java - Swipeable Views Null 异常错误 - Android Studio
问题描述
当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 方法以确保它可以处理这个问题,但应用程序仍然崩溃。该应用程序可以正常运行第一个视图,但在尝试滑动到第二个和第三个视图时崩溃。
如果有人能澄清错误是什么,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$3.run(ViewPager.java:267) at android.view.Choreographer$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)
解决方案
The problem is in this code:
@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; } }
Specifically, this line:
case 3:
This should be case 2
instead. As it is now, you'll return null
from the default
case for your third page.
Incidentally, I recommend changing how you handle situations like this going forward. Instead of returning null
from the default case, I think you should do this:
case default:
throw new IllegalArgumentException("unexpected position: " + position);
If you'd had this in place instead of return null
, your error would have been much more obvious. You'd have seen
IllegalArgumentException: unexpected position: 2
Which would have helped you realize the typo in case 3
.
In general, you should always throw an exception instead of returning some default value when your program is in a place it shouldn't be.
推荐阅读
- python - 带有彩条的情节甘特图
- javascript - 向导提交按钮问题,我认为js中的这个表单向导问题
- excel - ExcelVBA Q:如何替换字符的第一个实例并删除相同字符的第二个实例之后的所有内容
- angular - 何时注入自定义管道有用?
- google-maps - 基于数据标记值的 Google Map API 自定义标记
- python - 正确循环读取数据、比较数据和从 txt 文件写入数据
- r - 嵌入功能说明
- security - 为安全配置 apache2
- c# - WPF 拖放到/从 Windows 资源管理器,但不拖放到/从自身
- java - 如何将日期组合在一起并在 java 或 kotlin 中显示他们的数据