How is Java platform-independent when it needs a JVM to run

Question

I just started learning Java and I m confused about the topic of platform independence   Doesn t  independent  imply that Java code should run on any machine and need no special software to be installed  Yet the JVM needs to be present in the machine   For example  we need to have the Turbo C Compiler in order to compile C C   source code and then execute it  The machine has to have the C compiler   Could somebody please what is meant when Java is described as  platform independent

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bytecode is not plateform independent  but its JVM that makes bytecode independent  Bytecode is not the matchine code  bytecodes are compact numeric codes  constants  and references  normally numeric addresses  which encode the result of parsing and semantic analysis of things like type  scope  and nesting depths of program objects  They therefore allow much better performance than direct interpretation of source code  bytecode needs to be interpreted before execution which is always done by JVM interpreter

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No  it s the other way around  It s because you use the virtual machine that the Java program gets independend   The virtual machine is not independent  you have to install one that is specifically made for your type of system  The virtual machine creates an independent platform on top of the operating system

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1 jvm i e java virtual machine is a collection of programs which contains lot of files which provides various functionatiles present on a folder i e collections of programs on middle level format as called packages jvm helps not to be overloaded on o s where its helps to execute only the  class files or java applications only by itself only It helps to make its equalities middle level format after compliation by the java compiler then its provide the byte code   class file reprsentation which is not specific to o s and processor   2 jvm makes byte code to  exe file for proccessor to understandable and prsents memory allocation for every functions after recieving frm byte code  3 jvm also releases memory alocation from ram after control makes finishes thier execution

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Technical Article on How java is platform indepedent    Before going into the detail first you have to understand what is the mean of platform  Platform consists of the computer hardware mainly architecture of the microprocessor   and OS  Platform hardware Operating System  Anything that is platform indepedent can run on any operating system and hardware   Java is platform indepedent so java can run on any operating system and hardware  Now question is how is it platform independent   This is because of the magic of Byte Code which is OS indepedent  When java compiler compiles any code then it generates the byte code not the machine native code unlike C compiler   Now this  byte code needs an interpreter to execute on a machine  This interpreter is JVM  So JVM reads that byte code that is machine indepedent  amd execute it  Different JVM is designed for different OS and byte code is able to run on different OS   In case of C or C   language that are not platform indepedent  compiler generate the  exe file that is OS depedent so when we run this  exe file on another OS it will not run because this file is OS depedent so is not compatible with the another OS   Finally an intermediate OS indepedent Byte code make the java platform independent

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In c c   the source code c program file  after the compilation using a compiler is directly converted to native machine code which is understandable to particular machine on which u compiling the code   And hence the compiled code of c c   can not run on different OS   But in case of Java   the source file of java  java  will be compiled using JAVAC compiler present in JDK  which provides the Byte code  class file  which is understandable to any JVM installed on any OS Physical System    Here we need to have different JVM  which is platform dependent  for different operating Systems where we want to run the code  but the  class file compiled code Intermediate code remains same  because it is understandable to any of the JVM installed on any OS   In c c     only source code is machine independent  In Java   both the source code and the compiled code is platform independent   This makes Java Platform machine  independent

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java is not platform Independent  itself is a platform based on that platform java apps runs but java platform itself is platform dependent

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Edit  Not quite  See comments below   Java doesn t directly run on anything  It needs to be converted to bytecode by a JVM   Because JVMs exist for all major platforms  this makes Java platform-independent THROUGH the JVM

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Typically  the compiled code is the exact set of instructions the CPU requires to  execute  the program   In Java  the compiled code is an exact set of instructions for a  virtual CPU  which is required to work the same on every physical machine   So  in a sense  the designers of the Java language decided that the language and the compiled code was going to be platform independent  but since the code eventually has to run on a physical platform  they opted to put all the platform dependent code in the JVM   This requirement for a JVM is in contrast to your Turbo C example   With Turbo C  the compiler will produce platform dependent code  and there is no need for a JVM work-alike because the compiled Turbo C program can be executed by the CPU directly   With Java  the CPU executes the JVM  which is platform dependent   This running JVM then executes the Java bytecode which is platform independent  provided that you have a JVM available for it to execute upon   You might say that writing Java code  you don t program for the code to be executed on the physical machine  you write the code to be executed on the Java Virtual Machine   The only way that all this Java bytecode works on all Java virtual machines is that a rather strict standard has been written for how Java virtual machines work   This means that no matter what physical platform you are using  the part where the Java bytecode interfaces with the JVM is guaranteed to work only one way   Since all the JVMs work exactly the same  the same code works exactly the same everywhere without recompiling   If you can t pass the tests to make sure it s the same  you re not allowed to call your virtual machine a  Java virtual machine    Of course  there are ways that you can break the portability of a Java program   You could write a program that looks for files only found on one operating system  cmd exe for example    You could use JNI  which effectively allows you to put compiled C or C   code into a class   You could use conventions that only work for a certain operating system  like assuming     separates directories    But you are guaranteed to never have to recompile your program for a different machine unless you re doing something really special  like JNI

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when we compile java file the  class file of that program is generated that  class file contain the byte code That byte code is platform independent byte code can run on any operating system using java virtual machine  platform independence not about only operating system it also about the hardware  When you run you java application on a 16-bit machine that you made on 32-bit  you do not have to worry about converting the data types according to the target system  You can run your app on any architecture and you will get the same result in each

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When we compile C source data  it generate native code which can be understood by current operating system  When we move this source code to the other operating system  it can t be understood by operating system because of native code that means representation is change from O S to O S  So C or C   are platform dependent    Now in case of java  after compilation we get byte code instead of native code  When we will run the byte code  it is converted into native code with the help of JVM and then it will be executed   So Java is platform independent and C or C   is not platform independent

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The JVM abstracts from the concrete platform  Your program relies only upon the JVM and since the JVM is available for different platforms like Windows and Linux  your program is platform independent  but jvm depended

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well good question but when the source code is changed into intermediate native byte code by a compiler in which it converts the program into the byte code by giving the errors after the whole checking at once  if found  and then the program needs a interpreter which would check the program line by line and directly change it into machine code or object code and each operating system by default cannot have an java interpreter because of some security reasons so you need to have jvm at any cost to run it in that different O S platform independence as you said here means that the program can be run in any os like unix  mac  linux  windows  etc but this does not mean that each and every os will be able to run the codes without a jvm which saysspecification  implementation  and instance   if i advance then by changing the configuration of your pc so that you can have a class loader that can open even the byte code then also you can execute java byte code  applets  etc  -by nimish    best of luck

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JVM will be platform dependent  But whatever it will generate that will be platform independent   which we called as bytecode or simply you can say   the class file   for that why Java is called Platform independent  you can run the same class file on Mac as well on Windows but it will require JRE

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Java is platform independent in aspect of java developer but this is not the case for the end-user  who need to have platform dependent JVM to run java code  Basically  when java code is compiled  a bytecode is generated which is typically platform independent  Thus  the developer has to have write a single code for entire platform series  But  this benefit comes with a headache for end-user who need to install JVM in order to run this compiled code  This JVM is differnt for every platform  Thus  dependency comes into effect only for end-user

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The JVM is a  simulated machine  that can be installed on different systems  In this way  the same Java code can run on different systems  because it relies on the JVM  not on the operational system itself   That is to say  this allows the programmer to communicate with the virtual system  JVM  and utilize its functions  instead of the specific machine and OS functions  Since Java only relies on JVM  it is platform independent  if the platform has JVM installed    So in short  Java is not platform independent as such  it requires a JVM-installation for all systems it should run on  However  it will run on all systems that has the JVM installed

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Javac     compiler that converts source code to byte code   JVM- interpreter that converts byte code to machine language code    As we know java is both compile  r  amp    interpreter based language  Once the java code also known as source code is compiled  it gets converted to native code known as BYTE CODE which is portable  amp  can be easily executed on all operating systems  Byte code generated is basically represented in hexa decimal format  This format is same on every platform be it Solaris work station or Macintosh  windows or Linux  After compilation  the interpreter reads the generated byte code  amp  translates it according to the host machine    Byte code is interpreted by Java Virtual Machine which is available with all the operating systems we install  so to port Java programs to a new platform all that is required is to port the interpreter and some of the library routines   Hope it helps

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Doesn t independent means that Java code should be able to run on any machine and would need no special software to be installed  JVM in this case has to be present in the machine     With Java  you can compile source code on Windows and the compiled code  bytecode to be precise  can be executed  interpreted  on any platform running a JVM  So yes you need a JVM but the JVM can run any compiled code  the compiled code is platform independent    In other words  you have both portability of source code and portability of compiled code      Like  for example  we need to have Turbo C Compiler in order to compile C C   source code and then execute it   The machine has to have the C compiler    The machine doesn t have to have a C compiler  the machine has to use a platform specific binary  With C or C    the compiled code is specific to each architecture  it is platform independent    In other words  with C   C   you have portability of source code  with some discipline  but not portability of compiled code  you need to recompile for each architecture into platform specific binaries

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Java is not platform independent in that it runs on the JVM  Having said that  you gain platform independence via programming against a single abstract machine that has concrete realizations on most common OS platforms  and some embedded appliances    A related idea is the hardware abstraction layer present in many operating systems that allows the same OS to run on disparate hardware   In you original question  Turbo C is analagous to the javac program  and the JVM is the OS HAL

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Just a side note to the discussion about JVM and JIT Compilation  This is the same principle as with C  and the CLR and to some extent in Python  and when somebody says that the code runs  directly on hardware  that is actually true in that instructions that is already compiled will be able to take advantage of optimization on the machine cpu it s being run on  So even if the initial compilation of a module is rather slow  the next time this module is run  the code being executed runs at native speed and thus runs directly on hardware so to say

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App1 Java code ------ App1byteCode           JVM MacOS  help work   with App1 App2 App3        App2 Java Code ----- App2byteCode           JVM LinuxOS  help work   with App1 App2 App3        App3 Java Code ----- App3byteCode           JVM WindowsOS  help work   with App1 App2 App3    How this is happening    Ans  JVM have capability to read ByteCode and Response In Accordance with the underlying OS As the JVM is in Sync with OS   So we find  we need JVM with Sync with Platform   But the main thing is that the programmer dont have to know specific knowledge of the Platform and program his application keeping one specific platform in mind    This Flexibility of write Program in Java --- compile to ByteCode and run on any Machine  Yes need to have Platform DEPENDENT JVM to execute it  makes Java Platform Independent

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Java is platform-independent as it has JVM Java virtual machine   Let us illustrate it with a real life example  Let s assume you are free to your family members  But why    Because you know them well and they know you as well  But  you are not free to my family members  Because you don t know them and they don t know you either  But  if I m your friend and when I can introduce you to my family members  hence you will be able to talk to them freely   In a similar way  if you are a code and I am a JVM  Also  your family is windows platform and mine is the Linux platform  In the case you were a C or other platform-dependent languages  you only know your family members and vice versa  That s why only the platform on which you were written knows that Code and will support it  But if you are a JAVA code and when you come to my family viz  the Linux platform and if there you find me  JVM  then I can introduce you to my family  the Linux platform and you will be able to interact with it   For platform-dependent languages  there isn t any friend like JVM available to them to introduce themselves to any platform family  That is how Java is platform-independent

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In simple terms   Java programming language   is platform independent     JVM  is platform dependent

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JVM is os dependent  for every os JVM different     class  is same for all JVMs  so  every JVM understand that   class  file data   windows dependent JVM give windows dependent instruction to windows linux dependent JVM give linux dependent instruction to linux   that s like it for other operating systems  so java runs on any operating system   that s why java is os independent

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It means the Java programmer does not  in theory  need to know machine or OS details   These details do exist and the JVM and class libraries handle them   Further  in sharp contrast to C  Java binaries  bytecode  can often be moved to entirely different systems without modifying or recompiling

[java] How is Java platform-independent when it needs a JVM to run?

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