通过父FragmentActivity调用Fragment方法，获取NullPointerException

Question

我想从FragmentActivity调用一些Fragment的方法。 但是结果我总是得到NPE。 我做错了什么？ 这是我的源代码。

FragmentActivity：

    public class FragmentMenuActivity extends FragmentActivity implements
ActionBar.TabListener {

private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;
// Tab titles
private String[] tabs = { "Settings", "Image", "Text"};
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

setContentView(R.layout.activity_fragment_menu);

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

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

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


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) {
    }
});

TabText myTab= (TabText)mAdapter.getItem(2);
if (myTab!=null)
{   myTab.Test();

}
else
{
    Toast.makeText(getApplicationContext(), "null", Toast.LENGTH_LONG).show();
}
}

....
}

片段代码：

 public class TabText extends Fragment {


    public TabText()
    {

    }
    public void Test()
    {
        Toast.makeText(getActivity(), "TabText", Toast.LENGTH_LONG).show();
    }

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

        }
}

TabPagerAdapter代码：

    public class TabsPagerAdapter extends FragmentPagerAdapter {

    public TabsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int index) {

        switch (index) {
        case 0:
            // Top Rated fragment activity
            return new TabSettings();
        case 1:
            // Games fragment activity
            return new  TabImage();
        case 2:
            // Movies fragment activity
            return new TabText();
        }

        return null;
    }

    @Override
    public int getCount() {
        // get item count - equal to number of tabs
        return 3;
    }

}

结果，我总是得到NPE：

07-13 19:08:23.367: E/AndroidRuntime(529): FATAL EXCEPTION: main
07-13 19:08:23.367: E/AndroidRuntime(529): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.fragmentmenu/com.example.fragmentmenu.FragmentMenuActivity}: java.lang.NullPointerException
07-13 19:08:23.367: E/AndroidRuntime(529):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956)
07-13 19:08:23.367: E/AndroidRuntime(529):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
07-13 19:08:23.367: E/AndroidRuntime(529):  at android.app.ActivityThread.access$600(ActivityThread.java:123)
07-13 19:08:23.367: E/AndroidRuntime(529):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
07-13 19:08:23.367: E/AndroidRuntime(529):  at android.os.Handler.dispatchMessage(Handler.java:99)
07-13 19:08:23.367: E/AndroidRuntime(529):  at android.os.Looper.loop(Looper.java:137)
07-13 19:08:23.367: E/AndroidRuntime(529):  at android.app.ActivityThread.main(ActivityThread.java:4424)
07-13 19:08:23.367: E/AndroidRuntime(529):  at java.lang.reflect.Method.invokeNative(Native Method)
07-13 19:08:23.367: E/AndroidRuntime(529):  at java.lang.reflect.Method.invoke(Method.java:511)
07-13 19:08:23.367: E/AndroidRuntime(529):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
07-13 19:08:23.367: E/AndroidRuntime(529):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
07-13 19:08:23.367: E/AndroidRuntime(529):  at dalvik.system.NativeStart.main(Native Method)
07-13 19:08:23.367: E/AndroidRuntime(529): Caused by: java.lang.NullPointerException
07-13 19:08:23.367: E/AndroidRuntime(529):  at android.widget.Toast.<init>(Toast.java:92)
07-13 19:08:23.367: E/AndroidRuntime(529):  at android.widget.Toast.makeText(Toast.java:233)
07-13 19:08:23.367: E/AndroidRuntime(529):  at com.example.fragmentmenu.TabText.Test(TabText.java:16)
07-13 19:08:23.367: E/AndroidRuntime(529):  at com.example.fragmentmenu.FragmentMenuActivity.onCreate(FragmentMenuActivity.java:64)
07-13 19:08:23.367: E/AndroidRuntime(529):  at android.app.Activity.performCreate(Activity.java:4465)
07-13 19:08:23.367: E/AndroidRuntime(529):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
07-13 19:08:23.367: E/AndroidRuntime(529):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)
07-13 19:08:23.367: E/AndroidRuntime(529):  ... 11 more

Answer 1

一种解决方案是向TabText.test()方法添加上下文参数：

public void test(Context context)
{
    Toast.makeText(context, "TabText", Toast.LENGTH_LONG).show();
}

但是，您当前使用的TabText上的getActivity（）应该可以正常工作，但是如果该片段与活动分离，它将返回null。

因此，更好的解决方案是确保它不会与活动分离。