我的应用程序类代码包含一个空指针异常。我如何找到它？以及如何解决呢？

Question

我在代码中找到一个NullPointerException，无法找到它的发生位置。请帮忙。

public class SpinnerActivity extends AppCompatActivity implements
    OnItemSelectedListener {

// Spinner element
Spinner spinner, spinner2, spinner3, spinner4;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_spinner);
    Button proceed;

    // Spinner element
    spinner = findViewById(R.id.spinner);
    spinner2 = findViewById(R.id.spinner2);
    spinner3 = findViewById(R.id.spinner3);
    spinner4 = findViewById(R.id.spinner4);

    // Spinner click listener
    spinner.setOnItemSelectedListener(this);
    spinner2.setOnItemSelectedListener(this);
    spinner3.setOnItemSelectedListener(this);
    spinner4.setOnItemSelectedListener(this);
    proceed = (Button) findViewById(R.id.proceed);
    proceed.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            startActivity(new Intent(SpinnerActivity.this, Screenlayout.class));
        }
    });


    DatabaseHelper2 helper = new DatabaseHelper2(this);
    MovieObject a = new MovieObject();




    helper.insertMovie(a);

    // Loading spinner data from database
    loadSpinnerData();
}

/**
 * Function to load the spinner data from SQLite database
 * */
private void loadSpinnerData() {
    // database handler
    DatabaseHelper2 db = new DatabaseHelper2(getApplicationContext());

    List<String> dates = db.getAllDates();
    List<String> names = db.getAllNames();
    List<String> times = db.getAllTimes();
    List<String> seats = db.getAllSeats();

    ArrayAdapter<String> nameAdapter = setAdapter(names);

    ArrayAdapter<String> dateAdapter = setAdapter(dates);

    ArrayAdapter<String> timeAdapter = setAdapter(times);

    ArrayAdapter<String> seatAdapter = setAdapter(seats);

    // Drop down layout style - list view with radio button
    nameAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    dateAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    timeAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    seatAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);

    // attaching data adapter to spinner
    spinner.setAdapter(nameAdapter);
    spinner2.setAdapter(dateAdapter);
    spinner3.setAdapter(timeAdapter);
    spinner4.setAdapter(seatAdapter);

}

@Override
public void onItemSelected(AdapterView<?> parent, View view, int position, long id) { }

@Override
public void onNothingSelected(AdapterView<?> arg0) {
    // TODO Auto-generated method stub

}

public ArrayAdapter<String> setAdapter(List<String> strList){
    return new ArrayAdapter<>(this,
            android.R.layout.simple_spinner_item, strList);
}

错误：

Process: com.trivendra.gettickets, PID: 28164
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
        at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:457)
        at android.widget.ArrayAdapter.getView(ArrayAdapter.java:419)
        at android.widget.AbsSpinner.onMeasure(AbsSpinner.java:206)
        at android.widget.Spinner.onMeasure(Spinner.java:614)
        at android.support.v7.widget.AppCompatSpinner.onMeasure(AppCompatSpinner.java:420)
        at android.view.View.measure(View.java:24510)
        at android.support.constraint.ConstraintLayout.internalMeasureChildren(ConstraintLayout.java:1210)
        at android.support.constraint.ConstraintLayout.onMeasure(ConstraintLayout.java:1550)
        at android.view.View.measure(View.java:24510)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:141)
        at android.view.View.measure(View.java:24510)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
        at android.support.v7.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:400)
        at android.view.View.measure(View.java:24510)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at android.view.View.measure(View.java:24510)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1552)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:842)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:721)
        at android.view.View.measure(View.java:24510)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at com.android.internal.policy.DecorView.onMeasure(DecorView.java:736)
        at android.view.View.measure(View.java:24510)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:3004)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1833)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2122)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1721)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7595)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:966)
        at android.view.Choreographer.doCallbacks(Choreographer.java:790)
        at android.view.Choreographer.doFrame(Choreographer.java:725)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:951)
        at android.os.Handler.handleCallback(Handler.java:883)
        at android.os.Handler.dispatchMessage(Handler.java:100)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7319)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:934)
I/Process: Sending signal. PID: 28164 SIG: 9
Application terminated.