Methods with Parameters in Java

Question

Could someone explain why this works.

I have 2 classes in Eclipse. A Class called "Car" contains the following code..

public class Car {
    public void printOut(String variable1){
        System.out.println("Hello " +variable1);
    }
}

and another class , which is where my 'main' is, is called "House", the code inside it is

import java.util.Scanner;

class House {
    public static void main(String args[]){
        Scanner input = new Scanner(System.in);
        Car carObject = new Car();

        System.out.println("Enter name here: ");
        String variable2 = input.nextLine();

        carObject.printOut(variable2);
    }
}

When I run the code, it works, it writes "Enter name here" and when I type it out, it proceeds to say "Hello "name entered" "

My question is, do 'variable1' and 'variable2' have any relation to eachother, other than that they're both of String class.

because i'm confused as to why the code compiles correctly.

To me, it looks like variable 1 has no correlation to variable2, even though they're both of String class, it doesn't look like they ever interact with one another, and variable1 isn't used in the "House" class at all, yet it still knows to compile whatever I've entered. It's as if 'variable1' is replaced by 'variable2' and whatever variable2 contains gets printed out.

Answer 1

The method definition in class Car is sort of a prototype for when you use it. Have you ever been taught functions in maths with a 'black box'? You put in a number, and get output. So, you enter 3, if the function is f(x) = Xx2, the output will be 6. Before you call the method, var2 is completely different from var1. In the method however, var2 is passed and replaces all var1s you use in the method. Don't worry, I didn't get this either when I started Java

Answer 2

They're related only because you are passing variable2 into your Car.printOut method. Imagine this analogy for a moment: You know how to do math homework on command. You are an object, called FacelessVoid and you have a method called doHomework. doHomework takes a single parameter of type Work. In the real world, I would have a box of work, and I would dump the box of work onto your desk. Whatever I write on the box doesn't matter to you, but the contents of the box get dumped onto your desk, where you can call it whatever you want.

This is exactly what is happening in your code: You have a string called variable2, and its contents get "dumped" into variable1 inside printOut. Of note though is that the string doesn't actually get dumped into your method, a reference gets copied. So it's the same object, it's just called something different.