Firebase 回收器适配器显示空指针异常

Question

Firebase Recycler 视图

FirebaseRecyclerAdapter<Post, MyViewHolder> fadapter = new FirebaseRecyclerAdapter<Post, MyViewHolder>
(Post.class, R.layout.card_row, MyViewHolder.class, mDatabase) {
@Override
protected void populateViewHolder(MyViewHolder viewHolder, Post model, int position) {
}
};

视图持有人类

public static class MyViewHolder extends RecyclerView.ViewHolder {
View mview;
public MyViewHolder(View itemView) {
super(itemView);
//mview = itemView;
}
}

帖子.java

public class Post {
private String title;
private String image;
private String desc;
public Post(String title, String image, String desc) {
this.title = title;
this.image = image;
this.desc = desc;
}
public Post() {
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getImage() {
return image;
}
public void setImage(String image) {
this.image = image;
}
public String getDesc() {
return desc;
}
public void setDesc(String desc) {
this.desc = desc;
}

}

我收到此错误！

FATAL EXCEPTION: main
Process: cf.socialstack.socialstack, PID: 22343                                                                            java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{cf.socialstack.socialstack/cf.socialstack.socialstack.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database.ChildEventListener com.google.firebase.database.Query.addChildEventListener(com.google.firebase.database.ChildEventListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
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:898)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database.ChildEventListener com.google.firebase.database.Query.addChildEventListener(com.google.firebase.database.ChildEventListener)' on a null object reference
at com.firebase.ui.database.FirebaseArray.onCreate(FirebaseArray.java:69)
at com.firebase.ui.database.ObservableSnapshotArray.addChangeEventListener(ObservableSnapshotArray.java:64)
at com.firebase.ui.database.FirebaseRecyclerAdapter.startListening(FirebaseRecyclerAdapter.java:126)
at com.firebase.ui.database.FirebaseRecyclerAdapter.<init>(FirebaseRecyclerAdapter.java:71)
at com.firebase.ui.database.FirebaseRecyclerAdapter.<init>(FirebaseRecyclerAdapter.java:86)
at com.firebase.ui.database.FirebaseRecyclerAdapter.<init>(FirebaseRecyclerAdapter.java:108)
at cf.socialstack.socialstack.MainActivity$2.<init>(MainActivity.java:0)
at cf.socialstack.socialstack.MainActivity.<init>(MainActivity.java:134)
at java.lang.reflect.Constructor.newInstance(Native Method)
at java.lang.Class.newInstance(Class.java:1572)
at android.app.Instrumentation.newActivity(Instrumentation.java:1065)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) 
at android.app.ActivityThread.access$800(ActivityThread.java:144) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:135) 
at android.app.ActivityThread.main(ActivityThread.java:5221) 
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:898) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693) `

Answer 1

也许你的 DatabaseReference 返回 null，这就是你得到 NullPointException 的原因。

您确定您的数据库引用指向某个 url 时，该 url 实际上在您的回收器视图中有一些要填写的数据？

Answer 2

我不知道你想实现什么，但这是使用实时数据库实现FirebaseRecyclerView的正确方法。

首先创建查询以加载数据。

Query query = FirebaseDatabase.getInstance() .getReference().child("posts").limitToLast(50);

创建 FirebaseRecyclerView 选项的选项。

FirebaseRecyclerOptions<Post> options = new FirebaseRecyclerOptions.Builder<Post>().setQuery(query, Post.class).build();

创建适配器

FirebaseRecyclerAdapter adapter = new FirebaseRecyclerAdapter<Post, MyViewHolder>(options){
@Override 
public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) { // Create a new instance of the ViewHolder, in this case we are using a custom // layout called R.layout.message for each item 
View view = LayoutInflater.from(parent.getContext()) .inflate(R.layout.message, parent, false);
return new MyViewHolder(view);
}
@Override 
protected void onBindViewHolder(MyViewHolder holder, int position, Post model) { // Bind the Post object to the MyViewHolder // ...  
} 
};

就像使用recyclerView.setAdapter(adapter)将此适配器连接到回收器视图一样简单，并设置布局管理器。

Answer 3

将回收器适配器放在onCreate方法中