NullPointerException当调用findViewById方法从主活动在应用中使用ViewPager

Question

我尝试使用ViewPager编写应用程序，我想在我的应用程序中添加一个监听器到TextView。在片段代码中添加监听器很简单，但我如何在MainActivity中创建它而不获得NullPointerException ?

MainActivity:

import android.content.Context;
import android.support.v4.app.Fragment;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.view.ViewGroup;
import android.widget.HorizontalScrollView;
import android.widget.TabHost;
import android.widget.TextView;
import java.util.ArrayList;
import java.util.List;
public class MainActivity extends AppCompatActivity implements ViewPager.OnPageChangeListener, TabHost.OnTabChangeListener{
ViewPager viewPager;
TabHost tabHost;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    initTabHost();
    initViewPager();


    final TextView textView = (TextView) findViewById(R.id.txt_frag1);
    textView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            textView.setText("Changed text");
        }
    });
}


private void initTabHost() {
    tabHost = (TabHost) findViewById(R.id.tabHost);
    tabHost.setup();
    String[] tabNames = {"FRAG1","FRAG2"};
    for(int i=0; i<tabNames.length;i++){
        TabHost.TabSpec tabSpec;
        tabSpec = tabHost.newTabSpec(tabNames[i]);
        tabSpec.setIndicator(tabNames[i]);
        tabSpec.setContent(new FakeContent(getApplicationContext()));
        tabHost.addTab(tabSpec);
    }
    tabHost.setOnTabChangedListener(this);
}
public class FakeContent implements TabHost.TabContentFactory{
    Context context;
    public FakeContent(Context mcontext){
        context=mcontext;
    }
    @Override
    public View createTabContent(String tag) {
        View fakeView = new View(context);
        fakeView.setMinimumHeight(0);
        fakeView.setMinimumWidth(0);
        return fakeView;
    }
}
private void initViewPager() {
    viewPager = (ViewPager) findViewById(R.id.view_pager);
    List<Fragment> listFragments = new ArrayList<Fragment>();
    listFragments.add(new BlankFragment());
    listFragments.add(new BlankFragment2());

    MyFragmentAdapter myFragmentAdapter = new MyFragmentAdapter(getSupportFragmentManager(),listFragments);
    viewPager.setAdapter(myFragmentAdapter);
    viewPager.setOnPageChangeListener(this);
}

@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
}
@Override
public void onPageSelected(int position) {
}
@Override
public void onPageScrollStateChanged(int selectedItem) {
    tabHost.setCurrentTab(selectedItem);
}
@Override
public void onTabChanged(String tabId) {
    int selectedItem = tabHost.getCurrentTab();
    viewPager.setCurrentItem(selectedItem);
    //TabHost ustawianie na środku
    HorizontalScrollView hScrollView = (HorizontalScrollView) findViewById(R.id.h_scroll_view);
    View tabView = tabHost.getCurrentTabView();
    int scrollPos = tabView.getLeft() -(hScrollView.getWidth() -tabView.getWidth())/2;
    hScrollView.smoothScrollTo(scrollPos,0);
}
}

MyFragmentAdapter:

public class MyFragmentAdapter extends FragmentPagerAdapter {
List<Fragment> listFragments;
public MyFragmentAdapter(FragmentManager fm, List<android.support.v4.app.Fragment> listFragments) {
    super(fm);
    this.listFragments = listFragments;
}
@Override
public android.support.v4.app.Fragment getItem(int position) {
    return listFragments.get(position);
}
@Override
public int getCount() {
    return listFragments.size();
}

}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/activity_smart_led"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context="com.example.pietr.viewpagerr.MainActivity">

<TabHost
    android:id="@+id/tabHost"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">
        <HorizontalScrollView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:id="@+id/h_scroll_view"
            android:fillViewport="true"
            android:scrollbars="none">
            <TabWidget
                android:id="@android:id/tabs"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"></TabWidget>
        </HorizontalScrollView>
        <FrameLayout
            android:id="@android:id/tabcontent"
            android:layout_width="match_parent"
            android:layout_height="match_parent">
            <android.support.v4.view.ViewPager
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:id="@+id/view_pager"
                ></android.support.v4.view.ViewPager>
        </FrameLayout>
    </LinearLayout>
</TabHost>
</LinearLayout>

fragment是带有一个TextView的标准空白fragment。

在您的activity_main布局中没有id txt_frag1的视图。正因为如此，你得到了一个NPE。首先在activity_main xml中添加一个相同的textview，然后在java中访问它。

我有一个TextView的id txt_frag1定义在fragment_blank.xml。我想在寻呼机中显示这个片段，并从MainActivity访问它。