Inside into jvm

Java programs can run slower than an equivalent C++ program for many reasons:

   Interpreting bytecodes is 10 to 30 times slower than native execution.
   Just-in-time compiling bytecodes can be 7 to 10 times faster than interpreting, but still not quite as fast as native execution.
   Java programs are dynamically linked.
   The Java Virtual Machine may have to wait for class files to download across a network.
   Array bounds are checked on each array access.
   All objects are created on the heap (no objects are created on the stack).
   All uses of object references are checked at run-time for null.
   All reference casts are checked at run-time for type safety.
   The garbage collector is likely less efficient (though often more effective) at managing the heap than you could be if you managed it directly as in C++.
   Primitive types in Java are the same on every platform, rather than adjusting to the most efficient size on each platform as in C++.
   Strings in Java are always UNICODE. When you really need to manipulate just an ASCII string, a Java program will be slightly less efficient than an equivalent C++ program.