The big claim of Java was write-once-run-anywhere. It was achieved by creating an implementation of Java Runtime Environment (JRE) for practically all platforms, so the bytecode generated once from the source by Java compiler (javac tool) could be executed everywhere where JRE was installed, provided the version of the compiler javac was compatible with the version of JRE.
The first releases of JRE were primarily the interpreters of the bytecode and yielded slower performance than some other languages and their compilers, such as C and C++. However, over time, JRE was improved substantially and now produces quite decent results, on a par with many other popular systems. In big part, it is due to the JIT dynamic compiler that converts the bytecodes of the most frequently used methods to the native code. Once generated, the compiled methods (the platform-specific...