Question: When passing an object as an argument for a method, all changes that happen to the argument inside the method, affect the "original" object as well. That's because the argument is a reference to the object.

But I also want to do the same with variables- I want all changes that happen inside the method to affect the "original" variable. But I don't know how. I want to do this, because some times more than one local variables need to be processed in the same way.

How can I pass a reference to a variable as an argument for a method?

Answer: 
There is no other choice than to store the variable inside an object, and to pass the object. All arguments are passed by value in Java.

The object can be `this` (i.e. the variable is in fact a field of the current object).
Can you clarify what you mean by the word "variable"? Do you mean a local reference inside the method, a primitive that's passed in, or something else?

Java references and primitives are passed by value. You cannot alter them, no matter how much you may want to.