可滑动视图空异常错误 - Android Studio

Question

尝试 运行 应用程序并在视图之间滑动时，应用程序崩溃并出现空对象引用错误。我已经仔细检查了我的 getItem 方法以确保它能够处理这个问题，但应用程序仍然崩溃。应用程序 运行 第一个视图很好，但在尝试滑动到第二个和第三个时崩溃。

如果有人能澄清错误是什么，我们将不胜感激。

我还在代码下方包含了错误日志。

public class MainActivity extends AppCompatActivity {

private SectionsPagerAdapter mSectionsPagerAdapter;
private ViewPager mViewPager;

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

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    // Create the adapter that will return a fragment for each of the three
    // primary sections of the activity.
    mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

    // Set up the ViewPager with the sections adapter.
    mViewPager = (ViewPager) findViewById(R.id.container);
    mViewPager.setAdapter(mSectionsPagerAdapter);

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });

}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}


public class SectionsPagerAdapter extends FragmentPagerAdapter {

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

    @Override
    public Fragment getItem(int position) {
        //returning the current tabs
        switch (position){
            case 0:
                Tab1 tab1 = new Tab1();
                return tab1;
            case 1:
                Tab2 tab2 = new Tab2();
                return tab2;
            case 3:
                Tab3 tab3 = new Tab3();
                return tab3;
            default:
                return null;
        }
    }

    @Override
    public int getCount() {
        // Show 3 total pages.
        return 3;
    }
}
}

这是错误日志；

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:394) at android.support.v4.app.BackStackRecord.add(BackStackRecord.java:389) at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:104) at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:1002) at android.support.v4.view.ViewPager.populate(ViewPager.java:1216) at android.support.v4.view.ViewPager.populate(ViewPager.java:1084) at android.support.v4.view.ViewPager.run(ViewPager.java:267) at android.view.Choreographer$CallbackRecord.run(Choreographer.java:911) at android.view.Choreographer.doCallbacks(Choreographer.java:723) at android.view.Choreographer.doFrame(Choreographer.java:655) at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:897) at android.os.Handler.handleCallback(Handler.java:790) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:164) at android.app.ActivityThread.main(ActivityThread.java:6494) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

Answer 1

问题出在这段代码中：

@Override
public Fragment getItem(int position) {
    //returning the current tabs
    switch (position){
        case 0:
            Tab1 tab1 = new Tab1();
            return tab1;
        case 1:
            Tab2 tab2 = new Tab2();
            return tab2;
        case 3:
            Tab3 tab3 = new Tab3();
            return tab3;
        default:
            return null;
    }
}

具体来说，这一行：

case 3:

这应该是 case 2。就像现在一样，您将从 default 案例中 return null 作为您的第三页。

---

顺便说一句，我建议您改变处理此类情况的方式。而不是从默认情况下 returning null，我认为你应该这样做：

case default:
    throw new IllegalArgumentException("unexpected position: " + position);

如果你有这个而不是 return null，你的错误会更加明显。你会看到

IllegalArgumentException: unexpected position: 2

这会帮助您意识到 case 3 中的拼写错误。

一般来说，当您的程序位于不应该出现的位置时，您应该始终抛出异常而不是 return 设置一些默认值。