访问活动中的片段时，空指针exepction

Question

我正在尝试访问我的片段，因此我可以调用其中的方法。请提出任何建议吗？

这就是我在mainActivity中的onCreate中所做的：

mCurrent_forecast_fragment = (Current_forecast_fragment) getSupportFragmentManager().findFragmentById(R.id.current_fragment);

这是我的片段布局：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:id="@+id/current_fragment"
            android:background="@drawable/bg_gradient">

<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/iconImageView"
    android:src="@mipmap/clear_night"
    android:adjustViewBounds="true"
    android:scaleType="fitXY"
    android:contentDescription="@string/weatherIconDesc"
    android:layout_above="@+id/locationLabel"
    android:layout_marginBottom="20dp"
    android:layout_centerHorizontal="true"/>

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="requesting current location..."
    android:id="@+id/locationLabel"
    android:textColor="@android:color/white"
    android:textSize="22sp"
    android:layout_above="@+id/timeLabel"
    android:layout_centerHorizontal="true"
    android:layout_marginBottom="51dp"/>

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="--"
    android:id="@+id/temperatureLabel"
    android:layout_centerVertical="true"
    android:layout_centerHorizontal="true"
    android:textColor="@android:color/white"
    android:textSize="125sp"/>

<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/degreeImageView"
    android:layout_alignTop="@+id/temperatureLabel"
    android:layout_toRightOf="@+id/temperatureLabel"
    android:layout_toEndOf="@+id/temperatureLabel"
    android:src="@mipmap/degree"
    android:layout_marginTop="27dp"
    android:layout_marginLeft="10dp"/>

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="..."
    android:id="@+id/timeLabel"
    android:textStyle="bold"
    android:layout_above="@+id/degreeImageView"
    android:layout_centerHorizontal="true"
    android:textColor="#80ffffff"/>

<LinearLayout
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_below="@+id/temperatureLabel"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="10dp"
    android:id="@+id/linearLayout">

    <LinearLayout
        android:orientation="vertical"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_weight="1">

        <TextView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="HUMIDITY"
            android:id="@+id/humidityLabel"
            android:textColor="#80ffffff"
            android:gravity="center_horizontal"
            android:textIsSelectable="true"/>

        <TextView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="--"
            android:id="@+id/humidityValue"
            android:textColor="@android:color/white"
            android:textSize="24sp"
            android:gravity="center_horizontal"/>
    </LinearLayout>

    <LinearLayout
        android:orientation="vertical"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        >

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="WIND SPEED"
            android:id="@+id/windSpeedLabel"
            android:textColor="#80ffffff"
            android:gravity="center_horizontal"
            android:textIsSelectable="true"/>

        <LinearLayout
            android:orientation="horizontal"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content">

            <TextView
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:text="--"
                android:id="@+id/windSpeedValue"
                android:textSize="24sp"
                android:textColor="@android:color/white"
                android:gravity="center_horizontal"/>

            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:textAppearance="?android:attr/textAppearanceSmall"
                android:text="KM/h"
                android:id="@+id/kmh"
                android:textColor="#80ffffff"
                />
        </LinearLayout>

    </LinearLayout>

    <LinearLayout
        android:orientation="vertical"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_weight="1">

        <TextView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="RAIN?"
            android:id="@+id/precipLabel"
            android:textColor="#80ffffff"
            android:gravity="center_horizontal"/>

        <TextView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="--"
            android:id="@+id/precipValue"
            android:textColor="@android:color/white"
            android:textSize="24sp"
            android:gravity="center_horizontal"/>
    </LinearLayout>

</LinearLayout>

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Getting current weather..."
    android:id="@+id/summaryLabel"
    android:textColor="@android:color/white"
    android:textSize="18sp"
    android:layout_marginTop="15dp"
    android:gravity="fill_horizontal"
    android:layout_below="@+id/linearLayout"
    android:layout_centerHorizontal="true"/>


<RelativeLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_alignParentBottom="true"
    android:layout_marginBottom= "10dp"
    >

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/refreshImageView"
        android:src="@mipmap/refresh"
        android:scaleType="fitXY"
        android:adjustViewBounds="true"
        android:layout_alignLeft="@+id/progressBar"
        android:layout_alignStart="@+id/progressBar"
        android:layout_alignTop="@+id/progressBar"
        android:layout_alignBottom="@+id/progressBar"
        android:layout_alignRight="@+id/progressBar"/>

    <ProgressBar
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/progressBar"
        android:layout_centerHorizontal="true"/>

</RelativeLayout>

这是我的主要活动：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
          xmlns:tools="http://schemas.android.com/tools"
          android:layout_width="match_parent"
          android:layout_height="match_parent"
          android:orientation="vertical"
          tools:context=".MainActivity">

<koemdzhiev.com.stormy.SlidingTabLayout
    android:id="@+id/tabs"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@color/ColorPrimary"/>

<android.support.v4.view.ViewPager
    android:id="@+id/pager"
    android:layout_height="0dp"
    android:layout_width="match_parent"
    android:layout_weight="1">

</android.support.v4.view.ViewPager>

这是我的片段类：

public class Current_forecast_fragment extends Fragment {
private MainActivity mActivity;
@InjectView(R.id.timeLabel)
TextView mTimeLabel;
@InjectView(R.id.temperatureLabel)
TextView mTemperatureLabel;
@InjectView(R.id.humidityValue)
TextView mHumidityValue;
@InjectView(R.id.precipValue)
TextView mPrecipValue;
@InjectView(R.id.summaryLabel)
TextView mSummaryLabel;
@InjectView(R.id.locationLabel)
TextView mLocationLabel;
@InjectView(R.id.windSpeedValue)
TextView mWindSpeedValue;
@InjectView(R.id.iconImageView)
ImageView mIconImageView;
@InjectView(R.id.refreshImageView)
ImageView mRefreshImaveView;
@InjectView(R.id.progressBar)
ProgressBar mProgressBar;
@InjectView(R.id.degreeImageView)
ImageView mDegreeImageView;

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    mActivity = ((MainActivity) getActivity());
}

@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.current_forefast_fragment, container, false);
    ButterKnife.inject(this, v);


    return v;
}


public void toggleRefresh() {
    if (mProgressBar.getVisibility() == View.INVISIBLE) {
        mProgressBar.setVisibility(View.VISIBLE);
        mRefreshImaveView.setVisibility(View.INVISIBLE);
    } else {
        mProgressBar.setVisibility(View.INVISIBLE);
        mRefreshImaveView.setVisibility(View.VISIBLE);
    }

    // updates the dysplay with the data in the CUrrentWeather locaal object

}

public void updateDisplay() {
    Current current = mActivity.mForecast.getCurrent();
    //setting the current weather details to the ui
    mTemperatureLabel.setText(current.getTemperature() + "");
    mTimeLabel.setText("At " + current.getFormattedTime() + " it will be");
    mHumidityValue.setText(current.getHumidity() + "%");
    mPrecipValue.setText(current.getPrecipChange() + "%");
    mSummaryLabel.setText(current.getSummery());
    mWindSpeedValue.setText(current.getWindSpeed() + "");
    mLocationLabel.setText(current.getTimeZone());
    mActivity.getLocationName();
    Drawable drawable = ContextCompat.getDrawable(mActivity, current.getIconId());
    mIconImageView.setImageDrawable(drawable);

}

}

这是它崩溃的地方：

public class MainActivity extends AppCompatActivity {
ViewPager pager;
ViewPagerAdapter adapter;
SlidingTabLayout tabs;
CharSequence Titles[]={"Current","Hourly","Daily"};
int Numboftabs =3;
Current_forecast_fragment mCurrent_forecast_fragment;

public static final String TAG = MainActivity.class.getSimpleName();
public static final String LOCATION_KEY = "location_key";
public Forecast mForecast;
public static final String DAILY_FORECAST = "DAILY_FORECAST";
public static final String HOURLY_FORECAST = "HOURLY_FORECAST";
//default coordinates - Gotse Delchev, UK Lati:57.156866 ; Long:
private double latitude = 41.5667;
private double longitude = 23.7333;
private LocationManager locationManager;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    //-----------MY CODE STARTS HERE-----------------

    mCurrent_forecast_fragment = (Current_forecast_fragment) getSupportFragmentManager().findFragmentById(R.id.current_fragment);
    Log.d("MainActivity",mCurrent_forecast_fragment.getTag() + "Georgi");
}

Answer 1

current_fragment是你的片段RelativeLayout的id，而不是你片段的id

CREATE FUNCTION "public"."log_test_trigger" () RETURNS "trigger" AS 'BEGIN
...
   IF (TG_OP = ''UPDATE'') THEN
            IF (NEW.name <> OLD.name) THEN
                   INSERT INTO log_test (type, table, field, old_val, new_val, user, event) VALUES (NEW.type,  TG_RELNAME, ''name'', OLD.name, NEW.name, NEW.user, ''UPDATE'');
            END IF;
...
END;' LANGUAGE "plpgsql"
VOLATILE
CALLED ON NULL INPUT
SECURITY INVOKER

将返回null。如果你想通过id找到片段，请将它放在你的活动布局中，如下所示：

mCurrent_forecast_fragment = (Current_forecast_fragment) getSupportFragmentManager().findFragmentById(R.id.current_fragment);

希望这有帮助！

Answer 2

看起来您当前的xml代码中没有片段。 您需要在RelativeLayout中添加一个片段。 您可以在Android Developer的网站上看到完整的教程：http://developer.android.com/training/basics/fragments/creating.html

Answer 3

我将尝试改进ResolutioN帖子。问题可能是xml内容，这应该很容易解决。 样本布局：

<fragment
   android:name="Current_forecast_fragment"
   android:id="@+id/current_fragment"
   android:layout_width="wrap_content"
   android:layout_height="wrap_content" 
/>

注意：您需要插入android:name，以便将其类型转换为对象。

编辑： 在MainActivity中，它需要导入正确的类。 示例代码：

import package.Current_forecast_fragment;

注意：您需要通过指定正确的路径来引用片段类。

同样，这不应该太困难。玩得开心，祝你好运，不要放弃！