java.lang.NullPointerException:尝试在空对象引用上调用虚方法。我该怎么办? [英] java.lang.NullPointerException: Attempt to invoke virtual method on a null object reference. what should i do?
问题描述
frag_home.java
public class Frag_home extends Fragment {
private String mParam1;
private String mParam2;
动画fade_in,fade_out;
ViewFlipper viewFlipper;
private OnFragmentInteractionListener mListener;
public Frag_home(){
//必需的空公共构造函数
}
// TODO:重命名并更改类型和数字的参数
public static Frag_home newInstance(String param1,String param2){
Frag_home fragment = new Frag_home();
Bundle args = new Bundle();
args.putString(ARG_PARAM1,param1);
args.putString(ARG_PARAM2,param2);
fragment.setArguments(args);
返回片段;
}
@Override
public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
if(getArguments()!= null){
mParam1 = getArguments()。getString(ARG_PARAM1);
mParam2 = getArguments()。getString(ARG_PARAM2);
}
}
@Override
public View onCreateView(LayoutInflater inflater,ViewGroup容器,
Bundle savedInstanceState){
//填充此片段的布局
return inflater.inflate(R.layout.fragment_frag_home,container,false);
fade_in = AnimationUtils.loadAnimation(getActivity(),R.anim.fade_in);
fade_out = AnimationUtils.loadAnimation(getActivity(),R.anim.fade_out);
viewFlipper.setAnimation(fade_in);
viewFlipper.setAnimation(fade_out);
viewFlipper.setAutoStart(true);
viewFlipper.setFlipInterval(1000);
viewFlipper.startFlipping();
}
// TODO:重命名方法,更新参数和钩子方法到UI事件
public void onButtonPressed(Uri uri){
if(mListener!= null ){
mListener.onFragmentInteraction(uri);
}
}
@Override
public void onAttach(Context context){
super.onAttach(context);
if(Context instanceof OnFragmentInteractionListener){
mListener =(OnFragmentInteractionListener)context;
} else {
throw new RuntimeException(context.toString()
+必须实现OnFragmentInteractionListener);
}
}
@Override
public void onDetach(){
super.onDetach();
mListener = null;
}
public interface OnFragmentInteractionListener {
// TODO:更新参数类型和名称
void onFragmentInteraction(Uri uri);
}
Logcat:
04-09 18:29:02.196 27863-27863 / com.example.aastha.trying E / AndroidRuntime:FATAL EXCEPTION:main
进程:com.example.aastha.trying ,PID:27863
java.lang.NullPointerException:尝试在com上调用虚拟方法'void android.widget.ViewFlipper.setAnimation(android.view.animation.Animation)'一个空对象引用
。 example.aastha.trying.Frag_home.onCreate(Frag_home.java:70)
在android.support.v4.app.Fragment.performCreate(Fragment.java:1951)
在android.support.v4。 app.FragmentManagerImpl.moveToState(FragmentManager.j ava:1029)
在android.support.v4.app.BackStackRecord.ensureFragmentsAreInitialized(BackStackRecord.java:1077)
在android.support.v4.app.BackStackRecord.beginTransition(BackStackRecord.java:1032)
在android.support.v4.app.BackStackRecord.run(BackStackRecord.java:658)
在android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1617)
在android.support.v4.app.FragmentManagerImpl $ 1.run(FragmentManager.java:517)
在android.os.Handler.handleCallback(Handler.java:739)
在android.os.Handler.dispatchMessage(Handler.java:95)
在android.os.Looper.loop(Looper.java:145)
在android.app.ActivityThread.main(ActivityThread.java :5832)
在java.lang.reflect.Method.invoke(本机方法)
在java.lang.reflect.Method.invoke(Method.java:372)
在com.android .internal.os.ZygoteInit $ MethodAndArgsCaller.run(ZygoteInit.java:1399)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
上面的代码给了我t他的例外:
java.lang.NullPointerException:尝试调用虚方法'void android.widget.ViewFlipper.setAnimation(android。 view.animation.Animation)'在空对象引用
我应该如何解决这个问题?请帮助。
在你的onCreate() viewFlipper
相反,在您的onCreateView()中,您必须使用findViewById获取对ViewFlipper的引用,然后才能对其执行操作。
@Override
public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
if(getArguments()!= null){
mParam1 = getArguments()。getString(ARG_PARAM1);
mParam2 = getArguments()。getString(ARG_PARAM2);
}
fade_in = AnimationUtils.loadAnimation(getActivity(),R.anim.fade_in);
fade_out = AnimationUtils.loadAnimation(getActivity(),R.anim.fade_out);
}
@Override
public View onCreateView(LayoutInflater inflater,ViewGroup容器,
Bundle savedInstanceState){
//扩充此片段的布局
查看v = inflater.inflate(R.layout.fragment_frag_home,container,false);
viewFlipper = v.findViewById(R.id.< your_viewflipper_id>);
viewFlipper.setAnimation(fade_in);
viewFlipper.setAnimation(fade_out);
viewFlipper.setAutoStart(true);
viewFlipper.setFlipInterval(1000);
viewFlipper.startFlipping();
}
onCreate()在之前调用膨胀,因此您无法访问该视图。还需要通过onCreateView()中的findViewById()绑定每个视图。您应该看看片段生命周期(图2)。
I I have created an app that was working perfectly before I introduces ViewFlipper into it. I really need to use this, but now I'm stuck or this exception.
frag_home.java
public class Frag_home extends Fragment {
private String mParam1;
private String mParam2;
Animation fade_in, fade_out;
ViewFlipper viewFlipper;
private OnFragmentInteractionListener mListener;
public Frag_home() {
// Required empty public constructor
}
// TODO: Rename and change types and number of parameters
public static Frag_home newInstance(String param1, String param2) {
Frag_home fragment = new Frag_home();
Bundle args = new Bundle();
args.putString(ARG_PARAM1, param1);
args.putString(ARG_PARAM2, param2);
fragment.setArguments(args);
return fragment;
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (getArguments() != null) {
mParam1 = getArguments().getString(ARG_PARAM1);
mParam2 = getArguments().getString(ARG_PARAM2);
}
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_frag_home, container, false);
fade_in= AnimationUtils.loadAnimation(getActivity(), R.anim.fade_in);
fade_out=AnimationUtils.loadAnimation(getActivity(),R.anim.fade_out);
viewFlipper.setAnimation(fade_in);
viewFlipper.setAnimation(fade_out);
viewFlipper.setAutoStart(true);
viewFlipper.setFlipInterval(1000);
viewFlipper.startFlipping();
}
// TODO: Rename method, update argument and hook method into UI event
public void onButtonPressed(Uri uri) {
if (mListener != null) {
mListener.onFragmentInteraction(uri);
}
}
@Override
public void onAttach(Context context) {
super.onAttach(context);
if (context instanceof OnFragmentInteractionListener) {
mListener = (OnFragmentInteractionListener) context;
} else {
throw new RuntimeException(context.toString()
+ " must implement OnFragmentInteractionListener");
}
}
@Override
public void onDetach() {
super.onDetach();
mListener = null;
}
public interface OnFragmentInteractionListener {
// TODO: Update argument type and name
void onFragmentInteraction(Uri uri);
}
Logcat:
04-09 18:29:02.196 27863-27863/com.example.aastha.trying E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.aastha.trying, PID: 27863
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ViewFlipper.setAnimation(android.view.animation.Animation)' on a null object reference
at com.example.aastha.trying.Frag_home.onCreate(Frag_home.java:70)
at android.support.v4.app.Fragment.performCreate(Fragment.java:1951)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1029)
at android.support.v4.app.BackStackRecord.ensureFragmentsAreInitialized(BackStackRecord.java:1077)
at android.support.v4.app.BackStackRecord.beginTransition(BackStackRecord.java:1032)
at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:658)
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1617)
at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:517)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:5832)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
The code above gives me this exception:
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ViewFlipper.setAnimation(android.view.animation.Animation)' on a null object reference
How should i resolve this? Please help.
In your onCreate() viewFlipper
is still null.
Instead, in your onCreateView() you have to get a reference to your ViewFlipper with findViewById and only afterwards do operations on it.
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (getArguments() != null) {
mParam1 = getArguments().getString(ARG_PARAM1);
mParam2 = getArguments().getString(ARG_PARAM2);
}
fade_in= AnimationUtils.loadAnimation(getActivity(), R.anim.fade_in);
fade_out=AnimationUtils.loadAnimation(getActivity(),R.anim.fade_out);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View v = inflater.inflate(R.layout.fragment_frag_home, container, false);
viewFlipper = v.findViewById(R.id.<your_viewflipper_id>);
viewFlipper.setAnimation(fade_in);
viewFlipper.setAnimation(fade_out);
viewFlipper.setAutoStart(true);
viewFlipper.setFlipInterval(1000);
viewFlipper.startFlipping();
}
onCreate() is called before your layout is inflated, thus you cannot access views there. Also you need to bind every view via findViewById() in onCreateView(). You should have a look at the fragment lifecycle (figure 2).
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