Aliasing — How Java Runs

Student: Professor I wrote a program and I don’t get why it’s running that way.

Prof: Great! That probably means there’s something good to learn. Let’s see it!

Student: I made two Menu instances, but it seems like updating one updates the other:

record Menu(int[] pricesCents) {
  int[] discounted() { … }
  void discountInPlace() {
    int index = 0;
    for(int p: pricesCents) {
      pricesCents[index] = p * 90 / 100;
      index += 1;
    }
  }
}
int[] regular = { 2000, 1000, 500, 800 };
Menu lunch = new Menu(regular);
Menu dinner = new Menu(regular);
int[] lunchPrices = lunch.pricesCents;
int[] dinnerPrices = dinner.pricesCents;
void main() {
  lunch.discountInPlace();
}

Student: I’m confused because I thought discountInPlace would only update the lunch menu, not the dinner one. But when I show the arrays it seems like all the arrays updated.

Prof: Very interesting! I think what’s happening here is there is only one array.

Student: Only one array?

Prof: That’s right. Let’s look at a memory diagram at the end:

Prof: So that one array is created and stored in regular, and that same reference is used to initialize both lunch and dinner. Look at the two objects – they have different references but both of the pricesCents fields store the same array reference!

Student: I see, that one reference is referred to by five different places!

Prof: Right, and the array updates change the elements on the heap, so then if we access those elements through any of the references, we see the updated values. This idea has a name: it is called aliasing, when different names in a program store the same reference. Aliasing makes it so updates through one reference are visible through all the others.

Student: OK, I think I got it!

Practice

record Team(String[] players) {
  void replaceFirst(String name) {
    players[0] = name;
  }
}
String[] varsity = { "Ana", "Ben", "Cy" };
String[] jv = { "Dee", "Eli" };
Team red = new Team(varsity);
Team blue = new Team(jv);
Team green = new Team(red.players);
String[] roster = blue.players;
Team gold = new Team(roster);
void main() {
  green.replaceFirst("Zed");
}

This program creates two arrays. Which one is stored in each team’s players field?

red.players:
blue.players:
green.players:
gold.players:

After main runs, how many of the four teams have "Zed" as their first player?