Java r真正详细的解释！这对我帮助很大！我尝试使用Dao作为您给出的最后一个示例，但由于我不太清楚如何生成多个线程，因此我最终求助于LiveData，因为默认情况下，它在后台执行查询。你的解释让我很容易理解，为此我真的很感激！ ViewModelEntry re

r真正详细的解释！这对我帮助很大！我尝试使用Dao作为您给出的最后一个示例，但由于我不太清楚如何生成多个线程，因此我最终求助于LiveData，因为默认情况下，它在后台执行查询。你的解释让我很容易理解，为此我真的很感激！

ViewModelEntry recentEntry = new ViewModelEntry(RegisterActivity.this.getApplication());
int lastCadID = recentEntry.getLastEntry().getValue().getCadID();

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Integer com.xxxx.xxxx.entities.SchoolEntry.getCadID()' on a null object

repository = new ReportRepository(application);
public LiveData<SchoolEntry> getLastEntry() {return repository.getLastEntry(); }

ReportDatabase db = ReportDatabase.getDatabase(application);
schoolEntryDao = db.schoolEntryDao();
public LiveData<SchoolEntry> getLastEntry() { return schoolEntryDao.getLastEntry(); }

    @Query("SELECT * FROM SchoolEntry WHERE cadID = (SELECT MAX(cadID) from SchoolEntry)")
    LiveData<SchoolEntry> getLastEntry();

recentEntry.getLastEntry().observe(this, lastEntry ->
    {
        // do something with lastEntry.getCatId();
    });

recentEntry.getLastEntry().observe(this, new Observer<SchoolEntry>() {
        @Override
        public void onChanged(SchoolEntry lastEntry) {
            //do something with lastEntry.getCatId()
        }
    });

LiveData<SchoolEntry> lastEntryLiveData = recentEntry.getLastEntry();

// "this" is used here assuming this class is an Activity, as I inferred from your code above
lastEntryLiveData.observe(this, new Observer<SchoolEntry>() {
    @Override
    public void onChanged(SchoolEntry result) {
        int lastCadID = result.getCadID();

        // you can use lastCadID here, and this method may be called
        // multiple times whenever your database changes and the query
        // "SELECT * FROM SchoolEntry WHERE cadID = (SELECT MAX(cadID) from SchoolEntry)"
        // would return a new result
    }
});

@Query("SELECT * FROM SchoolEntry WHERE cadID = (SELECT MAX(cadID) from SchoolEntry)")
SchoolEntry getLastEntry();