How do I call a method from one thread but have another thread execute the method in Java?

Question

Let's say I have a thread:

Thread threadA = new Thread(() -> {
    int i = doSomething();
});

I want to call another method that takes the int 'i' as a parameter, but doing that from threadA slows down the thread too much. How do I do this:

Thread threadA = new Thread(() -> {
    int i = doSomething();
    doSomethingElse(i);
});

but have the method 'doSomethingElse()' be executed on another thread (threadB), and if threadB is busy, wait until it's not and execute then?