尝试在空对象引用上调用虚拟方法'android.view.View android.widget.EditText.findViewById（int）'

Question

每当我从MainActivity中单击FloatingActionButton时，我的应用程序就会崩溃。我的logcat中出现以下致命错误：

  

java.lang.RuntimeException：无法启动活动   ComponentInfo {com.androidmks.roomvmliverecyclerviewexample / com.androidmks.roomvmliverecyclerviewexample.AddNoteActivity}：   java.lang.NullPointerException：尝试调用虚拟方法   'android.view.View android.widget.EditText.findViewById（int）'在   空对象引用

我看到它来自一个EditText空对象引用，但是我不知道在哪里找到它。

package com.androidmks.roomvmliverecyclerviewexample;

import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.LifecycleOwner;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;
import androidx.lifecycle.ViewModelProviders;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Toast;

import com.google.android.material.floatingactionbutton.FloatingActionButton;

import java.util.List;

public class MainActivity extends AppCompatActivity implements LifecycleOwner {
public static final int ADD_NOTE_REQUEST=1;

    private NoteViewModel noteViewModel;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        FloatingActionButton buttonAddNote = findViewById(R.id.button_add_note);
        buttonAddNote.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(MainActivity.this, AddNoteActivity.class);
                startActivityForResult(intent,ADD_NOTE_REQUEST);
            }
        });

        RecyclerView recyclerView = findViewById(R.id.recycler_viewer);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        recyclerView.setHasFixedSize(true);

        final NoteAdapter adapter = new NoteAdapter();
        recyclerView.setAdapter(adapter);


        noteViewModel = ViewModelProviders.of(this).get(NoteViewModel.class);
        noteViewModel.getAllNotes().observe(this, new Observer<List<Note>>() {
            @Override
            public void onChanged(List<Note> notes) {
               adapter.setNotes(notes);
            }
        });
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if(requestCode== ADD_NOTE_REQUEST && resultCode == RESULT_OK){
            String title = data.getStringExtra(AddNoteActivity.EXTRA_TITLE);
            String description = data.getStringExtra(AddNoteActivity.EXTRA_DESCRIPTION);
            int priority = data.getIntExtra(AddNoteActivity.EXTRA_PRIORITY,1);

            Note note= new Note(title,description,priority);
            noteViewModel.insert(note);

            Toast.makeText(this, "Note Saved", Toast.LENGTH_SHORT).show();
        }else{
            Toast.makeText(this, "Note not Saved", Toast.LENGTH_SHORT).show();

        }
    }
}

AddNoteActivity.java

package com.androidmks.roomvmliverecyclerviewexample;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.widget.EditText;
import android.widget.NumberPicker;
import android.widget.Toast;

public class AddNoteActivity extends AppCompatActivity {
    public static final String EXTRA_TITLE =
            "com.androidmks.roomvmliverecyclerviewexample.EXTRA_TITLE";
    public static final String EXTRA_DESCRIPTION =
            "com.androidmks.roomvmliverecyclerviewexample.EXTRA_DESCRIPTION";
    public static final String EXTRA_PRIORITY =
            "com.androidmks.roomvmliverecyclerviewexample.EXTRA_PRIORITY";
    private EditText editTextTitle;
    private EditText editTextDescription;
    private NumberPicker numberPickerPriority;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_add_note);

        editTextTitle.findViewById(R.id.edit_text_title);
        editTextDescription.findViewById(R.id.edit_text_description);
        numberPickerPriority.findViewById(R.id.number_picker_priority);


        numberPickerPriority.setMinValue(1);
        numberPickerPriority.setMaxValue(10);

        getSupportActionBar().setHomeAsUpIndicator(R.drawable.ic_close);
        setTitle("Add Note");


    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater menuInflater = getMenuInflater();
        menuInflater.inflate(R.menu.add_note_menu, menu);
        return true;
    }

    private void saveNote() {
        String title = editTextTitle.getText().toString();
        String description = editTextDescription.getText().toString();
        int priority = numberPickerPriority.getValue();

        if (title.trim().isEmpty() || description.trim().isEmpty()) {
            Toast.makeText(this, "Please insert a title and description", Toast.LENGTH_SHORT).show();
            return;
        }
        Intent data = new Intent();
        data.putExtra(EXTRA_TITLE, title);
        data.putExtra(EXTRA_DESCRIPTION, description);
        data.putExtra(EXTRA_PRIORITY, priority);

        setResult(RESULT_OK, data);
        finish();
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case R.id.save_note:
                saveNote();
                return true;
            default:
                return super.onOptionsItemSelected(item);
        }
    }
}

activity_add_note.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="16dp"
    tools:context=".AddNoteActivity">

    <EditText
        android:id="@+id/edit_text_title"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Title"
        android:inputType="text"/>
    <EditText
        android:id="@+id/edit_text_description"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Description"
        android:inputType="textMultiLine"/>
    <TextView

        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Priority:"
        android:layout_marginTop="8dp"
        android:textAppearance="@style/TextAppearance.AppCompat.Medium"/>
    <NumberPicker
        android:id="@+id/number_picker_priority"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"></NumberPicker>
</LinearLayout>

LOGCAT

  

'2019-05-26 17：46：35.894   22330-22330 / com.androidmks.roomvmliverecyclerviewexample   E / AndroidRuntime：致命异常：main       流程：com.androidmks.roomvmliverecyclerviewexample，PID：22330       java.lang.RuntimeException：无法启动活动ComponentInfo {com.androidmks.roomvmliverecyclerviewexample / com.androidmks.roomvmliverecyclerviewexample.AddNoteActivity}：   java.lang.NullPointerException：尝试调用虚拟方法   'android.view.View android.widget.EditText.findViewById（int）'在   空对象引用           在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2913）           在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:3048）           在android.app.servertransaction.LaunchActivityItem.execute（LaunchActivityItem.java:78）           在android.app.servertransaction.TransactionExecutor.executeCallbacks（TransactionExecutor.java:108）           在android.app.servertransaction.TransactionExecutor.execute（TransactionExecutor.java:68）           在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1808）           在android.os.Handler.dispatchMessage（Handler.java:106）           在android.os.Looper.loop（Looper.java:193）           在android.app.ActivityThread.main（ActivityThread.java:6669）           在java.lang.reflect.Method.invoke（本机方法）           在com.android.internal.os.RuntimeInit $ MethodAndArgsCaller.run（RuntimeInit.java:493）           在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:858）        原因：java.lang.NullPointerException：尝试调用虚拟方法'android.view.View   空对象引用上的android.widget.EditText.findViewById（int）'           在com.androidmks.roomvmliverecyclerviewexample.AddNoteActivity.onCreate（AddNoteActivity.java:30）           在android.app.Activity.performCreate（Activity.java:7136）           在android.app.Activity.performCreate（Activity.java:7127）           在android.app.Instrumentation.callActivityOnCreate（Instrumentation.java:1271）           在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2893）           在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:3048）           在android.app.servertransaction.LaunchActivityItem.execute（LaunchActivityItem.java:78）           在android.app.servertransaction.TransactionExecutor.executeCallbacks（TransactionExecutor.java:108）           在android.app.servertransaction.TransactionExecutor.execute（TransactionExecutor.java:68）           在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1808）           在android.os.Handler.dispatchMessage（Handler.java:106）           在android.os.Looper.loop（Looper.java:193）           在android.app.ActivityThread.main（ActivityThread.java:6669）           在java.lang.reflect.Method.invoke（本机方法）           在com.android.internal.os.RuntimeInit $ MethodAndArgsCaller.run（RuntimeInit.java:493）           在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:858）'