Maybe I'm missing something but when would the 500 geese instances ever be stack allocated in Rust? That comparison seems unfair, the lifetime of that kind of object isn't going to be compatible with stack allocation.

Allocations are really really cheap in Java by the way, so I don't get how 500 allocations would even be an issue.

When you make a local variable with a Goose in Java, that's a heap allocation

    Goose jim = make_a_goose_somehow();  // Java, so jim is on the Heap, no way around it

    let jim : Goose = make_a_goose_somehow();  // Rust, jim is on the stack

    ArrayList<Goose> geese = new ArrayList<Goose>();
    // ... some loop eventually
      Goose a_goose = somehow_get_this_goose();   // That's an allocation
      geese.add(a_goose);

    let geese: Vec<Goose> = Vec::new();
    // ... some loop eventually
      let a_goose : Goose = somehow_get_this_goose(); // But this is not
      geese.push(a_goose);