我不完全确定为什么我会得到NullPointerException。如果有人能帮助我，那就太棒了。我在下面标注了第73行。
package com.shantanu.report;
import java.io.File;
import java.util.Locale;
import android.app.ActionBar;
import android.app.FragmentTransaction;
import android.content.ActivityNotFoundException;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.View;
import android.view.ViewGroup;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
import com.ipaulpro.afilechooser.utils.FileUtils;
public class MainActivity extends FragmentActivity implements
ActionBar.TabListener {
SectionsPagerAdapter mSectionsPagerAdapter;
ViewPager mViewPager;
Button b1, b2, b3, b4, b5;
private static final int REQUEST_CODE = 6384; // onActivityResult request code
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Set up the action bar.
final ActionBar actionBar = getActionBar();
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
b1 = (Button) findViewById(R.id.b1);
b2 = (Button) findViewById(R.id.b2);
b3 = (Button) findViewById(R.id.b3);
b4 = (Button) findViewById(R.id.b4);
b5 = (Button) findViewById(R.id.b5);
b5.setOnClickListener(new OnClickListener() { //line 73
public void onClick(View v) {
// Display the file chooser dialog
showChooser();
}
});
setContentView(b5);
mSectionsPagerAdapter = new SectionsPagerAdapter(
getSupportFragmentManager());
// Set up the ViewPager with the sections adapter.
mViewPager = (ViewPager) findViewById(R.id.pager);
mViewPager.setAdapter(mSectionsPagerAdapter);
// When swiping between different sections, select the corresponding
// tab. We can also use ActionBar.Tab#select() to do this if we have
// a reference to the Tab.
mViewPager
.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
@Override
public void onPageSelected(int position) {
actionBar.setSelectedNavigationItem(position);
}
});
// For each of the sections in the app, add a tab to the action bar.
for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
// Create a tab with text corresponding to the page title defined by
// the adapter. Also specify this Activity object, which implements
// the TabListener interface, as the callback (listener) for when
// this tab is selected.
actionBar.addTab(actionBar.newTab()
.setText(mSectionsPagerAdapter.getPageTitle(i))
.setTabListener(this));
}
}
private void showChooser() {
// Use the GET_CONTENT intent from the utility class
Intent target = FileUtils.createGetContentIntent();
// Create the chooser Intent
Intent intent = Intent.createChooser(
target, getString(R.string.chooser_title));
try {
startActivityForResult(intent, REQUEST_CODE);
} catch (ActivityNotFoundException e) {
// The reason for the existence of aFileChooser
}
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
switch (requestCode) {
case REQUEST_CODE:
// If the file selection was successful
if (resultCode == RESULT_OK) {
if (data != null) {
// Get the URI of the selected file
final Uri uri = data.getData();
try {
// Create a file instance from the URI
final File file = FileUtils.getFile(uri);
Toast.makeText(MainActivity.this,
"File Selected: "+file.getAbsolutePath(), Toast.LENGTH_LONG).show();
} catch (Exception e) {
Log.e("FileSelectorTestActivity", "File select error", e);
}
}
}
break;
}
super.onActivityResult(requestCode, resultCode, data);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public void onTabSelected(ActionBar.Tab tab,
FragmentTransaction fragmentTransaction) {
// When the given tab is selected, switch to the corresponding page in
// the ViewPager.
mViewPager.setCurrentItem(tab.getPosition());
}
@Override
public void onTabUnselected(ActionBar.Tab tab,
FragmentTransaction fragmentTransaction) {
}
@Override
public void onTabReselected(ActionBar.Tab tab,
FragmentTransaction fragmentTransaction) {
}
/**
* A {@link FragmentPagerAdapter} that returns a fragment corresponding to
* one of the sections/tabs/pages.
*/
public class SectionsPagerAdapter extends FragmentPagerAdapter {
public SectionsPagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int position) {
// getItem is called to instantiate the fragment for the given page.
// Return a DummySectionFragment (defined as a static inner class
// below) with the page number as its lone argument.
Fragment fragment;
fragment=null;
switch(position){
case 0:
fragment = new MyFragment0();
break;
case 1:
fragment = new MyFragment1();
break;
case 2:
fragment = new MyFragment2();
break;
}
//set args if necessary
Bundle args = new Bundle();
args.putInt(DummySectionFragment.ARG_SECTION_NUMBER, position + 1);
fragment.setArguments(args);
//return fragment
return fragment;
}
@Override
public int getCount() {
// Show 3 total pages.
return 3;
}
@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);
}
return null;
}
}
/**
* A dummy fragment representing a section of the app, but that simply
* displays dummy text.
*/
public static class DummySectionFragment extends Fragment {
/**
* The fragment argument representing the section number for this
* fragment.
*/
public static final String ARG_SECTION_NUMBER = "section_number";
public DummySectionFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main_dummy,
container, false);
TextView dummyTextView = (TextView) rootView
.findViewById(R.id.section_label);
dummyTextView.setText(Integer.toString(getArguments().getInt(
ARG_SECTION_NUMBER)));
return rootView;
}
}
}
错误日志如下：
08-14 18:21:09.375: E/AndroidRuntime(1303): FATAL EXCEPTION: main
08-14 18:21:09.375: E/AndroidRuntime(1303): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.shantanu.report/com.shantanu.report.MainActivity}: java.lang.NullPointerException
08-14 18:21:09.375: E/AndroidRuntime(1303): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
08-14 18:21:09.375: E/AndroidRuntime(1303): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
08-14 18:21:09.375: E/AndroidRuntime(1303): at android.app.ActivityThread.access$600(ActivityThread.java:141)
08-14 18:21:09.375: E/AndroidRuntime(1303): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
08-14 18:21:09.375: E/AndroidRuntime(1303): at android.os.Handler.dispatchMessage(Handler.java:99)
08-14 18:21:09.375: E/AndroidRuntime(1303): at android.os.Looper.loop(Looper.java:137)
08-14 18:21:09.375: E/AndroidRuntime(1303): at android.app.ActivityThread.main(ActivityThread.java:5103)
08-14 18:21:09.375: E/AndroidRuntime(1303): at java.lang.reflect.Method.invokeNative(Native Method)
08-14 18:21:09.375: E/AndroidRuntime(1303): at java.lang.reflect.Method.invoke(Method.java:525)
08-14 18:21:09.375: E/AndroidRuntime(1303): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
08-14 18:21:09.375: E/AndroidRuntime(1303): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
08-14 18:21:09.375: E/AndroidRuntime(1303): at dalvik.system.NativeStart.main(Native Method)
08-14 18:21:09.375: E/AndroidRuntime(1303): Caused by: java.lang.NullPointerException
08-14 18:21:09.375: E/AndroidRuntime(1303): at com.shantanu.report.MainActivity.onCreate(MainActivity.java:73)
答案 0 :(得分：0)
在我看来，那条线
(ViewPager) findViewById(R.id.pager);
返回null，因此语句mViewPager.setAdapter(mSectionsPagerAdapter);崩溃。确保您的xml布局中有一个ViewPager。
答案 1 :(得分：0)
我在这里看到2个问题，b5为null，检查你的xml布局以确保你的id正确
第二步，在您开始活动
之后，再次将内容视图设置为按钮setContentView(b5);
因此您将视图更改为我认为您不想做的按钮
答案 2 :(得分：0)
这可能是您的activity_main.xml文件中的一个问题。
我在你的评论中注意到你在xml中声明b5是这样的：
<Button android:id="@+id/b5" android:layout_width="wrap_content"
android:layout_height="wrap_content" android:layout_below="@id/b4"/>
我可以看到layout_below字段有问题。 id应该与id的声明方式完全匹配。所以，而不是＆＃34; @ id / b4＆＃34;，它应该是＆＃34; @ + id / b4＆＃34;。
我也建议你发布你的xml，因为这可能是问题所在。