我是Java / android / stackoverflow的新手，所以请原谅任何愚蠢的错误。
这是我正在处理的教程，这里有一些来自stackoverflow的帖子，我用它作为参考。
我的代码中有很多其他东西，但是现在我在片段上设置文本时遇到了麻烦。
片段A（fragment_class）嵌入了一个TextView，我想以编程方式更改
public class fragment_class extends Fragment {
public static final String ARG_SECTION_NUMBER = "section_number";
private static TextView tv;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.fragment, container, false);
tv = (TextView)v.findViewById(R.id.temperature);
return v;
}
public void setText(String str){
tv.setText(str);
}
}
片段B（BT）具有将创建更改的OnClickListener
public class BT extends Fragment{
public static final String ARG_SECTION_NUMBER = "section_number";
public fragCommander commander;
public interface fragCommander{
void sText(String str);
}
@Override
public void onAttach(Context context) {
super.onAttach(context);
try {
commander = (fragCommander)context;
}catch (ClassCastException e){
throw new ClassCastException(context.toString());
}
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.fragment_1, container, false);
final Button fbt = (Button)v.findViewById(R.id.fbtbutton);
fbt.setOnClickListener(
new View.OnClickListener() {
public void onClick(View v) {
commander.sText("chicken");
}
}
);
return v;
}
}
MainActivity实现了一个接口
public class MainActivity extends FragmentActivity implements ActionBar.TabListener, BT.fragCommander {
Context context;
SectionsPagerAdapter mSectionsPagerAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
getWindow().requestFeature(Window.FEATURE_ACTION_BAR);
setContentView(R.layout.sample_main);
context = getApplicationContext();
final ActionBar actionBar = getActionBar();
if (actionBar != null) {
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
}
mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
mViewPager = (ViewPager) findViewById(R.id.pager);
mViewPager.setAdapter(mSectionsPagerAdapter);
mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
@Override
public void onPageSelected(int position) {
assert actionBar != null;
actionBar.setSelectedNavigationItem(position);
}
});
for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
assert actionBar != null;
actionBar.addTab(
actionBar.newTab()
.setText(mSectionsPagerAdapter.getPageTitle(i))
.setTabListener(this));
}
}
@Override
public void sText(String str) {
fragment_class fc = (fragment_class) getSupportFragmentManager().findFragmentById(R.id.frag_layout);
fc.setText(str);
}
@Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
// When the given tab is selected, tell the ViewPager to switch to the corresponding page.
mViewPager.setCurrentItem(tab.getPosition());
}
@Override
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}
@Override
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}
public class SectionsPagerAdapter extends FragmentPagerAdapter {
public SectionsPagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int position) {
switch (position) {
case 0:
BT btFrag = new BT();
Bundle args = new Bundle();
args.putInt(btFrag.ARG_SECTION_NUMBER, position + 1);
btFrag.setArguments(args);
return btFrag;
case 1:
Fragment fragment2 = new DummySectionFragment2();
Bundle args2 = new Bundle();
args2.putInt(DummySectionFragment2.ARG_SECTION_NUMBER, position + 2);
fragment2.setArguments(args2);
return fragment2;
case 2:
Fragment object = new fragment_class();
Bundle args3 = new Bundle();
args3.putInt(fragment_class.ARG_SECTION_NUMBER, position + 3);
object.setArguments(args3);
return object;
case 3:
Fragment fragment4 = new DummySectionFragment4();
Bundle args4 = new Bundle();
args4.putInt(DummySectionFragment4.ARG_SECTION_NUMBER, position + 4);
fragment4.setArguments(args4);
return fragment4;
default:
return null;
}
}
@Override
public int getCount() {
return 4;
}
@Override
public CharSequence getPageTitle(int position) {
Locale l = Locale.getDefault();
switch (position) {
case 0:
return getString(R.string.title_section1).toUpperCase(l);
case 1:
return getString(R.string.title_section2).toUpperCase(l);
case 2:
return getString(R.string.title_section3).toUpperCase(l);
case 3:
return getString(R.string.title_section4).toUpperCase(l);
}
return null;
}
}
错误消息
10-27 22:56:39.712 19163-19163/com.example.android.horizontalpaging E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.android.horizontalpaging, PID: 19163
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.android.horizontalpaging.fragment_class.setText(java.lang.String)' on a null object reference
at com.example.android.horizontalpaging.MainActivity.sText(MainActivity.java:97)
at com.example.android.horizontalpaging.BT$1.onClick(BT.java:43)
at android.view.View.performClick(View.java:4785)
at android.view.View$PerformClick.run(View.java:19869)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:155)
at android.app.ActivityThread.main(ActivityThread.java:5696)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1028)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)
添加XML
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="horizontal" android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
android:id="@+id/frag_layout">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="Large Text"
android:id="@+id/temperature"
android:layout_gravity="center" />
</LinearLayout>
好的，我发现了问题，我试图在不同的片段上设置文本而不是显示的片段。寻呼机正在摧毁我引用的片段。所以在OnCreate方法下我添加了这行mViewPager.setOffscreenPageLimit（4）;.
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
getWindow().requestFeature(Window.FEATURE_ACTION_BAR);
setContentView(R.layout.sample_main);
context = getApplicationContext();
final ActionBar actionBar = getActionBar();
if (actionBar != null) {
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
}
// Create the adapter that will return a fragment for each of the three primary sections
// of the app.
mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
mViewPager = (ViewPager) findViewById(R.id.pager);
mViewPager.setAdapter(mSectionsPagerAdapter);
// added line
mViewPager.setOffscreenPageLimit(4);
之后我在片段中创建了一个设置文本的方法，然后我可以在任何片段之后设置文本。
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
v = inflater.inflate(R.layout.fragment, container, false);
tv = (TextView)v.findViewById(R.id.temperature);
return v;
}
public void settext(String str){
if (tv != null) {
tv.setText(str);
}
}