What does "There may be many references to the same object." mean?

Question

Section 4.3.1 of the Java Language Specification states that, "There may be many references to the same object.".

But it also states that "The reference values (often just references) are pointers to these objects, and a special null reference, which refers to no object.

So, my understanding is that "There may be many pointers to the same object.", but how can we have many pointers, which mean distinct addresses, that all refer to the same object? Can a certain object have different addresses?

It means that the reference (pointer) value can be stored in many places. We call each copy of the pointer a reference. So there are many references. — Stephen C, Dec 01 '21 at 10:40
A pointer is basically a variable, containing the address of the thing it is pointing at. You can have any amount of these pointers and they can all point to the same thing. — f1sh, Dec 01 '21 at 10:40

CodeAddict · Answer 1 · 2021-12-01T11:03:34.300

Obj newObj = new Obj();

This creats a new object in memory, from the class "Obj".

Obj newObj = new Obj();
Obj a1 = newObj;
Obj a2 = newObj;
Obj a3 = newObj;

a1,a2 and a3 all refrences the same object, meaning, they all point to the same object. If you, for example, changed a value in a2

System.out.println(a3.name) //prints carl
a2.name = "bob"
System.out.println(a3.name) //prints bob

It changes the value of the object in memory that a2 pointed to. In this case, that object was newObj.

Here is a tiny example you can play with:

  class Main{
  public static void main(String[] args) {
    Book b = new Book("ey");
    Book b1 = b;
    Book b2 = b;
    System.out.println(b1.title); //ey
    System.out.println(b2.title); //ey
    System.out.println(b.title); //ey
    b2.title = "ok";
    System.out.println(b1.title); //ok
    System.out.println(b2.title); //ok
    System.out.println(b.title); //ok
  }
}

 class Book {
  public String title;
  public Book(String t){
    title = t;
  }
}

Did this clear up stuff?

score 1 · Accepted Answer · answered Dec 01 '21 at 10:54

It just means 'many variables can all be referencing the same thing'.

List<String> list1 = new ArrayList<String>();
List<String> list2 = list1;

list1.add("Hello");
System.out.println(list2);

The above code prints 'Hello' even though we never invoked list2.add. That's because both the list1 variable and the list2 variable are referencing the same object, therefore, if you add something to the object you reach when dereferencing list1, it's also visible if you dereference list2 (which the println code ends up doing).

Variables in java (be it fields, locals, or parameters) of non-primitive types are like maps to treasure buried in the sand. The . operator (as well as [], synchronized (x) and a few others) 'dereference' - as in, they mean, in java-terms: Follow the map and dig.

Section 4.3.1 is trying to say: For any given treasure, there can be many many maps out there that all lead to it.

Don't think about references as having a numeric value that indicates where it lives in memory. That's a C thing (yes, java sort of works like that under the hood, but that's the point: It is under the hood; it's impossible to ever see this number, interact with it, do arithmetic on it, etc. In fact, on most JVMs, that 'number' isn't a memory address, not directly anyway).

You seem to interpret that as: For any particular object, many different 'memory addresses' can refer to it. That's not what it means. It just means: Multiple variables can hold the same value.

It's as trivial as:

int x = 5;
int y = 5;

Perhaps you're a tad confused because this seems incredibly obvious, but that's nevertheless what §4.3.1 is spelling out.

Thanks rzwitserloot, clear explanation. I think I was confused by the difference between reference value and reference variable. — constantem, Dec 01 '21 at 11:32

What does "There may be many references to the same object." mean?

2 Answers2