Android:总是在意图中传递的额外内容中接收空对象

Question

我正在我的应用程序中创建一个回收器视图，并从Firestore获取相同的数据。在此过程中，我将一些参数传递给单击回收器视图中的项目时调用的意图。

RecyclerView recyclerView = findViewById(R.id.my_recycler_view);
recyclerView.setHasFixedSize(true);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
recyclerView.setAdapter(adapter);
adapter.setOnItemClickListener(new LearningModuleAdapter.OnItemClickListener() {
@Override
public void onItemClick(DocumentSnapshot documentSnapshot, int position) {
LearningModule learningModule = documentSnapshot.toObject(LearningModule.class);
String id=documentSnapshot.getId();
String path = documentSnapshot.getReference().getPath();
String title = (String) documentSnapshot.get("title");
System.out.println("what did i get from document snapshot? id: "+id+" path:"+path+" title:"+title);
Intent intent = new Intent(learn.this,learn_content.class);
intent.putExtra("title",title);
startActivity(intent);
}
});

上面代码中 print 语句的输出如下：

what did i get from document snapshot? id: EsHW9FGjVe8A3pG4 path:LearningModules/EsHW9FGjVe8A3pG4 title:Front Load vs Top Load

被调用活动的onClick方法如下所示：

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.content_learn_view_pager);
viewPager = findViewById(R.id.pager);
progressBar = findViewById(R.id.progressBarViewPager);
Bundle extras;
extras = getIntent().getExtras();
System.out.println("captured form the first " + extras.getString("title"));

但是，这段代码会为 print 语句抛出错误NullPointerException。

我哪里做错了？

附加代码：

• 适配器

public class LearningModuleAdapter extends FirestoreRecyclerAdapter<LearningModule, LearningModuleAdapter.LearningModuleViewHolder> {
private OnItemClickListener listener;

public LearningModuleAdapter(@NonNull FirestoreRecyclerOptions<LearningModule> options) {
super(options);
}

@Override
protected void onBindViewHolder(@NonNull final LearningModuleViewHolder holder, final int position, @NonNull LearningModule model) {
holder.title.setText(model.getTitle());
//        holder.img.setImageResource(model.getImg());
}
@NonNull
@Override
public LearningModuleViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType){
View v= LayoutInflater.from(parent.getContext()).inflate(R.layout.list_item,parent,false);
return new LearningModuleViewHolder(v);
}

public class LearningModuleViewHolder extends RecyclerView.ViewHolder {
public View view;
public TextView title;
public ImageView img;
public LearningModuleViewHolder(View v) {
super(v);
view = v;
img = itemView.findViewById(R.id.list_image);
title = itemView.findViewById(R.id.list_text_title);
itemView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
System.out.println("The click was captured");
int position = getAdapterPosition();
if(position!=RecyclerView.NO_POSITION && null!=listener){
listener.onItemClick(getSnapshots().getSnapshot(position),position);
}
Intent intent = new Intent(view.getContext(), learn_content.class);
view.getContext().startActivity(intent);
}
});
}
}
public interface OnItemClickListener{
void onItemClick(DocumentSnapshot documentSnapshot,int position);
}
public void setOnItemClickListener(OnItemClickListener listener){
this.listener = listener;
}

}

• 错误堆栈跟踪

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.gigforce.app/com.gigforce.app.learn_content}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.os.Bundle.getString(java.lang.String)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.os.Bundle.getString(java.lang.String)' on a null object reference
at com.gigforce.app.learn_content.onCreate(learn_content.java:45)
at android.app.Activity.performCreate(Activity.java:7802)
at android.app.Activity.performCreate(Activity.java:7791)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
at android.os.Handler.dispatchMessage(Handler.java:107) 
at android.os.Looper.loop(Looper.java:214) 
at android.app.ActivityThread.main(ActivityThread.java:7356)

Answer 1

您调用startActivity两次导致问题。

选项 - 1：尝试删除内部itemView.setOnClickListener如下所示：

itemView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
System.out.println("The click was captured");
int position = getAdapterPosition();
if(position!=RecyclerView.NO_POSITION && null!=listener){
listener.onItemClick(getSnapshots().getSnapshot(position),position);
}
//Intent intent = new Intent(view.getContext(), learn_content.class);
//view.getContext().startActivity(intent);
}
});

选项 - 2：尝试在itemView.setOnClickListener内启动活动并删除OnItemClickListener相关代码，如下所示：

itemView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
System.out.println("The click was captured");
int position = getAdapterPosition();
/*if(position!=RecyclerView.NO_POSITION && null!=listener){
listener.onItemClick(getSnapshots().getSnapshot(position),position);
}*/
Intent intent = new Intent(view.getContext(), learn_content.class);
DocumentSnapshot documentSnapshot = getSnapshots().getSnapshot(position);
String title = (String) documentSnapshot.get("title");
intent.putExtra("title",title);
view.getContext().startActivity(intent);
}
});

Answer 2

试试这个：

Intent i = getIntent()
if (i == null) 
Log.d("***DEBUG****", "Intent was null");
else
Log.d("**** DEBUG ***", "Intent OK");
String title= i.getStringExtra("title"); //Exception points to this line
Log.d("*** DEBUG", rec + " " + title);

同样从项目视图单击您没有传递标题，请参见下文

itemView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
System.out.println("The click was captured");
int position = getAdapterPosition();
if(position!=RecyclerView.NO_POSITION && null!=listener){
listener.onItemClick(getSnapshots().getSnapshot(position),position);
}
Intent intent = new Intent(view.getContext(), 
learn_content.class);
intent.putExtra("title",title);  //This one
view.getContext().startActivity(intent);
}
});

Answer 3

我的替代解决方案：而不是将点击侦听器放在学习模块视图持有人类上，而是放在创建视图持有人上，所以现在视图不会为空。

@NonNull
@Override
public LearningModuleViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType){
View v= LayoutInflater.from(parent.getContext()).inflate(R.layout.list_item,parent,false);
LearningModuleViewHolder holder = new LearningModuleViewHolder(v);
v.setOnClickListener(v -> {
System.out.println("The click was captured");
Intent intent = new Intent(parent.getContext(), learn_content.class);
parent.getContext().startActivity(intent);
});
return holder;
}