我是Android新手，这是我的第一个项目，我花了一段时间试图解决这个问题并且已经达到了一个死胡同。我想要使用ViewPager打开一个片段，然后从它打开的Activity中访问该片段。我尝试在Activity中创建片段而不是ViewPager但是会引发不同的错误。
public class TorchActivity extends SherlockFragmentActivity {
private static TorchFragment torchFrag;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
ViewPager pager = (ViewPager) findViewById(R.id.pager);
pager.setAdapter(new ViewPagerAdapter(getSupportFragmentManager()));
}
public static class ViewPagerAdapter extends FragmentPagerAdapter {
public ViewPagerAdapter(FragmentManager mgr) {
super(mgr);
}
@Override
public int getCount() {
return (2);
}
@Override
public Fragment getItem(int position) {
if(position == 0){
torchFrag = new TorchFragment();
return torchFrag;
}else{
ColourPickerFragment colourPickerFrag = new ColourPickerFragment();
return colourPickerFrag;
}
}
}
}
main.xml中：
<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/pager"
android:layout_width="match_parent"
android:layout_height="match_parent" >
</android.support.v4.view.ViewPager>
当我稍后在同一个Activity中访问该片段时，我得到一个NullPointerException。
@Override
public void lightSensorCheck(boolean isThereALightSensor) {
torchFrag.setIsThereALightSensor(isThereALightSensor);
}
在片段中：
public void setIsThereALightSensor(boolean isThereALightSensor) {
this.isThereALightSensor = isThereALightSensor;
}
如果我将ViewPager代码输出并运行它，它可以正常工作，任何帮助都会受到赞赏。
02-17 16:01:30.132: E/AndroidRuntime(32006): FATAL EXCEPTION: main
02-17 16:01:30.132: E/AndroidRuntime(32006): java.lang.RuntimeException: Unable to start activity ComponentInfo{uk.co.bimrose.android.survivaltorch/uk.co.bimrose.android.survivaltorch.TorchActivity}: java.lang.NullPointerException
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2517)
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2574)
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.app.ActivityThread.access$600(ActivityThread.java:162)
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1413)
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.os.Handler.dispatchMessage(Handler.java:99)
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.os.Looper.loop(Looper.java:158)
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.app.ActivityThread.main(ActivityThread.java:5789)
02-17 16:01:30.132: E/AndroidRuntime(32006): at java.lang.reflect.Method.invokeNative(Native Method)
02-17 16:01:30.132: E/AndroidRuntime(32006): at java.lang.reflect.Method.invoke(Method.java:525)
02-17 16:01:30.132: E/AndroidRuntime(32006): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1027)
02-17 16:01:30.132: E/AndroidRuntime(32006): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:843)
02-17 16:01:30.132: E/AndroidRuntime(32006): at dalvik.system.NativeStart.main(Native Method)
02-17 16:01:30.132: E/AndroidRuntime(32006): Caused by: java.lang.NullPointerException
02-17 16:01:30.132: E/AndroidRuntime(32006): at uk.co.bimrose.android.survivaltorch.TorchActivity.lightSensorCheck(TorchActivity.java:199)
02-17 16:01:30.132: E/AndroidRuntime(32006): at uk.co.bimrose.android.survivaltorch.LightFragment.updateLightSensorStatus(LightFragment.java:180)
02-17 16:01:30.132: E/AndroidRuntime(32006): at uk.co.bimrose.android.survivaltorch.LightFragment.sensorCheck(LightFragment.java:134)
02-17 16:01:30.132: E/AndroidRuntime(32006): at uk.co.bimrose.android.survivaltorch.LightFragment.onAttach(LightFragment.java:65)
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:883)
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104)
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1460)
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:556)
02-17 16:01:30.132: E/AndroidRuntime(32006): at uk.co.bimrose.android.survivaltorch.TorchActivity.onStart(TorchActivity.java:109)
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1239)
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.app.Activity.performStart(Activity.java:5205)
02-17 16:01:30.132: E/AndroidRuntime(32006): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2490)
02-17 16:01:30.132: E/AndroidRuntime(32006): ... 11 more
以下是片段的代码：
public class TorchFragment extends SherlockFragment implements OnClickListener {
private Button buttonOn;
private Button buttonSos;
private Button buttonOff;
private TextView luxStatic;
private TextView luxMessage;
private TextView timerStatic;
private TextView timerMessage;
private TextView batteryStatic;
private TextView batteryMessage;
private TextView shutdownMessage;
private TextView shutdownMessageStatic;
private TextView timesUsed;
private ImageView img;
private TextView heading;
private SharedPreferences prefs;
private int lightSensitivity;
private boolean timerOn;
private int batteryPct;
private boolean isThereALightSensor;
public static int serviceCount;
private AlertResetListener alertResetListener;
private ServiceListener sListener;
private SetDataListener sDListener;
private ClearShutdownMessageListener cSMListener;
private ColourSchemeListener cSListener;
private int[] colourScheme = new int[4];
private LinearLayout dataCompassLayout;
int x = 0;
@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
// Makes sure that the container activity has implemented the callback interface
try {
alertResetListener = (AlertResetListener) activity;
} catch (ClassCastException e) {
throw new ClassCastException(activity.toString() + " must implement AlertReset");
}
// start / stop service
try {
sListener = (ServiceListener) activity;
} catch (ClassCastException e) {
throw new ClassCastException(activity.toString() + " must implement ServiceListener");
}
// Clear the dataFragment
try {
sDListener = (SetDataListener) activity;
} catch (ClassCastException e) {
throw new ClassCastException(activity.toString() + " must implement SetDataListener");
}
// Clear the dataFragment shutdown message
try {
cSMListener = (ClearShutdownMessageListener) activity;
} catch (ClassCastException e) {
throw new ClassCastException(activity.toString() + " must implement ClearShutdownMessageListener");
}
// Get the colourscheme
try {
cSListener = (ColourSchemeListener) activity;
} catch (ClassCastException e) {
throw new ClassCastException(activity.toString() + " must implement ColourSchemeListener");
}
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup parent, Bundle savedInstanceState) {
View result = inflater.inflate(R.layout.torchfrag, parent, false);
buttonOn = (Button) result.findViewById(R.id.button_on);
buttonSos = (Button) result.findViewById(R.id.button_sos);
buttonOff = (Button) result.findViewById(R.id.button_off);
buttonOn.setText("ON");
buttonOff.setText("OFF");
buttonSos.setText("SOS");
luxStatic = (TextView) result.findViewById(R.id.luxstatic);
luxMessage = (TextView) result.findViewById(R.id.luxmessage);
timerStatic = (TextView) result.findViewById(R.id.timerstatic);
timerMessage = (TextView) result.findViewById(R.id.timermessage);
batteryStatic = (TextView) result.findViewById(R.id.batterystatic);
batteryMessage = (TextView) result.findViewById(R.id.batterymessage);
shutdownMessage = (TextView) result.findViewById(R.id.shutdownmessage);
shutdownMessageStatic = (TextView) result.findViewById(R.id.shutdownmessagestatic);
timesUsed = (TextView) result.findViewById(R.id.timesused);
timesUsed.setText(Integer.toString(TorchActivity.timesUsed));
//Compass
img = (ImageView) result.findViewById(R.id.compass);
heading = (TextView) result.findViewById(R.id.heading);
dataCompassLayout = (LinearLayout) result.findViewById(R.id.datacompasslayout);
setColourScheme();
buttonOn.setOnClickListener(this);
buttonSos.setOnClickListener(this);
buttonOff.setOnClickListener(this);
return (result);
}
public void onResume() {
setColourScheme();
clearShutdownMessage();
getPreferences();
clearText();
setStaticMessage();
super.onResume();
}
@Override
public void onClick(View view) {
alertResetListener.alertReset();
TorchActivity.keepRunning = false;
cSMListener.clearShutdownMessage();
switch (view.getId()) {
case R.id.button_on:
serviceCount++;
sListener.startService("on");
break;
case R.id.button_sos:
serviceCount++;
sListener.startService("sosPreset");
break;
case R.id.button_off:
sListener.reset();
sDListener.setData();
break;
default:
throw new RuntimeException("Unknown button ID");
}
}
private void setColourScheme(){
//Calls the method from the activity which gets the colour scheme from the ColourSchemeFragment
colourScheme = cSListener.getColourScheme();
buttonOn.setBackgroundColor(getResources().getColor(colourScheme[0]));
buttonOff.setBackgroundColor(getResources().getColor(colourScheme[1]));
buttonSos.setBackgroundColor(getResources().getColor(colourScheme[2]));
dataCompassLayout.setBackgroundColor(getResources().getColor(colourScheme[3]));
}
public void setSosTextColour(String s){
int sosColour = 0;
if(s.equals("white")){
sosColour = R.color.button_text_sos;
}else{
sosColour = R.color.button_text_onoff;
}
try {
XmlResourceParser parser = getResources().getXml(sosColour);
ColorStateList colors = ColorStateList.createFromXml(getResources(), parser);
buttonSos.setTextColor(colors);
} catch (Exception e) {
// handle exceptions
}
}
// Called from TorchActivity whenever some new data is sent from one of the sensors
public void setMessage(int msg, String from) {
if (from.equals("light")) {
luxMessage.setText("Current LUX: " + Integer.toString(msg) + "\n");
} else if (from.equals("timer")) {
timerMessage.setText("Turns off in " + Integer.toString(msg) + " minutes\n");
} else if (from.equals("battery")) {
// Stops it updating battery % when the torch isn't on
if (TorchActivity.keepRunning) {
batteryMessage.setText("Current battery level: " + Integer.toString(msg) + "\n");
//toast("********");
}
}
}
public void setStaticMessage() {
if (lightSensitivity < 1000) {
luxStatic.setText("Turns off at " + Integer.toString(lightSensitivity) + " LUX");
}
if (timerOn) {
timerStatic.setText("Timer: ON");
} else {
timerStatic.setText("Timer: OFF");
}
if (batteryPct > 0) {
batteryStatic.setText("Turns off at " + Integer.toString(batteryPct) + "% battery");
}
shutdownMessageStatic.setText("Torch stopped by: ");
}
public void clearText() {
luxMessage.setText("\n");
luxStatic.setText("Light Monitor: OFF");
// only lets it clear the timer if the torch is off, && it's not the first time through. On the first time
// through it was getting cleared when it shouldn't have been.
if (!TorchActivity.keepRunning && !TorchActivity.autoOn) {
timerMessage.setText("\n");
}
timerStatic.setText("\nTimer: OFF");
batteryMessage.setText("\n");
batteryStatic.setText("Battery monitor: OFF");
}
public void updateCompass(String s, RotateAnimation rA){
img.startAnimation(rA);
heading.setText(s);
}
public void setImageWhite(){
img.setImageResource(R.drawable.compass_white);
}
public void setImageBlack(){
img.setImageResource(R.drawable.compass);
}
private void getPreferences() {
prefs = PreferenceManager.getDefaultSharedPreferences(getActivity());
lightSensitivity = Integer.valueOf(prefs.getString("lightsensitivity", "1000000"));
timerOn = prefs.getBoolean("timeron", false);
batteryPct = Integer.valueOf(prefs.getString("batterypct", "50"));
}
// From TorchActivity
public void setShutdownMessage(String s) {
shutdownMessage.setText("The " + s);
}
public void clearShutdownMessage() {
shutdownMessage.setText("");
}
public void setIsThereALightSensor(boolean isThereALightSensor) {
this.isThereALightSensor = isThereALightSensor;
}
public boolean getIsThereALightSensor() {
return isThereALightSensor;
}
// Container Activity must implement this interface
public interface AlertResetListener {
public void alertReset();
}
// Container Activity must implement this interface
public interface ServiceListener {
public void startService(String s);
// Resets the service and the timer
public void reset();
}
// Container Activity must implement this interface
public interface SetDataListener {
public void setData();
}
public interface ClearShutdownMessageListener {
public void clearShutdownMessage();
}
public interface ColourSchemeListener {
public int[] getColourScheme();
}
}
答案 0 :(得分：0)
在附加片段之前，您似乎正在调用setIsThereALightSensor。
您的评论已确认
引用
我通过启动fragments onAttach中的TorchFragment（通过托管活动）解决了这个问题，因此当调用方法时它已启动并运行