android - LiveData.getValue() returns null with Room

Question

Java POJO Object

public class Section {

    @ColumnInfo(name="section_id")
    public int mSectionId;

    @ColumnInfo(name="section_name")
    public String mSectionName;

    public int getSectionId() {
        return mSectionId;
    }

    public void setSectionId(int mSectionId) {
        this.mSectionId = mSectionId;
    }

    public String getSectionName() {
        return mSectionName;
    }

    public void setSectionName(String mSectionName) {
        this.mSectionName = mSectionName;
    }
}

My Query method

@Query("SELECT * FROM section")
LiveData<List<Section>> getAllSections();

Accessing DB

final LiveData<List<Section>> sections = mDb.sectionDAO().getAllSections();

On the next line I am checking sections.getValue() which is always giving me null although I have data in the DataBase and later I am getting the value in the onChanged() method.

sections.observe(this, new Observer<List<Section>>() {
    @Override
    public void onChanged(@Nullable List<Section> sections){

    }
});

But when I omit LiveData from the query I am getting the data as expected. Query Method:

@Query("SELECT * FROM section")
List<Section> getAllSections();

Accessing DB:

final List<Section> sections = mDb.sectionDAO().getAllSections();

Answer 1

On the next line I am checking sections.getValue() which is always giving me null although I have data in the DataBase and later I am getting the value in the onChanged() method.

This is a normal behavior, because queries that return LiveData, are working asynchronous. The value is null that moment.

So calling this method

LiveData<List<Section>> getAllSections();

you will get the result later here

sections.observe(this, new Observer<List<Section>>() {
@Override
public void onChanged(@Nullable List<Section> sections){

}
});

from documentation:

Room does not allow accessing the database on the main thread unless you called allowMainThreadQueries() on the builder because it might potentially lock the UI for a long periods of time. Asynchronous queries (queries that return LiveData or RxJava Flowable) are exempt from this rule since they asynchronously run the query on a background thread when needed.