DataSnapshot值未存储在变量中

Question

我正在尝试从Firebase数据库获取布尔值到我的Android应用程序中。我已经在一个布尔变量中得到了它，但是当我使用这个布尔值时，它说一个空对象引用。

我正在尝试根据Firebase数据库中的数据启动特定的Intent。但这给了我以下错误

public class SplashScreen extends AppCompatActivity {

Boolean rec_res;
@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_splash_screen);
        FirebaseDatabase database = FirebaseDatabase.getInstance();
        DatabaseReference rootRef = database.getReference();

        rootRef.child("rec_res").addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                rec_res = dataSnapshot.getValue(Boolean.class);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });

        int SPLASH_TIME_OUT = 2000;
        new Handler().postDelayed(new Runnable() {

            /*
             * Showing splash screen with a timer. This will be useful when you
             * want to show case your app logo / company
             */

            @Override
            public void run() {
                // This method will be executed once the timer is over
                // Start your app main activity



                ConnectivityManager connectivityManager
                        = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
                NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
                boolean isConnected = activeNetworkInfo != null && activeNetworkInfo.isConnected();


               /* if(String.valueOf(rec_res).equals("true")) {
                    Intent k = new Intent(SplashScreen.this, RecruitRegActivity.class);
                    startActivity(k);
                }*/

                if (rec_res && isConnected) {

                        Intent i = new Intent(SplashScreen.this, RecruitRegActivity.class);
                        startActivity(i);

                } else if(!isConnected){
                    Intent i = new Intent(SplashScreen.this, NoInternet.class);
                    startActivity(i);
                } else {
                    Intent i = new Intent(SplashScreen.this, HomePage.class);
                    startActivity(i);
                }

                // close this activity
                finish();
            }
        }, SPLASH_TIME_OUT);

这是我得到的错误

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.smec.tam, PID: 12815
    java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.Boolean.booleanValue()' on a null object reference
        at com.smec.tam.SplashScreen$2.run(SplashScreen.java:99)
        at android.os.Handler.handleCallback(Handler.java:751)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6316)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762)

Answer 1

问题是您在完成任务之前正在执行if (rec_res && isConnected)，因此这就是它抛出NullPointerException的原因。

您可以将逻辑放入onDataChange内，以便在完成任务并初始化rec_res东西后立即执行逻辑。

rootRef.child("rec_res").addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                rec_res = dataSnapshot.getValue(Boolean.class);
                int SPLASH_TIME_OUT = 2000;
        new Handler().postDelayed(new Runnable() {

            /*
             * Showing splash screen with a timer. This will be useful when you
             * want to show case your app logo / company
             */

            @Override
            public void run() {
                // This method will be executed once the timer is over
                // Start your app main activity



                ConnectivityManager connectivityManager
                        = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
                NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
                boolean isConnected = activeNetworkInfo != null && activeNetworkInfo.isConnected();


               /* if(String.valueOf(rec_res).equals("true")) {
                    Intent k = new Intent(SplashScreen.this, RecruitRegActivity.class);
                    startActivity(k);
                }*/

                if (rec_res && isConnected) {

                        Intent i = new Intent(SplashScreen.this, RecruitRegActivity.class);
                        startActivity(i);

                } else if(!isConnected){
                    Intent i = new Intent(SplashScreen.this, NoInternet.class);
                    startActivity(i);
                } else {
                    Intent i = new Intent(SplashScreen.this, HomePage.class);
                    startActivity(i);
                }

                // close this activity
                finish();
            }
        }, SPLASH_TIME_OUT);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });