setHomeButtonEnabled：java.lang.NullPointerException

Question

每当我构建Android应用时，都会产生以下错误：

03-16 10:01:01.329    1312-1312/nl.mikehagendoorn.efteltijden E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: nl.mikehagendoorn.efteltijden, PID: 1312
    java.lang.RuntimeException: Unable to start activity ComponentInfo{nl.mikehagendoorn.efteltijden/nl.mikehagendoorn.efteltijden.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5017)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at nl.mikehagendoorn.efteltijden.MainActivity.onCreate(MainActivity.java:85)
            at android.app.Activity.performCreate(Activity.java:5231)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5017)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
            at dalvik.system.NativeStart.main(Native Method)

我的MainActivity.java：

package nl.mikehagendoorn.efteltijden;

import android.app.ActionBar;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;
import android.widget.ListView;
import android.widget.ProgressBar;
import android.widget.TextView;

public class MainActivity extends FragmentActivity implements
        ActionBar.TabListener {

    private ListView listView1;
    private ProgressBar loadingDialog;
    private ViewPager viewPager;
    private TabsPagerAdapter mAdapter;
    private ActionBar actionBar;

    TextView tvIsConnected;
    TextView txtTitle;

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

        viewPager = (ViewPager) findViewById(R.id.pager);
        actionBar = getActionBar();
        mAdapter = new TabsPagerAdapter(getSupportFragmentManager());

        viewPager.setAdapter(mAdapter);
        actionBar.setHomeButtonEnabled(false);
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        // Tab titles
        String[] tabs = {
                getResources().getString(R.string.attractions),
                getResources().getString(R.string.shows),
                getResources().getString(R.string.horeca),
                getResources().getString(R.string.stores),
                getResources().getString(R.string.information)
        };

        // Adding Tabs
        for (String tab_name : tabs) {
            actionBar.addTab(actionBar.newTab().setText(tab_name)
                    .setTabListener(this));
        }


        /**
         * on swiping the viewpager make respective tab selected
         * */
        viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {

            @Override
            public void onPageSelected(int position) {
                // on changing the page
                // make respected tab selected
                actionBar.setSelectedNavigationItem(position);
            }

            @Override
            public void onPageScrolled(int arg0, float arg1, int arg2) {
            }

            @Override
            public void onPageScrollStateChanged(int arg0) {
            }
        });

    }

    @Override
    public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {
        // on tab selected
        // show respected fragment view
        viewPager.setCurrentItem(tab.getPosition());
    }

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

    }

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

    }
}

但是，当我为Android 5.0构建它时，它不会抛出错误，它只是起作用。 谁能告诉我我的代码有什么问题？

编辑 - 我正在使用SDK 16在模拟器中测试它.minSdkVersion为14，targetSdkVersion为21

Answer 1

要像您一样使用AppCompat主题，您的活动需要从ActionBarActivity而不是FragmentActivity扩展（内部ActionBarActivity是FragmentActivity的扩展），您需要使用android.support.v7.app.ActionBar代替of android.app.ActionBar。

而不是getActionBar（）使用getSupportActionBar（），就像@Kim已经提到的那样。

Answer 2

您的模拟器可能正在运行Android＆lt; 3.0，所以如果你想在那里使用ActionBar，请使用支持库和 actionBar = getActionBar();应该是actionBar = getSupportActionBar