What is the meaning of "Java is portable"?

Question

I'm confused about Java portability. If the Java language is portable, why is enum unknown in J2ME?

In C++, it's not important which platform or library is used. The "C++ language" doesn't change in all platforms.

My purpose is developing a Java library that just uses primitive types like int, String, or Array (something like a library for Genetic algorithms). I want to use this library in mobile and desktop applications. But it seems that enum and some other keywords do not exist in all platforms.

So I think I misunderstood the meaning of "Java portability". What does that mean?

Answer 1

Java provides three distinct types of portability:

Source code portability: A given Java program should produce identical results regardless of the underlying CPU, operating system, or Java compiler.

CPU architecture portability: the current Java compilers produce object code (called byte-code) for a CPU that does not yet exist. For each real CPU on which Java programs are intended to run, a Java interpreter, or virtual machine, "executes" the J-code. This non-existent CPU allows the same object code to run on any CPU for which a Java interpreter exists.

OS/GUI portability: Java solves this problem by providing a set of library functions (contained in Java-supplied libraries such as awt, util, and lang) that talk to an imaginary OS and imaginary GUI. Just like the JVM presents a virtual CPU, the Java libraries present a virtual OS/GUI. Every Java implementation provides libraries implementing this virtual OS/GUI. Java programs that use these libraries to provide needed OS and GUI functionality port fairly easily.

See this link

Answer 2

There are three flavors of Java: ME for mobile, SE for desktops, and EE for enterprise.

"Java is portable" refers to the SE version. It means that you can run Java bytecode on any hardware that has a compliant JVM.

It doesn't mean that ME is the same as SE is the same as EE. EE has EJBs, but SE and ME don't. That does not make them less portable.

C++ language doesn't change in all platforms.

This statement is not strictly correct. Microsoft adds extensions to their C++ that won't run elsewhere.

ANSI C++ might mean portable source code, as long as you stay away from platform-specific extensions. It does not mean portable bytecode; you may have to recompile and relink.

You want to run genetic algorithms on phones? I know that mobile devices have become pretty powerful, but I'm educated to think that GA would be a server-side functionality. Mobile devices feel more like view to me.

Answer 3

Portability refers to the ability to run a program on different machines. Running a given program on different machines can require different amounts of work (for example, no work whatsoever, recompiling, or making small changes to the source code). When people refer to Java applications and applets as portable, they usually mean the applications and applets run on different types of machines with no changes (such as recompilation or tweaks to the source code).

Answer 4

While C and C++ language syntax and semantic are standardised, to write a truly cross-platform application is extremely difficult, unless you limit yourself to extremely basic applications.

There are a number of high level and low level reason for this - from the endianness up to how to interact with the underlying operating system (eg. opening a window).

In addition, C/C++ source code only can be considered portable, not the result of the compilation - resulting executable code and libraries are not portable, with major difference between system architectures (different CPUs for example) and Operating Systems.

Java is a fairly successful attempt to solve both of these issues:

1. Java does not compile code to assembly, but to a more abstract "bytecode" - a pseudo-assembly language which is "interpreted" or "recompiled on the fly" by the virtual machine (JVM) into assembly. This conversion is usually fairly efficient as bytecode is mostly quite a low level language. Some version of the ARM processor can even execute bytecode natively. Thus, once a java app is compiled, the result can run on "any" architecture (provided a JVM is available for that machine)

2. Java comes bundled with a really large runtime library which provides not only an extensive implementation of the most common data structure (implemented in the JVM in the most efficent way for a particular architecture) but also provide an "hardware and software abstraction layer" - you can interact with the system in a standard way while coding, it is the JVM job to translate it into appropriate architecture and OS calls. As an example, Java provides the Swing framework, which allows you to create a GUI in a system independent way - ie, you open a window, and this is translated into Win32/MFC calls in Windows and XWin calls in Linux

Said that, there are different "types" of java:

1. JavaSE is the most common
2. JavaME is a cut down version with a limited library and not implementing the Java5.0 language changes
3. JavaEE for enterprise use, same as JavaSE but with a much larger runtime
4. Android Java, mostly compatible with JavaSE but with additional functionalities specific to android phones

However, you should be aware that the Java architecture has been designed to allow interoperability, in particularly to allow to mix libraries built for different versions or even different "types"

Answer 5

Every hardware architecture has its own somewhat unique instruction set (add ax, bx...) when you build a C++ code, the compiler turns it into a machine code specific to the system/architecture you are working on. So you have to customize and build your code for different architectures for it to work on them.

But What happens in java is, When you build it, it is compiled into a Byte code (as opposed to machine code). And the java virtual machine(JVM) interprets the Byte Code into an instruction that is understandable by the specific architecture you the program is running on. There is JVM for every major architecture and operating system so the code you write on windows will be interpreted and run on MAC-OS or linux without any source level modification by you. That is why Java is portable and that is where the Write Once Run Everywhere motto comes from

Answer 6

Java is known as a "portable language" because Java code can execute on all major platforms. What's more, once you've compiled your Java source to "byte-code" .class, those files can be used on any Java-supported platform without modification, unlike many other languages, which require compiling "machine code" for each platform, e.g. a separate ".exe" for 32-bit vs 64-bit environments.

Another meaning of "portable", used mainly in Windows environments, means that the Java run-time environment can be run from any arbitrary location in your filesystem and does not need to be "installed", that is, have important information stored in the Windows registry. This is also true for most Java applications, and enables them to be run from different drive letters, via for example an external storage device like a USB flash drive from any computer without having to install the application first.

Answer 7

it means that your java program written on one machine will run on any other machine provided that machine has JVM.

refer to this link.