java - 从一项 Activity 转移到另一项 Activity 时 Android 应用程序崩溃

Question

现在我有两个 Activity ，activity_bustracking和activity_main

当按下主 Activity 中的按钮时，它应该移动到总线跟踪 Activity ；但是，现在当我单击按钮时，应用程序崩溃并出现错误:

05-28 16:34:32.680  26130-26130/com.example.robertloggia.test1 D/ViewRootImpl﹕ ViewPostImeInputStage ACTION_DOWN
05-28 16:34:32.770  26130-26130/com.example.robertloggia.test1 D/AndroidRuntime﹕ Shutting down VM
05-28 16:34:32.770  26130-26130/com.example.robertloggia.test1 E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.robertloggia.test1, PID: 26130
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
            at android.content.ComponentName.<init>(ComponentName.java:77)
            at android.content.Intent.<init>(Intent.java:4562)
            at com.example.robertloggia.test1.MainActivity$1.onClick(MainActivity.java:67)
            at android.view.View.performClick(View.java:5162)
            at android.view.View$PerformClick.run(View.java:20873)
            at android.os.Handler.handleCallback(Handler.java:739)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:145)
            at android.app.ActivityThread.main(ActivityThread.java:5835)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1388)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1183)

我的 MainActivity.class 看起来像:

package com.example.robertloggia.test1;

import android.app.Activity;
import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
//import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.Spinner;
import android.widget.Toast;
import android.content.Context;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.widget.TextView;

public class MainActivity extends Activity implements LocationListener{

    //GPS Specific variables
    protected LocationManager locationManager;
    protected LocationListener locationListener;
    protected Context context;
    TextView txtLat;
    String lat;
    String provider;
    protected String latitude,longitude;
    boolean gps_enabled, network_enabled;



    Spinner routeChoice;
    Button btnSubmit;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        routeChoice = (Spinner) findViewById(R.id.route_choices);
        btnSubmit = (Button) findViewById(R.id.btn_submit);

        txtLat = (TextView) findViewById(R.id.textView1);
        locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);

        //get location and make sure it is enabled
        locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, this);
        if(!locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER) && !locationManager.isProviderEnabled(LocationManager.NETWORK_PROVIDER)) {
            Toast.makeText(MainActivity.this, "Please enable GPS", Toast.LENGTH_LONG).show();
        }
        addListenerOnButton();
    }


    public void addListenerOnButton() {

        btnSubmit.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {

//                    Toast.makeText(MainActivity.this,
//                            "OnClickListener : " +
//                                    "\nSpinner 1 : " + String.valueOf(routeChoice.getSelectedItem()),
//                            Toast.LENGTH_SHORT).show();

                     Intent intent = new Intent(context, BusTrackingActivity.class);
                     startActivity(intent);

            }

        });
    }



    /*
    * METHODS FOR GETTING LOCATION
    */
    @Override
    public void onLocationChanged(Location location) {
        txtLat = (TextView) findViewById(R.id.textView1);
        txtLat.setText("Latitude:" + location.getLatitude() + ", Longitude:" + location.getLongitude());
    }

    @Override
    public void onProviderDisabled(String provider) {
        Log.d("Latitude", "disable");
    }

    @Override
    public void onProviderEnabled(String provider) {
        Log.d("Latitude", "enable");
    }

    @Override
    public void onStatusChanged(String provider, int status, Bundle extras) {
        Log.d("Latitude", "status");
    }


}

我的 BusTrackingActivity.class 如下所示:

package com.example.robertloggia.test1;

import android.app.Activity;
import android.os.Bundle;
import android.widget.Button;

/**
 * Created by RobertLoggia on 5/28/15.
 */
public class BusTrackingActivity extends Activity {

    Button button;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_bustracking);
    }    
}

我的activity_main.xml:

<ImageView
    android:layout_width="330dp"
    android:layout_height="200.5dp"
    android:layout_marginTop="110dp"
    android:id="@+id/imageView"
    android:src="@drawable/ripta_logo"
    android:layout_gravity="center_horizontal"
    android:contentDescription="@string/logo" />

<Spinner
    android:layout_width="290dp"
    android:layout_height="50dp"
    android:id="@+id/route_choices"
    android:entries="@array/route_array"
    android:prompt="@string/route_prompt"
    android:layout_gravity="center_vertical"
    android:layout_below="@+id/imageView" />
<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/submit_text"
    android:id="@+id/btn_submit"
    android:layout_gravity="center_vertical"
    android:layout_alignBottom="@+id/route_choices"
    android:layout_toRightOf="@id/route_choices"/>

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Please make sure GPS is enabled"
    android:id="@+id/textView1"
    android:layout_below="@+id/route_choices"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

activity_bustracking.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

</LinearLayout>

我的 Android list 如下所示:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.robertloggia.test1" >

    <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
    <uses-permission android:name="android.permission.INTERNET" />


    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:label="@string/app_name"
            android:name=".BusTrackingActivity" >
        </activity>


    </application>

</manifest>

Answer 1

问题是您从未将context分配给this。

但是，没有理由在 Activity 中保存 Context，只需使用 MainActivity.this 而不是 context:

            @Override
            public void onClick(View v) {

//                    Toast.makeText(MainActivity.this,
//                            "OnClickListener : " +
//                                    "\nSpinner 1 : " + String.valueOf(routeChoice.getSelectedItem()),
//                            Toast.LENGTH_SHORT).show();

                     Intent intent = new Intent(MainActivity.this, BusTrackingActivity.class);
                     startActivity(intent);

            }