Instruction set

Core instructions

The JVM being well... a VM has bytecode which for simplicity I'll just call instructions. Let's go over a few of the most important ones.

Opcode	Type	Purpose	Frequency
iload	General	Loads a local variable of type primitive integer that was previously stored onto the stack.	High
istore	General	Stores a primitive integer value into a local variable that can later be loaded onto the stack using it's index.	High
goto	Flow	Jumps to a specified label.	Medium
ifeq,ne	Flow	Jumps to a specified label based on a condition.	Medium
dup	General	Duplicates the latest stack element.	Medium
pop	General	Pops the latest stack element off the stack.	High
ireturn	General	Returns an integer primitive from a method.	High
iadd	Arithmetic	Adds the latest 2 integer primitive type stack elements together and pushes the result.	Low
isub	Arithmetic	Subtracts the latest 2 integer primitive type stack elements and pushes the result.	Low
ixor	Bitwise	Performs an exclusive OR operation on the 2 latest integer primitive type stack elements.	Medium
invokestatic	Invocation	Invokes a static method from a class.	High
invokevirtual	Invocation	Invokes a method from a class using an instance of said class.	High