Java bytecode

copy from : https://www.ibm.com/developerworks/library/it-haggar_bytecode/

The information about bytecode, as well as the bytecode presented here, is based on the Java 2 SDK Standard Edition v1.2.1 javac compiler. The bytecode generated by other compilers may vary slightly.
Why understand bytecode?

Bytecode is the intermediate representation of Java programs just as assembler is the intermediate representation of C or C++ programs. The most knowlegable C and C++ programmers know the assembler instruction set of the processor for which they are compiling. This knowledge is crucial when debugging and doing performance and memory usage tuning. Knowing the assembler instructions that are generated by the compiler for the source code you write, helps you know how you might code differently to achieve memory or performance goals. In addition, when tracking down a problem, it is often useful to use a debugger to disassemble the source code and step through the assembler code that is executing.

An often overlooked aspect of Java is the bytecode that is generated by the javac compiler. 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.

The bytecode is your program. Regardless of a JIT or Hotspot runtime, the bytecode is an important part of the size and execution speed of your code. Consider that the more bytecode you have, the bigger the .class file is and the more code that has to be compiled by a JIT or Hotspot runtime. The remainder of this article gives you an in depth look at Java bytecode.
Generating bytecode
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javac Employee.java
javap -c Employee > Employee.bc
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Compiled from Employee.java
class Employee extends java.lang.Object {
public Employee(java.lang.String,int);
public java.lang.String employeeName();
public int employeeNumber();
}

Method Employee(java.lang.String,int)
0 aload_0
1 invokespecial #3