片段中的空指针异常 [英] Null Pointer Exception In Fragments
问题描述
从另一个片段开始一个片段时，我收到NullPointerException.我试图在动态启动Fragment之后调用第二个Fragment中的方法.
I am getting NullPointerException while Starting One Fragment From other Fragment. I am trying to call a method in the second Fragment after Starting the Fragment dynamically.
这是我的logcat:
Here is my logcat:
05-20 09:58:31.907: E/AndroidRuntime(2585): FATAL EXCEPTION: main
05-20 09:58:31.907: E/AndroidRuntime(2585): java.lang.NullPointerException
05-20 09:58:31.907: E/AndroidRuntime(2585): at com.exercise.FragmentTest.MyFragment3.setImage(MyFragment3.java:22)
05-20 09:58:31.907: E/AndroidRuntime(2585): at com.exercise.FragmentTest.FragmentTestActivity$1.onClick(FragmentTestActivity.java:48)
05-20 09:58:31.907: E/AndroidRuntime(2585): at android.view.View.performClick(View.java:4204)
05-20 09:58:31.907: E/AndroidRuntime(2585): at android.view.View$PerformClick.run(View.java:17355)
05-20 09:58:31.907: E/AndroidRuntime(2585): at android.os.Handler.handleCallback(Handler.java:725)
05-20 09:58:31.907: E/AndroidRuntime(2585): at android.os.Handler.dispatchMessage(Handler.java:92)
05-20 09:58:31.907: E/AndroidRuntime(2585): at android.os.Looper.loop(Looper.java:137)
05-20 09:58:31.907: E/AndroidRuntime(2585): at android.app.ActivityThread.main(ActivityThread.java:5041)
05-20 09:58:31.907: E/AndroidRuntime(2585): at java.lang.reflect.Method.invokeNative(Native Method)
05-20 09:58:31.907: E/AndroidRuntime(2585): at java.lang.reflect.Method.invoke(Method.java:511)
05-20 09:58:31.907: E/AndroidRuntime(2585): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
05-20 09:58:31.907: E/AndroidRuntime(2585): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
05-20 09:58:31.907: E/AndroidRuntime(2585): at dalvik.system.NativeStart.main(Native Method)
我的主要活动代码:
public class FragmentTestActivity extends FragmentActivity{
Fragment fragment;
Button btnFragment1, btnFragment2, btnFragment3;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
btnFragment1 = (Button)findViewById(R.id.displayfragment1);
btnFragment2 = (Button)findViewById(R.id.displayfragment2);
btnFragment3 = (Button)findViewById(R.id.displayfragment3);
// get an instance of FragmentTransaction from your Activity
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
//add a fragment
MyFragment myFragment = new MyFragment();
fragmentTransaction.add(R.id.myfragment, myFragment);
fragmentTransaction.commit();
btnFragment1.setOnClickListener(btnFragmentOnClickListener);
btnFragment2.setOnClickListener(btnFragmentOnClickListener);
btnFragment3.setOnClickListener(btnFragmentOnClickListener);
}
Button.OnClickListener btnFragmentOnClickListener
= new Button.OnClickListener(){
@Override
public void onClick(View v) {
if(v == btnFragment3){
MyFragment3 newfragment = new MyFragment3();
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack
transaction.replace(R.id.myfragment, newfragment);
transaction.addToBackStack(null);
// Commit the transaction
transaction.commit();
newfragment.setImage();
return;
}
// TODO Auto-generated method stub
Fragment newFragment = null;
// Create new fragment
if(v == btnFragment1){
newFragment = new MyFragment();
}
else if(v == btnFragment2){
newFragment = new MyFragment2();
}
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack
transaction.replace(R.id.myfragment, newFragment);
transaction.addToBackStack(null);
// Commit the transaction
transaction.commit();
// Create new transaction
}};
}
我的Fragment3类是:
And My Fragment3 class is:
public class MyFragment3 extends Fragment {
ImageView iv;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// TODO Auto-generated method stub
View myFragmentView = inflater.inflate(R.layout.fragmentlayout3, container, false);
iv = (ImageView)myFragmentView.findViewById(R.id.image);
return myFragmentView;
}
public void setImage(){
iv.setImageResource(R.drawable.penguins);
}
}
请帮助我克服这个问题.
Please help me to overcome this problem.
推荐答案
您的错误在这里发生:
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
newfragment.setImage();
并正在发生，因为变量iv仍然是null.
and it's happening because the variable iv is still null.
您必须等到片段"显示(放大)后才能访问被放大的视图.
you have to wait until after the Fragment is shown (inflated) in order to access the Views that were inflated into it.
有关更多信息，请尝试阅读有关片段生命周期的信息.
for more, try reading about the Fragment lifecycle.
onCreateView()
当片段第一次绘制其用户界面时，系统将调用此方法.要为片段绘制UI，必须从此方法返回一个View，它是片段布局的根.如果该片段不提供用户界面，则可以返回null.
onCreateView()
The system calls this when it's time for the fragment to draw its user interface for the first time. To draw a UI for your fragment, you must return a View from this method that is the root of your fragment's layout. You can return null if the fragment does not provide a UI.
创建此图像的一种非常好的方法是使用setArgs方法，并向Bundle传递有关图像的信息.
a very good way of creating this will be using the setArgs method and passing a Bundle with the information about your image.
尝试使用此代码:
public MyFragment3 extends Fragment {
public static final IMAGE_RES = "IMAGE_RES";
private int imageRes;
private ImageView iv;
public static MyFragment3 init(int imageRes) {
MyFragment3 frag = new MyFragment3();
Bundle args = new Bundle();
args.putInt(IMAGE_RES, imageRes);
frag.setArguments(args);
return frag;
}
public void onCreate(Bundle savedInstanceState) {
super.onCreate(SavedInstanceState);
imageRes = (getArguments() != null)? getArguments().getInt(IMAGE_RES) : 0;
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View myFragmentView = inflater.inflate(R.layout.fragmentlayout3, container, false);
iv = (ImageView)myFragmentView.findViewById(R.id.image);
iv.setImageResource(imageRes);
return myFragmentView;
}
}
这篇关于片段中的空指针异常的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
