findViewbyId 返回 null - 但不是当放入按钮 onClick 侦听器时

Question

我想访问TextView 来交换它的标签。 但是，findViewById 似乎在代码位于 onCreate()、onResume() 或 onStart() 中时返回 null。只有当

//if(Day != null) 

未被//注释。

将代码 sn-p 放入 onClickListener 或 Button 时可以正常工作，而不是 NullPointerException。

我确实已经阅读了很多解决方案（清理项目，将代码移动到 onCreateView 等），但没有一个对我有用。 所以我真的希望得到你的帮助：P

我的活动是

package edu.uahwi.smac;

import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBarActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.view.Window;
import android.widget.TextView;

public class HoursActivity extends ActionBarActivity {


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

    //hides the title bar
    requestWindowFeature(Window.FEATURE_NO_TITLE);

    setContentView(R.layout.activity_hours);

    final PlaceholderFragment fragment = new PlaceholderFragment();
    Bundle args = new Bundle();
    long ChosenDate = (long) getIntent().getExtras().getLong("DateLong");
    args.putLong("DateLong", ChosenDate);
    fragment.setArguments(args);
    getSupportFragmentManager().beginTransaction().add(R.id.container, fragment).commit();

    /*if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }
    */
    /*
    TextView Day1 = (TextView) findViewById(R.id.textViewDay);
    //if(Day != null)
        Day1.setText(String.valueOf(ChosenDate));*/

}

public void onStart()
{

}

public void Hour2onClick(View v)
{
    startActivity( new Intent("edu.uahwi.smac.BookingConfirmedActivity"));
}

public void buttonClick(View v)
{
    //startActivity( new Intent("edu.uahwi.smac.BookingConfirmedActivity"));


}


@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.hours, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}


/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {

    public PlaceholderFragment() {
    }

    private long chosenDate; // = 10l; 
                             //= (long) getIntent().getExtras().getLong("DateLong");

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_hours, container, false);

        TextView Day1 = (TextView) rootView.findViewById(R.id.textViewDay);
        Day1.setText(String.valueOf(chosenDate));

        return rootView;
    }

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

        Bundle args = getArguments();
        if (args != null)
            chosenDate = args.getLong("DateLong");
    }
}

}

XML-Code的开始：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/LinearLayout1"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="edu.uahwi.smac.HoursActivity$PlaceholderFragment" >

<TextView
    android:id="@+id/textViewDay1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center"
    android:gravity="center"
    android:paddingBottom="15dp"
    android:text="@string/ChosenDay"
    android:textSize="25sp" />

<ScrollView
    android:id="@+id/scrollView1"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content" >

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="208dp"
        android:orientation="vertical" >

最后是我的Stack-Trace：

05-19 23:12:01.649: W/dalvikvm(1596): threadid=1: thread exiting with uncaught 

exception (group=0xb2addba8)
05-19 23:12:01.669: E/AndroidRuntime(1596): FATAL EXCEPTION: main
05-19 23:12:01.669: E/AndroidRuntime(1596): Process: edu.uahwi.smac, PID: 1596
05-19 23:12:01.669: E/AndroidRuntime(1596): java.lang.RuntimeException: Unable to start activity ComponentInfo{edu.uahwi.smac/edu.uahwi.smac.HoursActivity}: java.lang.NullPointerException
05-19 23:12:01.669: E/AndroidRuntime(1596):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
05-19 23:12:01.669: E/AndroidRuntime(1596):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-19 23:12:01.669: E/AndroidRuntime(1596):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-19 23:12:01.669: E/AndroidRuntime(1596):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-19 23:12:01.669: E/AndroidRuntime(1596):     at android.os.Handler.dispatchMessage(Handler.java:102)
05-19 23:12:01.669: E/AndroidRuntime(1596):     at android.os.Looper.loop(Looper.java:136)
05-19 23:12:01.669: E/AndroidRuntime(1596):     at android.app.ActivityThread.main(ActivityThread.java:5017)
05-19 23:12:01.669: E/AndroidRuntime(1596):     at java.lang.reflect.Method.invokeNative(Native Method)
05-19 23:12:01.669: E/AndroidRuntime(1596):     at java.lang.reflect.Method.invoke(Method.java:515)
05-19 23:12:01.669: E/AndroidRuntime(1596):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-19 23:12:01.669: E/AndroidRuntime(1596):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-19 23:12:01.669: E/AndroidRuntime(1596):     at dalvik.system.NativeStart.main(Native Method)
05-19 23:12:01.669: E/AndroidRuntime(1596): Caused by: java.lang.NullPointerException
05-19 23:12:01.669: E/AndroidRuntime(1596):     at edu.uahwi.smac.HoursActivity.onCreate(HoursActivity.java:36)
05-19 23:12:01.669: E/AndroidRuntime(1596):     at android.app.Activity.performCreate(Activity.java:5231)
05-19 23:12:01.669: E/AndroidRuntime(1596):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-19 23:12:01.669: E/AndroidRuntime(1596):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
05-19 23:12:01.669: E/AndroidRuntime(1596):     ... 11 more

Answer 1

那个 TextView 在片段中。当您确定片段的视图存在时，您需要将设置文本值的代码移动到片段的 onCreateView()。像这样：

private long chosenDate;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
    View rootView = inflater.inflate(R.layout.your_fragment_layout, container, false);

    TextView Day1 = (TextView) findViewById(R.id.textViewDay1);
    Day1.setText(String.valueOf(chosenDate));

您可以将 selectedDate 传递给片段参数，以便片段可以使用它。从 Activity 开始这样的片段：

final PlaceholderFragment fragment = new PlaceholderFragment();
Bundle args = new Bundle();
long ChosenDate = (long) getIntent().getExtras().getLong("DateLong");
args.putLong("DateLong", ChosenDate);
fragment.setArguments(args);
fm.beginTransaction().add(R.id.container, fragment).commit();

然后在片段的 onCreate 中，您可以提取参数并将本地私有 long 设置为该值，以便稍后在 onViewCreate 调用中可用：

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

    Bundle args = getArguments();
    if (args != null)
        chosenDate = args.getLong("DateLong");