a really good converter :)

Theoretically it is possible. But as others pointed out the main problem is to translate libraries.
Some time ago I made Java to Tcl(XOTcl) and Java to Python translators to evaluate the translation posibility. Search by java2tcl and yava2python.
They convert syntax but do not make relevant constructions translations (e.g. Java file operations to Python ones). That would require more development time.
In general my opinion is what such a translation may be possible. But only if your translator covers classes/libraries of the converted project.

Does a Java to C++ converter/tool exist? [closed]

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I always asked myself if it would be possible to make a Java to C++ converter.
Maybe a tool that converts the Java syntax to the C++ syntax?
I am aware that the languages differ, but simple things like loops where the semantics match 1 to 1.
Is there such a tool? Or is it possible to make one?

It's possible to do anything given enough time, money and resources. Is it practical? Beyond trivial examples not really. Or rather it depends on what constitutes an acceptable error rate.
The real problem is that the idioms are different in Java to C++. Java to C# for example would actually be far easier (because the idioms are much more similar). The biggest of course is that C++ has destructors and manually managed memory. Java uses finally blocks for this kind of behaviour and has garbage collection.
Also Java has a common Object supertype. C++ doesn't.
The generics to templates would be nigh on impossible I would imagine.

The Firefox HTML5 parser is written in Java and converted to C++. But I think the converter used there is quite specific for this project. Interestingly, it turned out the resulting C++ parser was faster than the old parser written in C++.
I'm also writing a converter as part of the H2 database, under src/tools/org/h2/java. The idea is to allow converting a subset of the H2 database to C++, so this is also not a general purpose translater.
And there is the open source project J2C.
So there are ways to convert Java to C++. But don't expect the translator support all features, and don't expect the resulting code to be any faster than a good Java JVM.

Is is possible, no question, but it won't be so simple. It would be a Java compiler which generates C++.
If you want to do that from scratch, it will be very hard, you have to do all the work javac and the JVM do for you (e.g. garbage collection).
Btw. Google has a Java to JavaScript compiler (included in GWT)

There is one, bit I am not sure if it actually works.
Java to C++ Converter-Tangible Software Soulutions.
It is weird how there are c++ to java converters, but only 1 java to c++ converter.

As said it would be tough to convert Java to C++ but we can have an applicaiton or tool that generates code in Java and equivalnet C++ code.
I know one applicaiton which generates code in C++/Java/C# given a model which has its own way to deifine it.
That tool belongs to CA and name is CA Plex.
Search on www.ca.com

There are programs out there that claim they can do this, but none have gained enough popularity to be frequently mentioned, so we'll leave them at "attempts". Making a converter would require a lot of AI built into your program. The difficulty is increased tenfold when swing is involved because GTK/wxWidgets/Qt/win32 API all differ greatly from swing. But it is possible. Not that the code quality will be great, and no guarantees your program won't crash due to separate memory handling methods, but it's possible.

The main issue is that java is a language that is written and designed to talk to a VM. I suppose it would be possible, but all you would be left is a very poorly optimized application with a self translating layer doing what the VM already does. I mean, sure, it is possible, it still wouldn't be a solution for anything i could think of. If your looking to make your sluggish java app native, maybe your thinking too hard, just use an application like JET, its actually quite good, and will give you the benefits a native app would bring. Of course if the VM is already doing what the app is asking it to do just as well as native code could(it happens.. sometimes :P) it might change nothing.
Java to c#, tho, sounds more reasonable, as both the languages are written in similar ways, talking to a framework as such, but this would still leave code very much unoptimized as code written from scratch for a particular framework can not be bested.

http://www.tangiblesoftwaresolutions.com/Order/Order_Upgrade_Instant_CPlus_Java_Edition.htm
Depends on the domain of where the code will be used, from a learning perspective perhaps it might be interesting.
i just found this via a google as I remembered seeing one in Univeristy that created code based on uml.

Java to C would actually be the easiest. Remember you need to convert the language, If you do that, the required libraries can be converted by your new compiler. In other words Swing and AWT should not be a big problem...
I would start by taking a good look at the Java Native Interface (JNI). The JNI is a part of java which allows it to be used with C and C++. The reason I would start here is that it becomes fairly obvious how parts of Java may be implemented in C. Once I had a grasp on basic structures, like how Java Objects can be mapped onto C structures (struct) and how pretty much everything in Java is an Object including arrays, I might peek at the Open JDK source code.
The actual converter would have to convert all the imported Java libraries (and their imported libraries and so on...) which means you would need the source code for everything. This conversion no small task since the Java libraries are large.
The process would be time consuming, but no AI should be required. However, I see no reason to perform a conversion like this. It looses the portability of Java and would not gain the efficiency of C (except that it would be compiled to native code, but it would be better to compile the machine code directly from the Java).

Something neat would be a tool , that translate java to "C++ using Java API" (like GNU GCJ CNI), one problem remain is to manage array.length (array not vector) ...

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