Why is Java platform independent in theory and platform dependent in practice?
I know that one of the big things about Java is that it is platform independent in the sense that you can make a Java application and have it run in Windows, Linux, Mac, and so forth, as long as you don't use libraries specific to one OS, and as long as you have a JVM installed for the appropriate OS to interpret things correctly...
However, why can't a normal computer Java program (as in a simple Hello World in Java, for Windows or Linux for example) run just the same in a mobile phone, when mobile phones also have their specific JVM installed to interpret things correctly?
Why is it necessary to change the architecture of the program in some cases, such as Android development, or use Java ME to make applications specific for some general mobile phones?
I know that there are some functions that are related to certain functionalities of the OS, that might not apply in the mobile platforms for example, such as some things related with console, input methods, and so forth, but is this really the only reason that makes things not compatible? If so, why won't a simple application that just declares and initializes an integer variable be able to run across all non-mobile and mobile platforms that have a JVM available?
I am aware of other questions that have been posted before, such as this, but that do not focus the exact point I am aiming for here.
Answer
The unit of portability to look at is a class rather than an application. A class that declares and initialises an integer variable will run on all the platforms you describe, and many others too. They understand the same bytecode, even if they do execute it using different mechanisms ranging from bytecode interpreters, to JIT compilation, to Android's Dalvik (which translates JVM bytecode into its own instruction set).
Even if we look beyond a single integer variable, Java that uses "core" functionality will work on most of these devices. There's a great deal of common ground between J2ME, Android and J2SE (and particularly the latter two - J2ME was intended as a cut-down version of the standard Java APIs for devices with limited resources, so less of the standard API is available).
On a Windows/Mac/Linux system, an application is usually something that you explicitly start, use, and - when you're done - tell it to exit. Compare this with, say, an Android phone: the application might be started in response to an event occurring (perhaps an incoming SMS, or a specific type of file downloaded from the web) in which case it needs to know how and why it was started - a simple public static main(String[] args) just doesn't cut it. Once started, the app needs to be aware of events such as "low battery" or "entering standby mode" in order to free up resources or disable CPU-intensive features (such as GPS) that might otherwise drain the battery.
These aren't obscure functions - they're essential to a phone being useful as a phone - so all native applications must handle them.