从一个活动移动到另一个活动时，Android 应用程序崩溃

Question

Right now I have two activities, activity_bustracking and activity_main

When the button in the main activity is pressed it is supposed to move to the bustracking activity; however, right now it when I click the button the application crashes with the error:

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)

My MainActivity.class looks like:

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");
    }


}

My BusTrackingActivity.class looks like:

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);
    }    
}

My 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>

And my android manifest looks like:

<?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

The problem is that you never assign context to this .

However, there is no reason to save off a Context in an Activity, just use MainActivity.this instead of 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);

            }

Answer 2

When you are starting the new intent in this line:

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

The context object has not been initiated yet. You declared it earlier:

protected Context context;

Inside your onCreate() method, use this code:

context = this;

Answer 3

MainActivity 中的上下文为空。要么初始化它，要么使用 MainActivity.this 代替上下文。

Answer 4

Add the line of code from the 67th line in the MainActivity, as this is the line with the error.

However without this, I am guessing something went wrong with the button listener.

This being said, try removing all of this:

 public void addListenerOnButton() {

    btnSubmit.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {

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

        }

    });
}

Listeners are better put in the method they are used, or in this case on create. This can be done by putting this line of code:

    btnSubmit.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
                 Intent intent = new Intent(context, BusTrackingActivity.class);
                 startActivity(intent);

        }

    });

Really though, if this doesn't work I just need to like of code at the error. If you click the first blue thing in the logcat (the rest will be grey), I can help more.

Answer 5

this could be because of using card view in first activity or that activity on which your app got crashed while running.

used this code to implement card view in dependencies

implementation 'androidx.cardview:cardview:1.0.0'