android - onSetClickListener 的空指针异常

Question

好的，我想让我的应用程序执行以下操作。

1. 打开时，显示一个开始按钮。
2. 按下该按钮后，将启动两项服务，一项用于加速度计，一项用于 GPS，该按钮变为停止按钮。
3. 服务将继续运行，直到按下停止按钮，关闭监听器并停止服务。
4. 一旦按下停止按钮，应用程序就会显示应用程序在收集数据时执行的一些计算。

需要了解的重要事实:

1. 所有的计算都有效
2. 问题是在我更改 View 后出现的，最初只有一个 View 显示按钮和所有实时更新的计算。
3. 然后我将应用更改为具有 3 个 View 。每个按钮一个，结果一个。在我这样做之后，应用程序停止工作。然而!!!!在我更改 View 之前，我可能没有检查它们是否有效就做了一些更改，但我一直试图弄清楚问题出在哪里太久了，以至于我无法正确记住事件的准确顺序。

这是我当前的 onCreate 方法:

public void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.start);

    setGPS();
    context = this;

    Button startButton = (Button) findViewById(R.id.startbutton);
    Button stopButton = (Button) findViewById(R.id.stopbutton);

    Log.d("NULL POINTER", "NULL POINTER");
    startButton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            Log.d("NULL POINTER", "NULL POINTER");
            //setContentView(R.layout.stop);
            startService();

        }
    });

    stopButton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {

            //setContentView(R.layout.results);
            stopService();

        }
    });

好的，在这段代码中，您将看到两个 Log.d 输出，上面写着“NULL POINTER”。当我运行我的代码时，第一个显示但第二个不显示，这显然意味着中间的代码导致程序崩溃，这在日志 cat 中显示:

06-15 12:30:25.176: DEBUG/AndroidRuntime(1037): >>>>>>>>>>>>>> AndroidRuntime START <<<<<<<<<<<<<<
06-15 12:30:25.185: DEBUG/AndroidRuntime(1037): CheckJNI is ON
06-15 12:30:25.537: DEBUG/AndroidRuntime(1037): --- registering native functions ---
06-15 12:30:25.806: DEBUG/ddm-heap(1037): Got feature list request
06-15 12:30:26.147: INFO/ActivityManager(52): Starting activity: Intent {act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10000000 cmp=Hartford.gps/.GPSMain }
06-15 12:30:26.177: INFO/ActivityManager(52): Start proc Hartford.gps for activity Hartford.gps/.GPSMain: pid=1043 uid=10030 gids={}
06-15 12:30:26.196: DEBUG/AndroidRuntime(1037): Shutting down VM
06-15 12:30:26.196: DEBUG/dalvikvm(1037): DestroyJavaVM waiting for non-daemon threads to exit
06-15 12:30:26.196: DEBUG/dalvikvm(1037): DestroyJavaVM shutting VM down
06-15 12:30:26.196: DEBUG/dalvikvm(1037): HeapWorker thread shutting down
06-15 12:30:26.207: DEBUG/dalvikvm(1037): HeapWorker thread has shut down
06-15 12:30:26.207: DEBUG/jdwp(1037): JDWP shutting down net...
06-15 12:30:26.226: INFO/dalvikvm(1037): Debugger has detached; object registry had 1 entries
06-15 12:30:26.226: DEBUG/dalvikvm(1037): VM cleaning up
06-15 12:30:26.246: ERROR/AndroidRuntime(1037): ERROR: thread attach failed
06-15 12:30:26.317: DEBUG/dalvikvm(1037): LinearAlloc 0x0 used 638596 of 5242880 (12%)
06-15 12:30:26.456: DEBUG/ddm-heap(1043): Got feature list request
06-15 12:30:26.906: DEBUG/NULL POINTER(1043): NULL POINTER
06-15 12:30:26.916: DEBUG/AndroidRuntime(1043): Shutting down VM
06-15 12:30:26.916: WARN/dalvikvm(1043): threadid=3: thread exiting with uncaught exception (group=0x4001b188)
06-15 12:30:26.916: ERROR/AndroidRuntime(1043): Uncaught handler: thread main exiting due to uncaught exception
06-15 12:30:26.926: ERROR/AndroidRuntime(1043): java.lang.RuntimeException: Unable to start activity ComponentInfo{Hartford.gps/Hartford.gps.GPSMain}: java.lang.NullPointerException
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2496)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at android.app.ActivityThread.access$2200(ActivityThread.java:119)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at android.os.Handler.dispatchMessage(Handler.java:99)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at android.os.Looper.loop(Looper.java:123)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at android.app.ActivityThread.main(ActivityThread.java:4363)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at java.lang.reflect.Method.invokeNative(Native Method)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at java.lang.reflect.Method.invoke(Method.java:521)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at dalvik.system.NativeStart.main(Native Method)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043): Caused by: java.lang.NullPointerException
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at Hartford.gps.GPSMain.onCreate(GPSMain.java:70)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2459)
06-15 12:30:26.926: ERROR/AndroidRuntime(1043):     ... 11 more

我已经无计可施了!我已经尝试了所有我能想到的方法，但我认为在这个阶段我有隧道视觉，它只需要另一双眼睛来观察它。

如果需要，我可以发布更多代码

任何人都可以提供的任何帮助都很棒!

@马克

哦，好的，谢谢!你是什​​么意思他们可能是空的？这里是start.xml文件，和stop.xml是一样的

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
    <Button
        android:id="@+id/startbutton"
        android:layout_width="100px" 
        android:layout_height="100px" 
        android:text="Start"
        android:layout_centerInParent="true"
    />

</LinearLayout>

@叶戈尔和马赫

这是第 70 行标记的完整文件

import java.text.DecimalFormat;
import java.text.NumberFormat;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.os.PowerManager;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

public class GPSMain extends Activity   {

PowerManager powerManager;
PowerManager.WakeLock wL;

//text views to display latitude and longitude
static TextView latituteField;
static TextView longitudeField;
static TextView kmphSpeedField;
static TextView avgKmphField;
static TextView topKmphField;
static TextView accText;

//objects to store positional information
protected static double lat;
protected static double lon;

//objects to store values for current and average speed
protected static double kmphSpeed;
protected static double avgKmph;
protected static double totalKmph;
protected static double topKmph=0;

static String a, b, c;

private Context context;

//counter that is incremented every time a new position is received, used to calculate average speed
static int counter = 0;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.start);

    setGPS();
    context = this;

    Button startButton = (Button) findViewById(R.id.startbutton);
    Button stopButton = (Button) findViewById(R.id.stopbutton);

    Log.d("NULL POINTER", "NULL POINTER");
    startButton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            Log.d("NULL POINTER", "NULL POINTER");
            //setContentView(R.layout.stop);
            startService();

        }
    });

70      stopButton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {

            //setContentView(R.layout.results);
            stopService();

        }
    });

    latituteField = (TextView) findViewById(R.id.lat);
    longitudeField = (TextView) findViewById(R.id.lon);     
    kmphSpeedField = (TextView) findViewById(R.id.kmph);
    avgKmphField = (TextView) findViewById(R.id.avgkmph);
    topKmphField = (TextView) findViewById(R.id.topkmph);
    accText = (TextView) findViewById(R.id.acctext);

}

void setGPS(){
    powerManager = (PowerManager)getSystemService(Context.POWER_SERVICE);
    wL = powerManager.newWakeLock(PowerManager.SCREEN_BRIGHT_WAKE_LOCK,"My Tag");

}

static String roundTwoDecimalFloat(float a){

    String formattedNum;
    NumberFormat nf = new DecimalFormat();
    nf.setMaximumFractionDigits(2);
    nf.setMinimumFractionDigits(2);
    formattedNum = nf.format(a);
    return formattedNum;
}

void startService(){

    wL.acquire();
    Log.d("DEBUG", "start");
    startService(new Intent(this, AccelerometerReader.class));
    Toast.makeText(context, "Accessing Accelerometer", Toast.LENGTH_LONG).show();
    //startService(new Intent(this, Calculations.class));
    //Toast.makeText(context, "Acquiring GPS Locations", Toast.LENGTH_LONG).show();

}

void stopService(){

    wL.release();
    Calculations.locationManager.removeUpdates(Calculations.locationListener);
    stopService(new Intent(this, AccelerometerReader.class));
    Toast.makeText(context, "Terminating Accelerometer", Toast.LENGTH_LONG).show();
    //stopService(new Intent(this, Calculations.class));
    //Toast.makeText(context, "Terminating Connection", Toast.LENGTH_LONG).show();

}

static void update(){

    latituteField.setText("Current Latitude: "+String.valueOf(lat));
    longitudeField.setText("Current Longitude: "+String.valueOf(lon));
    kmphSpeedField.setText("Cuttent Speed (kmph): "+String.valueOf(kmphSpeed));
    avgKmphField.setText("Average Speed (kmph): "+String.valueOf(avgKmph));
    topKmphField.setText("Top Speed (kmph): "+String.valueOf(topKmph));
    accText.setText("Accelerometer Values: "+" x: " + a + " y: " + b + " z: " + c);

}

}

Answer 1

您断言这两个 Log 语句证明错误发生在它们之间是错误的。 onClick() 代码将在稍后执行。

您的错误最可能的原因是 startButton 或 stopButton 为空，并且您的一个或两个 findViewById 查找是失败。尝试将它们转储到日志语句中，或在调试器下运行它并检查它们以查看哪个为空。

编辑:看到您的 XML 文件，问题现在很清楚了。您正在膨胀 start.xml，其中包含 R.id.startbutton 的定义。但它不包含 R.id.stopbutton 的定义。因此，您的 Button stopButton = (Button) findViewById(R.id.stopbutton); 将 stopButton 设置为 null，因为它在您的布局中不存在。然后，您尝试向这个空对象添加一个 OnClickListener，这会导致您的 NPE。