What is the difference between map() and switchMap() methods?

As per the documentation

Transformations.map()

Applies a function on the value stored in the LiveData object, and propagates the result downstream.

Transformations.switchMap()

Similar to map, applies a function to the value stored in the LiveData object and unwraps and dispatches the result downstream. The function passed to switchMap() must return a LiveData object.

In other words, I may not be 100% correct but if you are familiar with RxJava; Transformations#map is kind of similar to Observable#map & Transformations#switchMap is similar to Observable#switchMap.

Let’s take an example, there is a LiveData which emits a string and we want to display that string in capital letters.

One approach would be as follows; in an activity or fragment

Transformations.map(stringsLiveData, String::toUpperCase)
    .observe(this, textView::setText);

the function passed to the map returns a string only, but it’s the Transformation#map which ultimately returns a LiveData.

The second approach; in an activity or fragment

Transformations.switchMap(stringsLiveData, this::getUpperCaseStringLiveData)
            .observe(this, textView::setText);

private LiveData<String> getUpperCaseStringLiveData(String str) {
    MutableLiveData<String> liveData = new MutableLiveData<>();
    liveData.setValue(str.toUpperCase());
    return liveData;
}

If you see Transformations#switchMap has actually switched the LiveData. So, again as per the documentation The function passed to switchMap() must return a LiveData object.

So, in case of map it is the source LiveData you are transforming and in case of switchMap the passed LiveData will act as a trigger on which it will switch to another LiveData after unwrapping and dispatching the result downstream.