What is portability? How is java more portable than other languages?
I wonder how Java is more portable than C, C++ and .NET and any other language. I have read many times about java being portable due to the interpreter and JVM, but the JVM just hides the architectural differences in the hardware, right? We'd still need different JVMs for different machine architectures. What am I missing here? So if someone writes an abstraction layer for C for the most common architectures, let's say the CVM, then any C program will run on those architectures once CVM is installed, isn't it?
What exactly is this portability? Can .NET be called portable?
Answer
Portability isn't a black and white, yes or no kind of thing. Portability is how easily one can I take a program and run it on all of the platforms one cares about.
There are a few things that affect this. One is the language itself. The Java language spec generally leaves much less up to "the implementation". For example, "i = i++" is undefined in C and C++, but has a defined meaning in Java. More practically speaking, types like "int" have a specific size in Java (eg: int is always 32-bits), while in C and C++ the size varies depending on platform and compiler. These differences alone don't prevent you from writing portable code in C and C++, but you need to be a lot more diligent.
Another is the libraries. Java has a bunch of standard libraries that C and C++ don't have. For example, threading, networking and GUI libraries. Libraries of these sorts exist for C and C++, but they aren't part of the standard and the corresponding libraries available can vary widely from platform to platform.
Finally, there's the whole question of whether you can just take an executable and drop it on the other platform and have it work there. This generally works with Java, assuming there's a JVM for the target platform. (and there are JVMs for many/most platforms people care about) This is generally not true with C and C++. You're typically going to at least need a recompile, and that's assuming you've already taken care of the previous two points.
Yes, if a "CVM " existed for multiple platforms, that would make C and C++ more portable -- sort of. You'd still need to write your C code either in a portable way (eg: assuming nothing about the size of an int other than what the standard says) or you'd write to the CVM (assuming it has made a uniform decision for all of these sorts of things across all target platforms). You'd also need to forgo the use of non-standard libraries (no networking, threading or GUI) or write to the CVM-specific libraries for those purposes. So then we're not really talking about making C and C++ more portable, but a special CVM-C/C++ that's portable.
Once again, portability isn't a black and white thing. Even with Java there can still be incompatibilities. The GUI libraries (especially AWT) were kind of notorious for having inconsistent behavior, and anything involving threads can behave differently if you get sloppy. In general, however, it's a lot easier to take a non-trivial Java program written on one platform and run it on another than it is to do the same with a program written in C or C++.