Develop Reference

Why is Java platform independent?

When I used C++ programs, I needed Turbo C complier; and when I have a Java program, I need to have JVM. Still C++ isn't platform independent, but Java is!
If any Java program require a JVM running in order to execute, why does Java is said to be Platform Independent?

Java is operating-system independent because it runs on the Java platform (the JVM): The mantra is "write once, run anywhere" because you write your code using the JDK API, compile that once, and it runs on any operating system that has a JVM available. You write your code, wrap it up in a jar, and that jar runs wherever you want to use it, within reasonable bounds. The job of the JDK and JVM are to abstract away the differences in environments.
In contrast, particularly back when Java was created, writing C or C++ for multiple operating systems was a big pain and usually required additional toolkits (of course, the JDK and JVM are a toolkit of sorts), and even today still requires at least recompiling for the target system.
There's nothing magic about Java's OS-independence. It would be entirely possible to build the same thing for C or C++: Compile to an intermediary form, provide a runtime that knows how to interpret or recompile that intermediary form for different environments and provide a library that abstracts away environmental differences. Java just...did that, with its own spin on the language. Later, so did the .Net platform.

No software is really "independent". Eventually, your program has to call the underlying OS in order to make some basic operations, like allocating memory, create new threads etc.
The way to achieve an executable which is "cross platform" is to create specific executable for each OS. Common practice is to write different code for each OS, and then "hide" it in a cross platform interface and compile the relevant code to the relevant OS. For example, std::thread is "cross-platform" to the user who uses this class, but behind the scenes it will call different functions based on the OS which was specified on compile time (such as CreateThread on Windows, but pthread_create on *nix OS's).
So basically, the JVM is a C/C++ executable that was written with different set of functions for each OS, and was compiled separately for each OS. A JVM executable which works on Linux, will not work on Windows, and vice-versa.
That JVM compiles .class files to machine code based on the OS and CPU it currently operating on, so that's why Java programs can "run anywhere".
But basically, it's a lie. It's like saying a human being can live on Mars....
if he lives inside a sealed spaceship with proper temperature, water, food, air and sunlight supply
So Java program can run anywhere.... if the JVM already is installed and running on the computer.

Firstly, I'd like to link to this question which has a lot of good information.
https://softwareengineering.stackexchange.com/questions/85175/what-is-the-exact-meaning-of-platform-independence
In the question above are comments about what it means to be "Platform independent," but the one thing I wanted to mention, but is summed up nicely here is
You're right, platform independence means that the same program works on any platform (operating system) without needing any modification.
The code we write is known as "Write once, run anywhere" or as someone else said "run once, test everywhere."
Our Java code SHOULD run everywhere, but sometimes there are little native bugs that cause issues i.e., someone was having issues with printing on Mac OSX with JavaFX-8 Printing, while it works fine on Windows. There was also a bug report on this to fix this "Mac specific Java issue."
So.... For the most part, the underlying JavaSE code SHOULD work across all platforms.....
** however, if you do have an Application running on multiple computers and work with the File System, you will have to do checks to understand which OS you are working with i.e., (System.getProperty("os.name").contains("Windows")); **
more info on that here https://docs.oracle.com/javase/tutorial/essential/environment/sysprop.html
There is also another thing to note.
Certain components do not work cross platform, i.e., JavaSE vs JavaEE. JSP/JSF is what is used in JavaEE, on the web, that is specific for running code on a server, and in a webpage, but cannot be used in the Desktop (to my knowledge).
However, JavaSE has GUIs such as Swing, and JavaFX, which also cannot work on the web, either in the Client, or in the Server.
Android has it's own set of commands and things it can/cannot do, and other "Platforms" have specifics to it as well.
Overall, the underlying Java Architecture is what is used across all platforms, where certain "Java Specifics" are used in certain platforms i.e., JSP.
So what does Java do differently?
In the case of Java the application runs in a Java Virtual Machine which itself isn't platform independent. This has to be the interface between the actual machine (operating system) and the Java code you've written.
I am not really knowledgeable that much of the JVM, but it seems that each JVM is specifically tailored to each "Platform" (Which is why there are soo many versions to install), and does a lot of heavy lifting in the background, whereas C you might have to do this all yourself (not really sure how it works), for each OS.
Want a JVM for Windows? Np... Linux? Np.... That new car you just bought with all that fancy tech in it? JVM for that.... Or how about that new parking meter where you just parked your car? Yeah, there's one for that too...
For example, here is an Answer from this site on how Java is converted to Dalvik for Android.
https://stackoverflow.com/a/24570735/3599960
Hope this helps and makes sense.

C or C++ program gets compiled into native-code which is close to the metal where metal is the OS (earlier it could be hardware also in DOS era). There is no further re-compilation necessary to run the executable on target platform. But, a developer must build executables for all platforms he/she indents the program should run on.
It doesn't just mean different OSes, but bit-ness of particular OS. For example a 64-bit binary (EXE) cannot run on a 32-bit OS (vice-versa is possible, mostly, however). Here Java/.NET and other platform virtualization engine are boon for the developers - they just build once (for example "AnyCPU" for C# module), and don't need to give multiple binaries (EXE files). The runtime installed on given OS would re-compiler (JIT - Just In Time compilation).
I'm not sure about Java, but a.NET program can be compiled into specific platform. It is also possible by the .NET engine to JIT compile the intermediate (or byte-code in Java) into native format just once (and keep the EXE for direct run). The advantage is that .NET JIT compiler can take advantage of current hardware and latest CPU instructions, which C++ program cannot (it won't have JIT/re-compilation).

If it's "Write once run anywhere", why do so many Java programs tell me to pick the version for my OS?

I am fairly new to Java or programming in general. On my journeys through the internet to master this language I have come up the saying "write once run anywhere" multiple times.
But I have found many software that requires you to pick the right version for your OS. Sometimes there is only one version available.
Could you explain to me why that is so?

[expanded per the comments]
Java runs on a Virtual Machine, the JVM. In an ideal world this means that the Operating System is abstracted away behind this and you only have to make sure your code works with the JVM which will make it work with the underlying OS. This can already be undone by using the wrong path separators or line endings; it is not an absolute truth.
An application can use many Operating System-specific approaches/libraries/functions/etc that might make it not feasible to restrict yourself to one general codebase. Instead they might want to leverage some advantages provided by a platform and create a separate application with it.
The statement should probably be somewhere along the lines of "Write once in a general fashion, run anywhere" but that's not as snappy.
This statement is often linked to Java but there are also other languages that incorporate this: weblanguages like Javascript and HTML will run on any browser because the browser itself forms the abstraction between the language and the underlying OS.
Other languages don't have this (entirely?) since they work differently: C# will use the underlying .NET framework which as it is only exists for Windows. There exists a cross platform variant (Mono) but it would be an overstatement to consider C# truly cross platform.

The Java program or source code is compiled to generate "bytecodes" ( an intermediate binary format). Second, the bytecodes is executed by an interpreter which is part of the Java Virtual Machine (JVM).
"Write Once, Run Everywhere" refers to the fact that an application written is Java can be run on any hardware which has the Java Virtual Machine (JVM), and that the JVM is now licensed to hundreds of operating systems vendors systems including Microsoft for Windows.

Well, some stuff is really cross-platform (most of standard Java library), while some other stuff may need right version for the OS. Generally, this applies to software that uses dynamic libraries, that aren't written in Java. In this case, versions for different OSes are packed with libraries for this OS.
In case there's version only for one OS - it could be because required libraries don't exists for other OSes or developer didn't port it. There is still possibility that it's actually cross-platform, but developer tested it only under one OS.

Your question is more based on platform dependency/independency.
Java is a programming language which is platform independent which means the code which you write will produce the same output on all machines running Windows, Linus, Unix, etc... without any changes to the code. To run a Java program you need to have JVM (Java Virtual Machine) installed. Now what does JVM do.? Well, it translates your code into Machine code which the Operating System could understand. Therefore JVM is platform dependent since every OS has a different Machine code.
So, basically you write a Java Program only once and can be used/run everywhere.

you can write source code in one platform and run it any where.some times you can encounter problem if a new java version is available but still current version runs the code.

How to speed up Java applications?

I need to use Java for a desktop application. I heard that there are many tools that compile java natively, is this true? does these tools compile java program into machine code?
THank you!

Since the (Sun/Oracle) Java VM has a good JIT (just-in-time) compiler, you don't have to compile your Java program to machine code yourself. The compiler will do that on the fly when it's necessary.
So: Speed up your Java programs just as every other program:
reduce algorithmic complexity
exploit parallelism
compute at the right moment
find and remove bottlenecks
...
Since Java is a garbage collected language, there is one important point to more speed: reduce allocations! Reducing allocations will help you at least twice: The allocation itself isn't done and the garbage collector will have to do less work (which will save time).

I agree with the others that compiling to machine code does not make much sense: mind that C free/malloc have same or higher costs than Java new/garbage collection.
The NetBeans IDE comes with a built-in Profiler; so you could profile your application in that IDE to find bottlenecks.

are you coding the app or it's someone's else?
It looks you're trying to run an java app that is slow. Try increasing the memory when running it. You can change the shell script specifying these params:
java -Xms64m -Xmx512m

I need to use Java for a desktop application. I heard that there are
many tools that compile java natively, is this true? does these tools
compile java program into machine code?
Such programs do exist, but may come with tradeoffs when using some of the more dynamic capabilities of the Java platform like you may lose the ability to load new classes at runtime. The JVM may have a slow start up, but it's plenty fast enough once it gets going.
That said, one solution that I didn't see anyone mention here is to replace code written in Swing with SWT. The SWT toolkit uses native code underneath.

What is the role of the OS when JVM executes a Java application? And why do we need the OS?

I have done some reading on the internet and some people say that Java application is executed by the java virtual machine (JVM). The word "execute" confuses me a little bit. As I know, a non-Java application (i.e: written in C, C++...) can be executed by the Operating system. At the lower level, it means the OS will load the binary program into memory, then direct the CPU to execute instructions in memory.
So now with a JVM, what would happen? As I know, JVM (contains a run-time environment) would be called first by the OS. From that point on, the JVM will spawns one (or many) threads for the application. I wonder if the role of the OS comes into play any more? It seems to me that the JVM has "by-passed" the OS and directly instruct the CPU to execute the application. If so, why do we need the OS?
Taking a little bit further, the JVM will use its JIT to compile the application's byte codes into machine codes, then execute those machine codes. Since it is already machine codes, do we need the JVM any more? Because instead of JVM, the OS can be able to instruct the CPU to execute those machine codes. Am I making any mistake here?
I would like to learn more from people here. Please correct me if I am wrong. Thank you so much!

We need the OS for all the things a C or C++ program would. The JVM does a few more things by default, but it doesn't replace anything the OS does. The only difference might be that sometimes you have Your Code [calls the] JVM [calls the] OS, or with compiled code you can have Your Code [calls the] OS
Similarly in C++ you might have Your Code [calls the] Boost [calls the] OS.
When your program is running in native code, it doesn't need the JVM as such. This is good because the JVM knows when to "stand back" and let the application run. However, not all the program will be compiled to native code for the rest of the life of the application, so you still need it.
Its is possible to use kernel by-pass devices/drivers with JNI, but Java doesn't directly support this sort of feature.

It seems to me that the JVM has "by-passed" the OS and directly
instruct the CPU to execute the application. If so, why do we need the
OS?
All C/C++ binaries (not just the JVM) run directly on the CPU. Once running, these programs can call into more machine code provided by the operating system to do useful things like reading files, starting threads, or using the network.
The JVM translates a Java program into instructions that run on the CPU. Behind the scenes, though, Java's threads, file i/o, and network sockets (to name a few) all contain instructions that call into the code provided by the operating system for threads/files/etc. This is one of the reasons you still need the OS.
Since it is already machine codes, do we need the JVM any more?
The JVM provides features that you don't get from the JIT compiler. At the end of the day the JVM is just running a lot of machine code, but not all of that machine code comes from the JIT (or from the interpreter). Some of that machine code does garbage collection, for example. That's why you need the JVM.

The underlying base O/S still has to do almost everything for the JVM, not least:
Input / Output
Memory management
Creation of threads (if using native threads)
Time sharing - i.e. allowing more than one process to run
and lots more besides!

Well, I want to keep this simple.
How you coded in ZX Spectrum, that is in old days, when really you don't use OS (even before DOS era, in pre-PC era). You write your code, and you have to manage all. In many cases there were no compiler, so your program was interpreted.
Next, it was realized that OS is great thing and the programs became simpler. Also, compiler was in broader use. I am talking about C++, for example. In those programs if you need to call to some OS function you added needed library and makes your call. One of the drawbacks where that now, your program is OS-depended, another problem was that your programs includes OS DLL in some fixed version. If another program on same station required that DLL in different version you were in trouble.
In the early days of the JVM history no JIT compiler where in used. So, your program run in interpreted mode. Your application has no longer needed to call OS directly, instead it use JVM for all it needs. JVM instead redirect some of the application calls to the OS. Think about JVM as mediator. One of the best features of the JVM where it's universality. You where not needed to stick to the specific OS (while in practice you do need to make some minor adjustments, when you are not stick to the Java requirement while your program "occasionally" works in some specific OS, for example you use C:\ for the files or assumptions upon Thread scheduler that is happen to be true on current OS, but generally JVM is not guaranteed to be true). Programmers of the JVM develop such API that can be easy in use for Java developer in one hand and it will be possible to map to any OS system calls on another hand.
JVM provides you more that simple wrapper to the OS. It has it's own memory model (thread synchronization) for example, that has some week grantees on it's own (it was totally revised in JDK 1.5, because it was broken). It also have garbage collection, it's initialize variables to null values (int i; i will be initialize to 0). That is JVM besides being moderator to OS is acting as helper code for your own application.
At some point JIT was added. It was added to reduce overhead that JVM creates. When some assumptions holds, usually after one execution of the code, command interpretation can be compiled to the machine code (I skip phase of byte code). It is optimization, and I don't know any case where you can effected.
In JDK 1.6 another optimization where added. Now, some objects in some circumstances can be allocated at the stack and not on the heap. I don't know, may be it has some side-effects, but it is example of what JVM can do for you.
And my last remark. When you compile your code, what really happens, your program is checked to be syntactically correct and then it byte-code generated (.class file). Java language use subset of the existing byte-codes (this is how AOP was implemented, using existing byte-codes that where not part of Java language). When java program is executed these byte codes are interpreted, they are translated on-the-fly to the machine instructions. If JIT is on, than some of the execution lines can be compiled to the machine language and reused instead of on-the-fly interpretation.

Since it is already machine codes, do we need the JVM any more?
compiled java programs are not the machine codes. [javac] compile [.java] file into bytecode [.class] file.Then these bytecodes are given to JRE [Java Run-time Environment].
Now the java interpreter comes in action that interpret the bytecode into native machine code that run on the CPU.

As we know OS does not Execute any program it provide Environment for processor to Execute if we talk about Environment it allocate Memory Loading file Giving instruction to the Processor,Manage the Address
of loaded data method work of the Processor is only Executing program this thing happen in c or any procedural programming Language if we see than OS Playing a very vital Role in this overhead on OS
Because if we write a small simple program in c like Hello World which contains only one Main function when will compile it generate .exe file of more than one function which is taken from Library function so manage all thing by OS is tedious job so in JVM has given Relief to OS here the work of OS is only to
load the JVM from hard disk to RAM and make the jvm Execute and allocate space for JVM to execute java Program here Momery allocation ,Loading on Byte code file from Hard disk,Address Management ,Memory Allocation and De-allocation is Done by JVM itself so OS is free it can do other work.jvm allocate or Deallocate memory based on What ever OS has given to Execute the java Program.
if we talk about Execution JVM Contains Interpreter as well as JIT compiler which converting the Byte code into Machine code of Required Function after Execution of the method Executable code of that method is destroyed thats why we can say java does have .EXE File

Java based usb application

I working on a Java based application which can be carried around by the user in a USB flash Drive. At the startup I want to check for existing JVMs on the machine the USB is plugged-in.
How can I achieve it ???

Investigate JSmooth -- http://jsmooth.sourceforge.net/ --
or Java Web Start in newest Java 6.
http://java.sun.com/j2se/1.5.0/docs/guide/javaws/developersguide/cdinstall.03.06.html

If you wrap the program in an .exe wrapper - as Launch4j - it can check for JVM installation,
before each execution.

Isn't this a chicken-and-egg situation?
Your Java program needs to be launched in a JVM before it can run its logic to look for JVMs. And at the point it can look, a JVM has already been found so I don't think you could do anything particularly useful with it. (Unless your "main" app is actually a tiny bootstrapper, which chooses the JVM it wants and uses Runtime.exec to launch the real application.)
In any case, I would suggest that it's not the responsibility of the application to choose which JVM it wants to run in; the user will set up his environment to run with the JVM he wants, and I'd consider it rude to ignore this and force your own choice on him (which is likely to be made with less knowledge about the relative merits of those JVMs).