How can any single-threaded program be a valid multithreaded program?

Question

I was reading the book "Java Concurrency In Practice". In second chapter, I read the statement

any single-threaded program is also a valid multithreaded program

I could not understand this statement.

Kindly share your thoughts on it so that my confusion can be cleared. Thanks in advance.

Answer 1

Note that the statement doesn't imply that any correct single threaded program is also correct multi threaded program, the author simply states that it is possible to transform any single threaded program to be executed in multi-threaded environment.

However, if the program is already not correct in single threaded environment then it cannot possibly be correct in more complex multi threaded environment.

From the book:

Since any single threaded program is also a valid multithreaded program, it cannot be thread safe if it is not even correct in a single threaded environment.

If an object is correctly implemented, no sequence of operations-calls to public methods and reads or writes of public fields should be able to violate any of its invariants or post conditions. No set of operations performed sequentially or concurrently on instances of a thread safe class can cause an instance to be in an invalid state.

And this:

If the loose use of "correctness" here bothers you, you may prefer to think of a thread safe class as one that is no more broken in a concurrent environment than in a single threaded environment.

@yshavit Before you go off thinking about multithreaded correctness, make sure you at least have single-threaded correctness.