Why does my variable contain "2122" and not "2322"?

Question

This was a question on an exam. Luckily I picked the right answer, but I still can't see why it's right.

Consider this program:

class D {
  protected C c;
  public D(C c) {
    this.c = new C(c);
  }
  public C getC() {
    return c;
  }
  public void setC(C c) {
    this.c = c;
  }
}
class C {
  protected String s;
  public C(String s) {
    this.s = s;
  }
  public C(C c) {
    this(c.s);
  }
  public String getS() {
    return s;
  }
  public void setS(String s) {
    this.s = s;
  }

  public static void main(String[] args) {
    C c1 = new C("1");
    C c2 = new C("2");
    D[] d = {
      new D(c1), new D(c1), new D(c2), new D(c2)
    };
    d[0] = d[3];
    c1.setS("3");
    String r = "";
    for (D i: d) {
      r += i.getC().getS();
    }
    System.out.println(r);
  }

}

It'll print 2122. I would expect 2322 however (I'm clearly wrong when you run the code). My reasoning behind that:

In the third line of the main method, four instances of D get initialized. The constructor of D makes a new instance of C. An instance of C has a String variable which points somewhere to a spot in the memory. Now the instance variable c, let's call it c3, of the object in d[1] has a instance variable (type String), let's call it s3, pointing to the same memory as the String s1, variable of c1.

So when we change s1, I'd expect the value of s3 also to change, since it's pointing to the same spot in the memory.

On a side note, if you change the constructor of D, see below, you'll get 2322 instead. Which I'd expect, since now the variable c3 in d[1] is pointing directly towards the memory location of c1.

public D(C c) {
  this.c = c;
}

My thoughts so far on the explanation (could be wrong):

1. When initializing the instance variable s1/s3, new String objects get made (so far I assumed they were pointing towards "1" in the String pool, since the constructor of C makes it look that way)
2. When changing s1, it's pointer will be redirected towards "3" in the String pool. Rather than "1" becoming "3" in the pool.

Could anyone explain this behaviour? What are the errors in my (faulty) reasoning?

Answer 1

This is not related to String pooling at all. Main answer: Is Java "pass-by-reference" or "pass-by-value"?

That's because D creates a new instance of C based on C#c. This mean that the instance of D#c is not the same instance as parameter C passed in constructor D, thus modifying that instance won't affect the current instance in D#c.

---

Re explaining all this in nice terms.

Here's what you're testing:

class Surprise {
    String item;
    public Surprise(String item) {
        this.item = item;
    }
    //this is called copy constructor
    //because you receive an object from the same class
    //and copy the values of the fields into the current instance
    //this way you can have a "copy" of the object sent as parameter
    //and these two object references are not tied by any mean
    public Surprise(Surprise another) {
        //here you just copy the value of the object reference of another#item
        //into this#item
        this.item = another.item;
    }
}

class Box {
    Surprise surprise;
    public Box(Surprise surprise) {
        //here you create a totally new instance of Surprise
        //that is not tied to the parameter surprise by any mean
        this.surprise = new Surprise(surprise);
    }

    public static void main(String[] args) {
        Surprise surprise1 = new Surprise("1");
        Surprise surprise2 = new Surprise("2");
        Box[] boxes = {
            new Box(surprise1),
            new Box(surprise1),
            new Box(surprise2),
            new Box(surprise2)
        };
        boxes[0] = boxes[3];
        //you update surprise1 state
        //but the state of Box#surprise in the boxes that used surprise1
        //won't get affected because it is not the same object reference
        surprise1.item = "3";
        //print everything...
        System.out.println("Boxes full of surprises");
        //this code does the same as the printing above
        for (Box box : boxes) {
            System.out.print(box.surprise.item);
        }
        System.out.println();
    }
}