A little further out in left field, it isn't even necessary to know Java to write Java programs. Intermetrics is beta testing an ADA-95 to Java byte code compiler. Other such cross-compilers are probably possible including ANSI Fortran-77, COBOL, and Basic. However the one most people probably want, a C/C++ to Java byte code compiler, is probably not possible due to Java's lack of pointers.
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