Vector2.Zero unexpected behaviour when used to initialize Vector2

Question

I have base class which contains a Vector2 velocity field, which I was initializing in the constructor with Vector2.Zero. But that caused an issue so I had to replace it with new Vector2(0, 0)

public abstract class BaseEntity {
   private Vector2 velocity;

   public BaseEntity() {
     // velocity = Vector2.Zero; // this causes the issue
     velocity = new Vector2(0, 0);  // this I am using now
   }
}

The problem was that when I was adding other entities like Ship, Rock which extend the BaseEntity they were always moving in the same direction. So when I was moving the player, rocks and other entities which extend that BaseEntity class were following movement of my player.

I am not able to understand why that is; could someone explain to me why that issue occurred?

DMGregory · Accepted Answer · 2022-01-19 10:25:22Z

This is obvious from the semantics of Java.

Unlike C# structs, Java class instances are reference types. When you write:

Vector2 a = new Vector2(0, 0);
Vector2 b = a;

...you have not copied the vector a into b, creating two zero vectors. You have copied a reference to the same vector into b, so now you have two aliases that refer to the same logical object / the same bit of memory.

If I then write

b.x = 5;
DebugPrint(a.x);

...it will print "5" — because mutating b mutates a, because they're just two names for the same object.

This is what your code does when it takes a reference to Vector2.Zero. It's not initializing a new vector with zero for each component, it's saying "I am taking the definition of zero itself, to change how I please". If you mutate this value by setting its x/y components, you have mutated the shared value of Vector2.Zero observed anywhere in your code. If multiple objects have all "borrowed" zero in this way, they'll all see each other's mutations in their "own version" because they don't have multiple versions of the vector at all, just a bunch of references to the same one.

So, lesson: if you want to initialize a new mutable vector to zero, just write new Vector2() — the default constructor already does exactly that, and it's no more characters to type than what you were doing already.

Use Vector2.Zero as a read-only value when you want to do something with a zero vector without constructing a temporary. That's all it's for.

Thanks for good explanation. I think now I now what was causes the issue. — noname
– noname, Commented Jan 19, 2022 at 10:30

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