Why a virtual machine is needed execute a java program.? [duplicate]

Answer 1

Because any machine that has an implementation of the JVM can run any program that's been compiled into Java Byte Code (in theory). Java byte code also acts as a higher-level language that different languages can compile into. Of course, different languages can be compiled into the same machine-code, but that ties compiled code to one type of machine, instead of non-machine-specific byte code.

The JVM implementation also takes care of the machine specific details instead of the app programmer. This allows Java code to do away with the 32/64 bit code variations that are typically resolved using preprocessors in c/c++.

Answer 2

To make Java code platform independent.

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If a C-programme is compiled on a particular platform, then it would produce compiled code that is capable of running only on the platform on which it was compiled. A C-program compiled on windows machine cannot be run on Linux/Unix/MacOS machines.

One of the main design goals of Java was to make the code platform-independent. This was achieved by adding an extra step in between code compilation and code execution. That is the step of generating the 'byte code'.

Compiling a Java programme results into what is called as "byte code" file, having an extension of .class. The byte code actually contains instructions that can be interpreted by Java Virtual Machine. That is to say that 'javac' converts Java programme into a set of instructions that a JVM can understand.

JVM - Java Virtual Machine, runs on top of the OS and is implemented for each platform (OS and underlying hardware) separately.

With this design it is possible to compile a Java program in Windows machine and run the generated .class file on a Linux box, thus achieving platform independence.

Answer 3

Because that way Java can be implemented for the JVM which has a defined architecture and instruction set. Different machines can have their own JVM on which Java programs can be executed without having to worry about machine on which JVM is running. In addition many optimizations are also added in JVM.