java - Android Studio - 带选项卡布局的滑动 View

Question

我正在尝试使用 ViewPager 和 Fragments 编写一个具有选项卡布局的应用程序，但每次运行它时，它都会因 nullPointerException 崩溃。

这是我的主要 Activity 的代码:

import android.annotation.TargetApi;
import android.app.ActionBar;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;

@TargetApi(11)
public class MainActivity extends FragmentActivity {
    ViewPager Tab;
    TabPagerAdapter TabAdapter;
    ActionBar actionBar;

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

        TabAdapter = new TabPagerAdapter(getSupportFragmentManager());

        Tab = (ViewPager)findViewById(R.id.pager);
        Tab.setOnPageChangeListener(
                new ViewPager.SimpleOnPageChangeListener() {
                    @Override
                    public void onPageSelected(int position) {

                        actionBar = getActionBar();
                        actionBar.setSelectedNavigationItem(position);                    }
                });

        Tab.setAdapter(TabAdapter);

        actionBar = getActionBar();
        //Enable Tabs on Action Bar
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
        ActionBar.TabListener tabListener = new ActionBar.TabListener(){

            @Override
            public void onTabReselected(android.app.ActionBar.Tab tab,
                                        FragmentTransaction ft) {
                // TODO Auto-generated method stub

            }

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

                Tab.setCurrentItem(tab.getPosition());
            }

            @Override
            public void onTabUnselected(android.app.ActionBar.Tab tab,
                                        FragmentTransaction ft) {
                // TODO Auto-generated method stub

            }};
        //Add New Tab

        actionBar.addTab(actionBar.newTab().setText("Android").setTabListener(tabListener));
        actionBar.addTab(actionBar.newTab().setText("iOS").setTabListener(tabListener));
        actionBar.addTab(actionBar.newTab().setText("Windows").setTabListener(tabListener));

    }

TabPagerAdapter.java

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentStatePagerAdapter;

public class TabPagerAdapter extends FragmentStatePagerAdapter {
    public TabPagerAdapter(FragmentManager fm) {
        super(fm);
        // TODO Auto-generated constructor stub
    }

    @Override
    public Fragment getItem(int i) {
        switch (i) {
            case 0:
                //Fragement for Android Tab
                return new Android();
            case 1:
                //Fragment for Ios Tab
                return new Ios();
            case 2:
                //Fragment for Windows Tab
                return new Windows();
        }
        return null;

    }

    @Override
    public int getCount() {
        // TODO Auto-generated method stub
        return 3; //No of Tabs
    }
}

这就是我的 logcat 的样子:

07-11 12:27:21.803  11967-11967/com.example.diana.myapplication3 D/dalvikvm﹕ Late-enabling CheckJNI
07-11 12:27:21.943  11967-11967/com.example.diana.myapplication3 E/ResourceType﹕ 0x5acd8640: Failed to ResTable::remove() cookie = 0x3, not last table. mHeaders.size() = 4. Warning for spontaneous crashes when the garbage collector runs.
07-11 12:27:21.943  11967-11967/com.example.diana.myapplication3 E/asset﹕ Error removing runtime skin resource (cookie 0x3)
07-11 12:27:21.943  11967-11967/com.example.diana.myapplication3 I/asset﹕ Problem removing all runtime skin resources
07-11 12:27:22.043  11967-11967/com.example.diana.myapplication3 D/AndroidRuntime﹕ Shutting down VM
07-11 12:27:22.043  11967-11967/com.example.diana.myapplication3 W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x415478b0)
07-11 12:27:22.053  11967-11967/com.example.diana.myapplication3 E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.diana.myapplication3/com.example.diana.myapplication3.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2261)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2311)
            at android.app.ActivityThread.access$600(ActivityThread.java:149)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1293)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5214)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:739)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:555)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.example.diana.myapplication3.MainActivity.onCreate(MainActivity.java:35)
            at android.app.Activity.performCreate(Activity.java:5133)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2225)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2311)
            at android.app.ActivityThread.access$600(ActivityThread.java:149)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1293)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5214)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:739)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:555)
            at dalvik.system.NativeStart.main(Native Method)

我认为这里的问题是我的 actionBar 返回 null，因此我添加了

 getWindow().requestFeature(Window.FEATURE_ACTION_BAR);

但我遇到了同样的错误。

有什么想法吗？

Answer 1

我发现问题所在了。我删除了 getWindow().requestFeature(Window.FEATURE_ACTION_BAR);，并使用 actionBar = getSupportActionBar(); 和 MainActivity extends ActionBarActivity 代替 actionBar = getActionBar();。

这解决了我的问题，希望它也能帮助其他人。