Hello, I read kotlin docs and it says that object declaration is thread safe. But then why we use vo...

# getting-started

[Ömer Sungur]

Hello, I read kotlin docs and it says that object declaration is thread safe. But then why we use volatile and synchronized keywords in companion object? I thought object declariton is not thread safe and we should use that structure in companion object.

[Youssef Shoaib [MOD]]

the object itself is thread-safe, as in you're guaranteed that you'll get the same object across threads, but there's no guarantee as to the data itself inside the object

[Ömer Sungur]

So, is it a good case for data thread safe?

Object A {
@Volatile var count = 0
}

[Nick Kleban]

guaranteed that you'll get the same object across

Unless you serialize\deserialize object and end up with 2 object instances (should be fixed in 1.9.0)

[Youssef Shoaib [MOD]]

It's not actually fixed. Instead, the == checks are fixed

[Nick Kleban]

@Youssef Shoaib [MOD] Hm interesting, thank you for clarification

[Nick Kleban]

@Ömer Sungur thread safe is quite vague concept, depends on what you want to achieve with this code

[Ömer Sungur]

I mean only being able to access it from one place

[Nick Kleban]

Still not sure what it means

Marks the JVM backing field of the annotated var property as volatile, meaning that reads and writes to this field are atomic and writes are always made visible to other threads. If another thread reads the value of this field (e.g. through its accessor), it sees not only that value, but all side effects that led to writing that value.

[Nick Kleban]

@Ömer Sungur What does it mean “from one place”?

[Ömer Sungur]

I think this means that when a change is made , that change is valid in all threads.

[Ömer Sungur]

I mean in method or class.

[Nick Kleban]

It will not be cached, but resulting value still could be inconsistent if you will increment it from different threads. Increment = read\increment\write, 3 operations. You probably will need to use AtomicInt or synchronized block to increment this counter.

[Ömer Sungur]

Okay thank you for replying @Nick Kleban