Instance Method call in Java

Question

In Java, for each object, a new copy of instance variables is created which can be accessed using the object reference. But in case of an instance method, only one copy of it(instance method) exists. How is this method accessed by various object references?

Answer 1

The byte code (or native code if it's JIT'd) for the method is stored in one location. When the method is called, a pointer (under the hood, aka reference at a higher level) to the instance object is passed as the first argument so the method code can operate on that specific instance - have access to its fields, etc. In order to save space without additional performance cost, the calling mechanism in Java is quite a bit more complicated than C++, especially for interface methods.

Answer 2

Methods and fields are completely different. Methods apply to all instances of the object, but fields are per instance.

One way to think of it:
pretend the method is "global" to all instances, but it is "passed" an instance of the object via the "this" reference.

Methods can change the state of a particular instance, but they themselves are stateless.

Answer 3

the implied reference "this" is passed in to each method, which of course you can reference explicitly

Answer 4

Behind the scenes a reference to the object is passed to the method as part of the call. It may be useful to look at Java's reflection classes, Method.invoke() in particular.

Answer 5

I'm assuming you're meaning on a simplistic level, as in how you actually do the call. I'm also assuming you're refering to a method that has the static modifier in its signature, ie:

public static int getNum() 
{
  // code in here 
  return num;
}

If this is what you mean, and this was part of a class called 'SomeClass', then it would be accessed via the method call SomeClass.getNum(). ie, you put the actual class name before the method.

If this is not what you mean, ignore my answer :)