为什么我的Android程序第73行出现Null指针异常？

Question

I am not entirely sure why I am getting a NullPointerException . If anybody could help me out that would be great. I labeled Line 73 below.

    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;
    }
}

}

The Error log is below:

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)

Answer 1

It looks to me, that line

(ViewPager) findViewById(R.id.pager);

returns null and therefore the statement mViewPager.setAdapter(mSectionsPagerAdapter); crashes. Make sure, there is a ViewPager in your xml layout.

Answer 2

there are 2 problems I see here, b5 is null, check your xml layout to make sure you have the id right

second you are setting the content view again to a button after you already set it when you started the activity

setContentView(b5);

so you are changing the view to a button which I assume you dont want to do

Answer 3

It's probably an issue in your activity_main.xml file.

I noticed in your comment that you declare b5 in your xml like this:

<Button android:id="@+id/b5" android:layout_width="wrap_content"
    android:layout_height="wrap_content" android:layout_below="@id/b4"/>

I can see a problem with your layout_below field. The id should match exactly how the id was declared. So, instead of "@id/b4", it should be "@+id/b4".

I would recommend posting your xml as well, because that's probably where the problem lies.