Difference of setValue() & postValue() in MutableLiveData

All of the above answers are correct. But one more important difference. If you call postValue() and after that you call getValue(), you may not receive the value that you set in postValue(). If the main thread had already set the value, then you will get the value that you posted, but if the main thread hadn't set the value yet, then you don't get the value that you posted. So be careful if you work in background threads.

setValue() is called directly from caller thread, synchronously notifies observers and changes LiveData value immediately. It can be called only from MainThread.
postValue() uses inside something like this new Handler(Looper.mainLooper()).post(() -> setValue()), so it runs setValue via Handler in MainThread. It can be called from any thread.

Based on the documentation:

setValue():

Sets the value. If there are active observers, the value will be dispatched to them. This method must be called from the main thread.

postValue():

Posts a task to a main thread to set the given value. If you called this method multiple times before a main thread executed a posted task, only the last value would be dispatched.

To summarize, the key difference would be:

setValue() method must be called from the main thread. But if you need set a value from a background thread, postValue() should be used.