Java bytecode is the form of instructions that the Java virtual machine executes. Each bytecode instruction or opcode is one byte in length; however, not all of the possible 256 instructions are used. In fact, Sun Microsystems, the original creators of the Java programming language, the Java virtual machine and other components of the Java Runtime Environment (JRE), have set aside 3 values to be permanently unimplemented. 
Relation to Java Edit
A Java programmer does not need to be aware of or understand Java bytecode at all. However, as suggested in the IBM developerWorks journal, "Understanding bytecode and what bytecode is likely to be generated by a Java compiler helps the Java programmer in the same way that knowledge of assembler helps the C or C++ programmer." 
Java bytecode is designed to be executed in a Java virtual machine. There are several virtual machines available today both free or commercial.
Further information: Java Virtual Machine
If executing Java bytecode in a Java virtual machine is not desirable, a developer can also compile Java source code or Java bytecode directly to native machine code with tools such as the GNU Compiler for Java. Some ARM processors have the ability to execute bytecode directly.
See Also Edit
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