Question

好的，该应用应该做什么：

显示一个开始按钮，
按下两个服务开始运行，开始按钮（视图）变为停止按钮
当按下停止按钮时，代码应该再次改变视图以显示所有结果

目前正在发生的事情是代码一直在运行，直到最后一个方法调用（update（）;在代码中标记）并且logcat在第146行报告了一个nullPointerException，我已经标记了在代码中。

需要注意！

如果我删除“update（）;”方法调用代码执行完美，并在xml本身中显示给予文本视图的值。它只是当我尝试更新它崩溃的textviews时。我已经尝试将它们放在文件中的其他位置，但我仍然得到相同的错误

昨天我遇到了一个非常类似的问题，但我尝试做同样的事情来解决这个问题但是没有用。我发布了它正在访问的xml布局文件。我昨天遇到的错误是布局文件没有其中一个按钮的定义（我现在删除了）。

这是我的主要课程。

    public class GPSMain extends Activity   {

    protected PowerManager powerManager;
    protected PowerManager.WakeLock wL;

    //text views to display latitude, longitude, speed and G force values
    static TextView latituteField, longitudeField, kmphSpeedField, avgKmphField, topKmphField, accText, totalDistance;

    //objects to store information for  GPS locations and current, top, total and average speed
    protected static double lat = 0, lon = 0, kmphSpeed = 0, avgKmph = 0, totalKmph = 0, topKmph = 0;

    protected static float distanceBetweenPoints = 0.0f;

    protected static Location previousLocation;

    //accelerometer values
    static String a = "x", b = "y", c = "z";

    //context for Toast views
    private Context context;

    //The stop and start button
    static Button button;

    //switch to alternate between stop and start features
    private int buttonSwitch = 2;

    static int counter = 0;


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

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

    context = this;

    //gives the button the start value
    button = (Button) findViewById(R.id.startbutton);

    //calls the method to the initial button listener
    switchButton();

    //sets up the text views to display results
    latituteField = (TextView) findViewById(R.id.lat);
    longitudeField = (TextView) findViewById(R.id.lon); 
    totalDistance = (TextView) findViewById(R.id.totaldistance);
    kmphSpeedField = (TextView) findViewById(R.id.kmph);
    avgKmphField = (TextView) findViewById(R.id.avgkmph);
    topKmphField = (TextView) findViewById(R.id.topkmph);
    accText = (TextView) findViewById(R.id.acctext);




    }

    //defines the button functions
    void switchButton(){

    button.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {

            if(buttonSwitch%2==0){

                buttonSwitch++;
                startService();
            }

            else

            {

                buttonSwitch++;
                stopService();
                update();            //update method call

            }

        }

    });

    }

    //rounds the float values from the accelerometer
    static String roundTwoDecimalFloat(float a){

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

    //starts the 2 services and changes the button from a start button to a stop button
    void startService(){

    setContentView(R.layout.stop);
    GPSMain.button = (Button) findViewById(R.id.stopbutton);
    switchButton();

    Toast.makeText(context, "Accessing Accelerometer", Toast.LENGTH_LONG).show();
    startService(new Intent(this, AccelerometerReader.class));

    Toast.makeText(context, "Acquiring GPS Locations", Toast.LENGTH_LONG).show();
    startService(new Intent(this, Calculations.class));

    }

    //stops the two services
    void stopService(){

    setContentView(R.layout.results);

    Toast.makeText(context, "Terminating Accelerometer", Toast.LENGTH_LONG).show();
    AccelerometerReader.myManager.unregisterListener(AccelerometerReader.mySensorListener);
    stopService(new Intent(this, AccelerometerReader.class));

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

    }

    //prints the results to the screen
    static void update(){

146 latituteField.setText("Current Latitude: "+String.valueOf(lat));
    longitudeField.setText("Current Longitude: "+String.valueOf(lon));
    totalDistance.setText("Total Distance: "+String.valueOf(distanceBetweenPoints/1000));
    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);

    }

    }

这是xml：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:id="@+id/linearlayout1"
 android:orientation="vertical"
 android:layout_width="fill_parent"
 android:layout_height="fill_parent"
 >
<TextView  
    android:id="@+id/hello"
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="@string/hello"
    />
<TextView  
    android:id="@+id/lat"
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="Current Latitude:     unknown"
    />
<TextView  
    android:id="@+id/lon"
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="Current Longitude:        unknown"
    />
<TextView
    android:id="@+id/totaldistance"  
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="Total Distance (km):      unknown"
    />
<TextView
    android:id="@+id/kmph"  
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="Current Speed (kmph):     unknown"
    />
<TextView
    android:id="@+id/avgkmph"  
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="Average Speed (kmph):     unknown"
    />
<TextView
    android:id="@+id/topkmph"  
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="Top Speed (kmph):     unknown"
    />   
<TextView
    android:id="@+id/acctext"  
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="Accelerometer Values:     unknown"
    />
</LinearLayout>

Answer 1

问题是你在启动TextView之前调用switchButton()方法即latituteField = (TextView) findViewById(R.id.lat);，因为你有空指针异常。所以首先在调用switchButton()之后编写所有视图元素的初始化。我修改了你的代码。请尝试使用以下代码

对onCreate方法

使用以下代码

@Override
    public void onCreate(Bundle savedInstanceState) {

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

        context = this;

        // gives the button the start value
        button = (Button) findViewById(R.id.startbutton);

        // calls the method to the initial button listener


        // sets up the text views to display results
        latituteField = (TextView) findViewById(R.id.lat);
        longitudeField = (TextView) findViewById(R.id.lon);
        totalDistance = (TextView) findViewById(R.id.totaldistance);
        kmphSpeedField = (TextView) findViewById(R.id.kmph);
        avgKmphField = (TextView) findViewById(R.id.avgkmph);
        topKmphField = (TextView) findViewById(R.id.topkmph);
        accText = (TextView) findViewById(R.id.acctext);

        switchButton();

    }

Answer 2

进入更新方法尝试使用txt=(TextView)findviewbyId(R.id.yourtextid);

Answer 3

当你调用setContentView（）时，对先前建立的任何视图的引用都不再有效。

在onCreate（）中你setContentView（R.layout.results）然后你建立latitudeField（和其他引用），假设它们没有返回null，就可以了。（你没有提到你为xml提供的布局）。

接下来安装按钮侦听器，它调用startService（）或stopService（），每个都设置一个新的内容视图，这两个视图都不会更新View引用;所有先辈在这一点上都是无效的。然后调用update（），它尝试使用无效的引用。

更改视图时更新引用，或者在需要时获取它们。

尝试更新文本视图时出现空指针异常

3 个答案:

对onCreate方法