When to use MutableLiveData and LiveData

LiveData has no public method to modify its data.

LiveData<User> getUser() {
    if (userMutableLiveData == null) {
        userMutableLiveData = new MutableLiveData<>();
    }
    return userMutableLiveData
}

You can't update its value like getUser().setValue(userObject) or getUser().postValue(userObject)

So when you don't want your data to be modified use LiveData If you want to modify your data later use MutableLiveData

Let's say you're following MVVM architecture and having LiveData as observable pattern from ViewModel to your Activity. So that you can make your variable as LiveData object exposing to Activity like below :

class MyViewModel : ViewModel() {
    // LiveData object as following
    var someLiveData: LiveData<Any> = MutableLiveData()

    fun changeItsValue(someValue: Any) {
        (someLiveData as? MutableLiveData)?.value = someValue
    }
}

And now at Activity part, you can observe LiveData but for modification you can call method from ViewModel like below :

class SomeActivity : AppCompatActivity() {
    // Inside onCreateMethod of activity
    val viewModel = ViewModelProviders.of(this)[MyViewModel::class.java]
    viewModel.someLiveData.observe(this, Observer{
        // Here we observe livedata
    })
    viewModel.changeItsValue(someValue) // We call it to change value to LiveData
    // End of onCreate
}