what does # do after invokevirtual

Question

For example if int-variables a and b are stored in local variables 1 and 2, this code would compute this.f(3+b*a). (this-pointer is stored in variable 0)

iload_1
aload_0
icons_3
iload_1
iload_1
imul
iadd
invokevirtual #4

for what does #4 stand here ?

Answer 1

This is an index into the constant pool of the classfile; it says that the operand of invokevirtual is stored at index #4 of the constant pool (which will be a Constant_MethodRef_info.) If you invoke javap with the -v flag, it will dump out the constant pool and you'll see at index #4 what class and method it is invoking.