Question

我直到现在才遇到过这个问题，因为我刚刚将我的项目更新为API21（来自API20），我在行NullPointerException上得到#62 #63 #64 #65 #66 #67。你可以帮帮我吗？这是我的build.gradle：

android {
compileSdkVersion 21
buildToolsVersion "21.1.2"

defaultConfig {
    applicationId "active_news.activenews"
    minSdkVersion 10
    targetSdkVersion 21
    versionCode 1
    versionName "1.0"
}
buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
     }
   }
 }

   dependencies {
      compile fileTree(dir: 'libs', include: ['*.jar'])
      compile 'com.android.support:appcompat-v7:21.0.3'
      compile 'com.squareup.picasso:picasso:2.1.1'
      compile 'com.astuetz:pagerslidingtabstrip:1.0.1'
      compile 'com.afollestad:material-dialogs:0.5.1'

MainActivity：

public class MainActivity extends FragmentActivity implements
    ActionBar.TabListener, android.app.ActionBar.TabListener {
CollectionPagerAdapter mCollectionPagerAdapter;
ViewPager mViewPager;
SharedPreferences mPrefs;
final String welcomeScreenShownPref = "welcomeScreenShown";


public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
    setProgressBarIndeterminateVisibility(true);
    setProgressBarIndeterminateVisibility(false);
    setContentView(R.layout.activity_main);

    mPrefs = PreferenceManager.getDefaultSharedPreferences(this);

    Boolean welcomeScreenShown = mPrefs.getBoolean(welcomeScreenShownPref, false);

    if (!welcomeScreenShown) {

        MaterialDialogCompat.Builder dialogBuilder = new MaterialDialogCompat.Builder(this);
        dialogBuilder.setMessage(getString(R.string.Dialog_message));
        dialogBuilder.setTitle(getString(R.string.Dialog_title));
        dialogBuilder.setNegativeButton(getString(R.string.start), new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                dialog.dismiss();
            }
        });
        dialogBuilder.create().show();
        SharedPreferences.Editor editor = mPrefs.edit();
        editor.putBoolean(welcomeScreenShownPref, true);
        editor.commit(); // Very important to save the preference
    }

    mCollectionPagerAdapter = new CollectionPagerAdapter(
            getSupportFragmentManager());

    final android.app.ActionBar actionBar = getActionBar();
    assert actionBar != null;
    actionBar.setDisplayShowTitleEnabled(true);   //line#62
    actionBar.setDisplayUseLogoEnabled(false);    //line#63
    actionBar.setHomeButtonEnabled(false);       //line#64
    actionBar.setIcon(R.drawable.ic_action_icon3);  //line#65
    actionBar.setTitle(R.string.app_name2);     //line#66
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);    //line#67

    mViewPager = (ViewPager) findViewById(R.id.pager);
    mViewPager.setAdapter(mCollectionPagerAdapter);

    PagerSlidingTabStrip tabs = (PagerSlidingTabStrip) findViewById(R.id.tabs);
    tabs.setViewPager(mViewPager);
    tabs.setTextColor(getResources().getColor(android.R.color.white));
    tabs.setTextSize((int) getResources().getDimension(R.dimen.tab_text));

}

public void onTabUnselected(ActionBar.Tab tab,
                            FragmentTransaction fragmentTransaction) {

}

public void onTabSelected(ActionBar.Tab tab,
                          FragmentTransaction fragmentTransaction) {
    mViewPager.setCurrentItem(tab.getPosition());

}

public void onTabReselected(ActionBar.Tab tab,
                            FragmentTransaction fragmentTransaction) {

}

@Override
public void onTabSelected(android.app.ActionBar.Tab tab, android.app.FragmentTransaction ft) {
    mViewPager.setCurrentItem(tab.getPosition());

}

@Override
public void onTabUnselected(android.app.ActionBar.Tab tab, android.app.FragmentTransaction ft) {

}

@Override
public void onTabReselected(android.app.ActionBar.Tab tab, android.app.FragmentTransaction ft) {

}

public class CollectionPagerAdapter extends FragmentPagerAdapter {

    //final int NUM_ITEMS = 11; // number of tabs

    public CollectionPagerAdapter(FragmentManager fm) {
        super(fm);

    }

    @Override
    public Fragment getItem(int position)
    {

        switch (position) {
            case 0:
                return new TAB1();
            case 1:
                return new TAB2();
            case 2:
                return new TAB3();
            case 3:
                return new TAB4();
            case 4:
                return new TAB5();
            case 5:
                return new TAB6();
            case 6:
                return new TAB7();
            case 7:
                return new TAB8();

        }

        return null;
    }

    @Override
    public int getCount()
    {
        return 8;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        switch (position)
        {
            case 0:
                return getString(R.string.label1);
            case 1:
                return getString(R.string.label2);
            case 2:
                return getString(R.string.label3);
            case 3:
                return getString(R.string.label4);
            case 4:
                return getString(R.string.label5);
            case 5:
                return getString(R.string.label6);
            case 6:
                return getString(R.string.label8);
            case 7:
                return getString(R.string.label10);
            //case 8:
              //  return getString(R.string.label9);
            //case 9:
              //  return getString(R.string.label7);
        }
        return null;
    }
}

@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 boolean onOptionsItemSelected(MenuItem item) {
    // Handle item selection
    int i = item.getItemId();
    if (i == R.id.actiorn) {
        Intent intent = new Intent(MainActivity.this, SettingsActivity.class);
        MainActivity.this.startActivity(intent);
        return true;
    } else {
        return super.onOptionsItemSelected(item);
    }
}

这是我的logcat（它在第62行显示NullPointerException）：

12-30 01:00:24.249  25790-25790/active_news.activenews E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to start activity ComponentInfo{active_news.activenews/active_news.activenews.MainActivity}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2304)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2354)
    at android.app.ActivityThread.access$600(ActivityThread.java:150)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1244)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:5191)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:562)
    at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
    at active_news.activenews.MainActivity.onCreate(MainActivity.java:62)
    at android.app.Activity.performCreate(Activity.java:5104)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2258)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2354)
    at android.app.ActivityThread.access$600(ActivityThread.java:150)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1244)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:5191)
    at java.lang.reflect.Method.invokeNative(Native Method)

Answer 1

你正在使用AppCompat库，所以我想你必须调用getSupportActionBar（）而不是getActionBar（）

编辑：正如我之前所说，如果您将min SDK更新为11，那么您不需要使用getSupportActionBar。如果getActionBar方法中仍然存在错误，那么请确保您没有使用主题android：theme =＆＃34; @ style / Theme.NoTitle＆＃34;在您的应用程序清单上您需要在主题中定义一个带有标题的操作栏，例如android：theme =＆＃34; @android：style / Theme.WithActionBar＆＃34;。或者使用getWindow（）。requestFeature（Window.FEATURE_ACTION_BAR）;在setContentView之后。有许多解决方案：getActionBar returns null

NullPointerException - 将项目更新为API21

1 个答案: