线程以未捕获的表达式退出

Question

我有一个应用程序，它由简单的抽屉导航和菜单列表组成。在其中一个菜单中，我想添加一个带有选项卡布局的页面 - 在 this 上找到可滑动视图 -- 此应用程序运行没有任何错误。

我决定在其中一个菜单中放置一个按钮，按下该按钮时会将用户引导至 TabLayout 的 MainActivity。

现在在我的主项目中，我有两个 MainActivity 类，一个是 MainActivity.java（这是 NavDrawer 的）和 MainActivity2.java（这是 TabLayout 的），而且我管理将所有必需的文件复制粘贴到我的主项目中，包括适配器。

我把按钮放在我的 HomeFragment.java 中，如下所示：

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {

        View rootView = inflater.inflate(R.layout.fragment_home, container, false);
        Button mButton = (Button) rootView.findViewById(R.id.button1);
        mButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // here you set what you want to do when user clicks your button,
                Intent intent = new Intent(getActivity(), MainActivity2.class);
                startActivity(intent);
            }
        });
        return rootView;
    }
}

但不幸的是，当我点击按钮时，应用程序崩溃并显示 logcat

04-08 05:58:52.106: E/AndroidRuntime(1317): FATAL EXCEPTION: main
04-08 05:58:52.106: E/AndroidRuntime(1317): Process: my.jlm, PID: 1317
04-08 05:58:52.106: E/AndroidRuntime(1317): java.lang.RuntimeException: Unable to start activity ComponentInfo{my.jlm/my.jlm.MainActivity2}: java.lang.NullPointerException
04-08 05:58:52.106: E/AndroidRuntime(1317):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2176)
04-08 05:58:52.106: E/AndroidRuntime(1317):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)
04-08 05:58:52.106: E/AndroidRuntime(1317):     at android.app.ActivityThread.access0(ActivityThread.java:135)
04-08 05:58:52.106: E/AndroidRuntime(1317):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)
04-08 05:58:52.106: E/AndroidRuntime(1317):     at android.os.Handler.dispatchMessage(Handler.java:102)
04-08 05:58:52.106: E/AndroidRuntime(1317):     at android.os.Looper.loop(Looper.java:137)
04-08 05:58:52.106: E/AndroidRuntime(1317):     at android.app.ActivityThread.main(ActivityThread.java:4998)
04-08 05:58:52.106: E/AndroidRuntime(1317):     at java.lang.reflect.Method.invokeNative(Native Method)
04-08 05:58:52.106: E/AndroidRuntime(1317):     at java.lang.reflect.Method.invoke(Method.java:515)
04-08 05:58:52.106: E/AndroidRuntime(1317):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
04-08 05:58:52.106: E/AndroidRuntime(1317):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
04-08 05:58:52.106: E/AndroidRuntime(1317):     at dalvik.system.NativeStart.main(Native Method)
04-08 05:58:52.106: E/AndroidRuntime(1317): Caused by: java.lang.NullPointerException
04-08 05:58:52.106: E/AndroidRuntime(1317):     at my.jlm.MainActivity2.onCreate(MainActivity2.java:32)

它说错误可能是由第 32 行的 MainActivity2.java 引起的，但我不知道如何处理它

这是第 32 行

viewPager.setAdapter(mAdapter);

这是我完整的 MainActivity2

public class MainActivity2 extends FragmentActivity implements
ActionBar.TabListener {

private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;
// Tab titles
private String[] tabs = { "Top Rated", "Games", "Movies" };

@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) {
}

}

感谢您的光临。

Answer 1

viewPager 是 null。你这样初始化它：

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

findViewById returns 找到视图，否则 null。

public View findViewById (int id) Added in API level 1

Finds a view that was identified by the id attribute from the XML that was processed in onCreate(Bundle). Returns
The view if found or null otherwise.

Source.

你的问题是没有找到视图，可能是因为它还没有创建。

线程以未捕获的表达式退出

Thread Exiting With Uncaught Expression

android

android-intent

uncaught-exception

android-fragments