java.lang.RuntimeException：無法啟動活動

Question

我的應用程序啟動時總是崩潰。 這是Android Monitor上的錯誤。 以下代碼摘自我的片段，應從應用程序的開頭開始：

01-30 23:42:55.281 5767-5767/silverbeach.meintieralter E/AndroidRuntime: FATAL EXCEPTION: main
                                                                         Process: silverbeach.meintieralter, PID: 5767
                                                                         java.lang.RuntimeException: Unable to start activity ComponentInfo{silverbeach.meintieralter/silverbeach.meintieralter.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                                                                             at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3254)
                                                                             at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3350)
                                                                             at android.app.ActivityThread.access$1100(ActivityThread.java:222)
                                                                             at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1795)
                                                                             at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                             at android.os.Looper.loop(Looper.java:158)
                                                                             at android.app.ActivityThread.main(ActivityThread.java:7229)
                                                                             at java.lang.reflect.Method.invoke(Native Method)
                                                                             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
                                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
                                                                          Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                                                                             at silverbeach.meintieralter.FragmentOne.onCreate(FragmentOne.java:36)
                                                                             at android.app.Fragment.performCreate(Fragment.java:2242)
                                                                             at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:947)
                                                                             at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1163)
                                                                             at android.app.BackStackRecord.run(BackStackRecord.java:793)
                                                                             at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1552)
                                                                             at android.app.FragmentController.execPendingActions(FragmentController.java:325)
                                                                             at android.app.Activity.performStart(Activity.java:6914)
                                                                             at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3217)
                                                                             at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3350) 
                                                                             at android.app.ActivityThread.access$1100(ActivityThread.java:222) 
                                                                             at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1795) 
                                                                             at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                             at android.os.Looper.loop(Looper.java:158) 
                                                                             at android.app.ActivityThread.main(ActivityThread.java:7229) 
                                                                             at java.lang.reflect.Method.invoke(Native Method) 
                                                                             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
                                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 
01-30 23:42:55.401 5767-5767/silverbeach.meintieralter I/Process: Sending signal. PID: 5767 SIG: 9

碼

 package silverbeach.meintieralter;

import android.app.Fragment;
import android.content.Context;
import android.os.Bundle;
import android.os.Vibrator;
import android.support.annotation.Nullable;
import android.support.design.widget.Snackbar;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Spinner;
import android.widget.TextView;
import android.view.View.OnClickListener;

import static android.content.Context.VIBRATOR_SERVICE;

public class FragmentOne extends Fragment {

private TextView out;
private EditText alter;
private Spinner s;
private Button button;



@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);



    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {




            String text = s.getSelectedItem().toString();
            int newage = 404;
            double doublenewage = 4.04;
            double doubleage;
            int intage;
            doubleage = Double.parseDouble(alter.getText().toString());
            intage = (int) doubleage;

            switch (text) {
                case "Pferd": {
                    doublenewage = intage / 3.1;
                    long longpferdage = Math.round(doublenewage);
                    newage = (int) longpferdage;

                    if (newage < 31)
                        out.setText("Du bist ein " + Integer.toString(newage) + " Jahre altes Pferd");
                    else
                        out.setText("Du bist höchstwahrscheinlich schon tot, aber theoretisch wärst du ein " + newage + "-Jähriges Pferd.");
                }break;
                case "Hund": {
                    newage = intage / 7;
                    if (newage < 15)

                        out.setText("Du bist ein " + Integer.toString(newage) + " Jahre alter Hund.");
                    else
                        out.setText("Du bist höchstwahrscheinlich schon tot, aber theoretisch wärst du ein " + newage + "-Jähriger Hund.");

                }break;
                case "Meerschweinchen": {
                    newage = intage / 12;
                    if (newage < 8)

                        //out.setText("Du bist ein " + Integer.toString(newage) + " Jahre altes Meerie.");

                        out.setText( "Du bist ein "+Integer.toString(newage)+" Jahre altes Meerie.");





                    else
                        out.setText("Du bist höchstwahrscheinlich schon tot, aber theoretisch wärst du ein " + newage + "-Jähriges Meerie.");

                }break;


            }



        }


    });

}





@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

    View layout = inflater.inflate(R.layout.fragment_one, null);

     out = (TextView) layout.findViewById(R.id.out);
     alter = (EditText) layout.findViewById(R.id.altereingabe);
     s = (Spinner) layout.findViewById(R.id.spinner);
     button = (Button) layout.findViewById(R.id.button);

   // tvText1 = (TextView) layout.findViewById(R.id.tvFragmentOne);

    return layout;
}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);

    //tvText1.setText("Ok View 1 wurde geändert.");

}

}

Answer 1

onCreate 在 onCreateView 之前被稱為BEFORE ，因此在將偵聽器分配給視圖時，其仍為null；

因此，我建議將您的button.setOnClickListener移到onCreateView() 。 方法

Answer 2

button為null。 在設置偵聽器之前，您將需要使用button = (Button)findViewById(R.id.<id of the button>) 。