由于 java.lang.NullPointerException，当我尝试从一个活动移动到另一个活动时，我的应用程序崩溃了

Question

这是我的第一个安卓项目。我对android开发不太了解。请给我一个详细的解决方案。我正在尝试从我的登录活动转移到 recyclerview 活动，但是在单击登录按钮后，我的应用程序崩溃了。

这是原木猫

    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
    at android.view.LayoutInflater.from(LayoutInflater.java:284)
    at com.example.trackmybusadmin.recyclerAdapter.onCreateViewHolder(recyclerAdapter.java:27)
    at com.example.trackmybusadmin.recyclerAdapter.onCreateViewHolder(recyclerAdapter.java:13)
    at androidx.recyclerview.widget.RecyclerView$Adapter.createViewHolder(RecyclerView.java:7078)
    at androidx.recyclerview.widget.RecyclerView$Recycler.tryGetViewHolderForPositionByDeadline(RecyclerView.java:6235)
    at androidx.recyclerview.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:6118)
    at androidx.recyclerview.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:6114)
    at androidx.recyclerview.widget.LinearLayoutManager$LayoutState.next(LinearLayoutManager.java:2303)
    at androidx.recyclerview.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1627)
    at androidx.recyclerview.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1587)
    at androidx.recyclerview.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:665)
    at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep2(RecyclerView.java:4134)
    at androidx.recyclerview.widget.RecyclerView.dispatchLayout(RecyclerView.java:3851)
    at androidx.recyclerview.widget.RecyclerView.onLayout(RecyclerView.java:4404)
    at android.view.View.layout(View.java:22088)
    at android.view.ViewGroup.layout(ViewGroup.java:6325)
    at androidx.constraintlayout.widget.ConstraintLayout.onLayout(ConstraintLayout.java:1855)
    at android.view.View.layout(View.java:22088)
    at android.view.ViewGroup.layout(ViewGroup.java:6325)
    at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
    at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
    at android.view.View.layout(View.java:22088)
    at android.view.ViewGroup.layout(ViewGroup.java:6325)
    at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1829)
    at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1673)
    at android.widget.LinearLayout.onLayout(LinearLayout.java:1582)
    at android.view.View.layout(View.java:22088)
    at android.view.ViewGroup.layout(ViewGroup.java:6325)
    at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
    at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
    at android.view.View.layout(View.java:22088)
    at android.view.ViewGroup.layout(ViewGroup.java:6325)
    at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1829)
    at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1673)
    at android.widget.LinearLayout.onLayout(LinearLayout.java:1582)
    at android.view.View.layout(View.java:22088)
    at android.view.ViewGroup.layout(ViewGroup.java:6325)
    at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
    at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
    at com.android.internal.policy.DecorView.onLayout(DecorView.java:812)
    at android.view.View.layout(View.java:22088)
    at android.view.ViewGroup.layout(ViewGroup.java:6325)
    at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:3191)
    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2701)
    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1819)
    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7781)
    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1031)
    at android.view.Choreographer.doCallbacks(Choreographer.java:854)
    at android.view.Choreographer.doFrame(Choreographer.java:789)
    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1016)
    at android.os.Handler.handleCallback(Handler.java:883)
    at android.os.Handler.dispatchMessage(Handler.java:100)
    at android.os.Looper.loop(Looper.java:224)
    at android.app.ActivityThread.main(ActivityThread.java:7590)

这是我的 recylerAdapter.java 文件

public class recyclerAdapter extends RecyclerView.Adapter<recyclerAdapter.MyViewHolder> {
String data1[],data2[],data3[];
int img[];
Context context;
public recyclerAdapter(Context ct, String title[],String description[],String count[], int imageicons[]){
data1=title;
data2=description;
data3=count;
img=imageicons;

}
@NonNull
@Override
public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
    LayoutInflater inflater = LayoutInflater.from(context);
    View view = inflater.inflate(R.layout.additembutton,parent,false);
    return new MyViewHolder(view);
}

@Override
public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
    holder.text1.setText(data1[position]);
    holder.text2.setText(data2[position]);
    holder.text3.setText(data3[position]);
    holder.img1.setImageResource(img[position]);

}

@Override
public int getItemCount() {
    return data1.length;
}

public class MyViewHolder extends RecyclerView.ViewHolder{
    TextView text1,text2,text3;
    ImageView img1;
    public MyViewHolder(@NonNull View itemView) {
        super(itemView);
        text1 = itemView.findViewById(R.id.textViewtitle);
        text2 = itemView.findViewById(R.id.textViewdescription);
        text3 = itemView.findViewById(R.id.textViewCount);
        img1 = itemView.findViewById(R.id.imageView);
    }
}

}

这是我的 MainActivity2.java

public class MainActivity2 extends AppCompatActivity {
String title[],description[],count[];
int imageicons[] = {R.drawable.bus,R.drawable.student};
RecyclerView recyclerView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main2);
    title = getResources().getStringArray(R.array.Title);
    description = getResources().getStringArray(R.array.Description);
    count = getResources().getStringArray(R.array.Count);
    recyclerView = findViewById(R.id.recycleview);

    recyclerAdapter ra = new recyclerAdapter(this, title, description,count,imageicons);
    recyclerView.setAdapter(ra);
    recyclerView.setLayoutManager(new LinearLayoutManager(this));


}

}

Answer 1

你应该使用：

LayoutInflater inflater = LayoutInflater.from(parent.getContext());

代替：

LayoutInflater inflater = LayoutInflater.from(context);

Answer 2

似乎您在新活动中有一个回收器视图和 onCreateViewHolder 方法，您尝试使用空上下文来 getSystemService .. 所以如果您可以提供代码为此以及如何从回收器视图适配器获取上下文