选项卡布局实现上的 NullPointerException

Question

我已经实现了选项卡布局，但我的应用程序没有启动。我在这一行得到 NullPointerException：

actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

我认为一切都做得很好。我找不到我的错误。

主活动：

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

import com.msh.numeral_system_converter_adapter.TabsPagerAdapter;

public class MainActivity extends FragmentActivity implements
    ActionBar.TabListener {

private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;

// Tab titles
private String[] tabs = { "Converter", "Calculator"};

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

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

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

    // 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 onTabReselected(Tab tab, FragmentTransaction ft) {
}

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

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

}

日志文件：

04-11 17:44:02.553    1545-1545/com.msh.numeral_system_converter E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.msh.numeral_system_converter, PID: 1545
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.msh.numeral_system_converter/com.msh.numeral_system_converter.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 com.msh.numeral_system_converter.MainActivity.onCreate(MainActivity.java:34)
            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)

Answer 1

如果您的窗口没有ActionBar，getActionBar() 可以重新运行null。就像您选择Theme.NoTitleBar 样式作为活动/应用程序主题一样。

改用此代码 (picked from there)：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

// The Action Bar is a window feature. The feature must be requested
// before setting a content view. Normally this is set automatically
// by your Activity's theme in your manifest. The provided system
// theme Theme.WithActionBar enables this for you. Use it as you would
// use Theme.NoTitleBar. You can add an Action Bar to your own themes
// by adding the element <item name="android:windowActionBar">true</item>
// to your style definition.
    getWindow().requestFeature(Window.FEATURE_ACTION_BAR);
    setContentView(R.layout.main);

// experiment with the ActionBar 
    ActionBar actionBar = getActionBar();
}

这个link 也可能有帮助。

Answer 2

这段代码：

actionBar = getActionBar();
mAdapter = new TabsPagerAdapter(getSupportFragmentManager());

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

您从 getActionBar() 方法分配 actionBar，但您没有提供该代码。我的猜测是 getActionBar() 正在返回 null。 NullPointerException 来自尝试在 null 对象上调用 setNavigationMode，可能不是来自引用 ActionBar 上的字段。

Answer 3

使用上述修复可能会出现另一个错误：-

在添加内容之前必须调用requestFeature()

要解决该问题，请使用此修复：-

不要在requestFeature() 之前调用setContentView()。

Answer 4

当您选择没有ActionBar 的主题时会出现此异常。 尝试使用类似的主题，

<style name="AppTheme" parent="@android:style/Theme.Holo.Light">
    <!-- Customize your theme here. -->
</style>