java - 为什么我的 "Notes"选项卡崩溃了？

Question

我尝试寻找答案，但我对这个编程领域和使用 Android Studio 还比较陌生。首先，这是模拟器崩溃时收到的错误。请耐心等待，因为有很多代码，这是我第一次发帖。

--------- beginning of crash
08-16 16:59:46.930    1861-1861/com.example.richard.stopandsearch E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.richard.stopandsearch, PID: 1861
    java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
            at com.example.richard.stopandsearch.MainActivity$NotesListAdapter.getView(MainActivity.java:157)
            at android.widget.AbsListView.obtainView(AbsListView.java:2347)
            at android.widget.ListView.measureHeightOfChildren(ListView.java:1270)
            at android.widget.ListView.onMeasure(ListView.java:1182)
            at android.view.View.measure(View.java:17547)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
            at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
            at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
            at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
            at android.view.View.measure(View.java:17547)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
            at android.view.View.measure(View.java:17547)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
            at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
            at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
            at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
            at android.view.View.measure(View.java:17547)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
            at android.view.View.measure(View.java:17547)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
            at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
            at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
            at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
            at android.view.View.measure(View.java:17547)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
            at android.support.v7.internal.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:124)
            at android.view.View.measure(View.java:17547)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
            at android.support.v7.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:444)
            at android.view.View.measure(View.java:17547)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
            at android.view.View.measure(View.java:17547)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
            at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
            at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
            at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
            at android.view.View.measure(View.java:17547)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
            at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2615)
            at android.view.View.measure(View.java:17547)
            at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2015)
            at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1173)
            at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1379)
            at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1061)
            at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5885)
            at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
            at android.view.Choreographer.doCallbacks(Choreographer.java:580)
            at android.view.Choreographer.doFrame(Choreographer.java:550)
            at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
            at android.os.Handler.handleCallback(Handler.java:739)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.Activ

MainActivity.Java

package com.example.richard.stopandsearch;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.TabHost;
import android.widget.TextView;
import android.widget.Toast;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    EditText LocationTxt, TimeTxt, DateTxt, OfficerTxt, DetailsTxt;
    List<Notes> Notes = new ArrayList<Notes>();
    ListView notesListView;
    databaseHandler dbHandler;



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

        LocationTxt = (EditText) findViewById(R.id.txtLocation);
        TimeTxt = (EditText) findViewById(R.id.txtTime);
        DateTxt = (EditText) findViewById(R.id.txtDate);
        OfficerTxt = (EditText) findViewById(R.id.txtOfficer);
        DetailsTxt = (EditText) findViewById(R.id.txtDetails);
        notesListView = (ListView) findViewById(R.id.listView);
        dbHandler = new databaseHandler(getApplicationContext());
        TabHost tabHost = (TabHost) findViewById(R.id.tabHost);

        tabHost.setup();

        TabHost.TabSpec tabSpec = tabHost.newTabSpec("addNote");
        tabSpec.setContent(R.id.tabAddNote);
        tabSpec.setIndicator("AddNote");
        tabHost.addTab(tabSpec);

        tabSpec = tabHost.newTabSpec("notes");
        tabSpec.setContent(R.id.tabNotes);
        tabSpec.setIndicator("Notes");
        tabHost.addTab(tabSpec);

        tabSpec = tabHost.newTabSpec("info");
        tabSpec.setContent(R.id.tabInfo);
        tabSpec.setIndicator("Info");
        tabHost.addTab(tabSpec);

        final Button addBtn = (Button) findViewById(R.id.btnAdd);
        addBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Notes notes = new Notes(dbHandler.getNotesCount(), String.valueOf(LocationTxt.getText()), String.valueOf(TimeTxt.getText()), String.valueOf(DateTxt.getText()), String.valueOf(OfficerTxt.getText()), String.valueOf(DetailsTxt.getText()));
              if (!noteExists(notes)) {
                  // Notes.add (new Notes(0, LocationTxt.getText().toString(), TimeTxt.getText().toString(), DateTxt.getText().toString(), OfficerTxt.getText().toString(), DetailsTxt.getText().toString()));
                  dbHandler.createNote(notes);
                  Notes.add(notes);

                  // populateList();
                  Toast.makeText(getApplicationContext(), "Your note has been added.", Toast.LENGTH_SHORT).show();
                  return;
              }
                Toast.makeText(getApplicationContext(), String.valueOf(LocationTxt.getText()) + " already exists. Please enter a different location.", Toast.LENGTH_SHORT).show();
            }
        });

        DetailsTxt.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {
                addBtn.setEnabled(String.valueOf(DetailsTxt.getText()).trim().length() >0);

           // @Override
          //  public void afterTextChanged(Editable s) {

            }

            @Override
            public void afterTextChanged(Editable s) {

            }
        });

       // List<Notes> addableNotes = dbHandler.getAllNotes();
       // int noteCount = dbHandler.getNotesCount();

       // for (int i=0; i < noteCount; i++) {
       //     Notes.add(addableNotes.get(i));
       // }
        if (dbHandler.getNotesCount()!=0)
        Notes.addAll(dbHandler.getAllNotes());

       // if (!addableNotes.isEmpty())
            populateList();
    }

    private boolean noteExists(Notes notes) {
        String location = notes.getLocation();
        int notesCount = Notes.size();

        for (int i=0; i < notesCount; i++) {
            if(location.compareToIgnoreCase(Notes.get(i).getLocation()) ==0)
                return true;
        }
        return false;
    }

    private void populateList() {
        ArrayAdapter<Notes> adapter = new NotesListAdapter();
        notesListView.setAdapter(adapter);
    }


   // private void addNotes(String location, String time, String date, String officer, String details) {
        //Notes.add(new Notes(location, time, date, officer, details));

  //  }

    public class NotesListAdapter extends ArrayAdapter<Notes> {
        public NotesListAdapter() {

            super(MainActivity.this, R.layout.listviewitem, Notes);
        }

    @Override
    public View getView(int position, View view, ViewGroup parent) {
        if (view == null)
            view = getLayoutInflater().inflate(R.layout.listviewitem, parent, false);

        Notes currentNote = Notes.get(position);

        TextView location = (TextView) view.findViewById(R.id.txtLocationView);
        location.setText(currentNote.getLocation());
        TextView time = (TextView) view.findViewById(R.id.txtTimeView);
        time.setText (currentNote.getLocation());
        TextView date = (TextView) view.findViewById(R.id.txtDateView);
        date.setText (currentNote.getLocation());
        TextView officer = (TextView) view.findViewById(R.id.txtOfficer);
        officer.setText(currentNote.getLocation());
        TextView details = (TextView) view.findViewById(R.id.txtDetailsView);
        details.setText(currentNote.getLocation());

        return view;
    }

}

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

databaseHandler.java

package com.example.richard.stopandsearch;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

import java.util.ArrayList;
import java.util.List;

public class databaseHandler extends SQLiteOpenHelper {

    private static final int DATABASE_VERSION = 1;
    private static final String DATABASE_NAME = "notes",
    TABLE_NOTES = "notes",
    KEY_ID = "id",
    KEY_LOCATION = "location",
    KEY_TIME = "time",
    KEY_DATE = "date",
    KEY_OFFICER = "officer",
    KEY_DETAILS = "details";

    public databaseHandler(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);

    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE " + TABLE_NOTES + "(" + KEY_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " + KEY_LOCATION + " TEXT, " + KEY_TIME + " TEXT, " + KEY_DATE + " TEXT, " + KEY_OFFICER + " TEXT, " + KEY_DETAILS + " TEXT)");
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_NOTES);
        onCreate(db);
    }

    public void createNote(Notes notes) {
        SQLiteDatabase db = getWritableDatabase();

        ContentValues values = new ContentValues();

        values.put(KEY_LOCATION, notes.getLocation());
        values.put(KEY_TIME, notes.getTime());
        values.put(KEY_DATE, notes.getDate());
        values.put(KEY_OFFICER, notes.getOfficer());
        values.put(KEY_DETAILS, notes.getDetails());

        db.insert(TABLE_NOTES, null, values);
        db.close();
    }

    public Notes getNotes(int id) {
        SQLiteDatabase db = getReadableDatabase();

        Cursor cursor = db.query(TABLE_NOTES, new String[] { KEY_ID, KEY_LOCATION, KEY_TIME, KEY_DATE, KEY_OFFICER, KEY_DETAILS }, KEY_ID + "=?", new String [] {String.valueOf(id) }, null, null, null, null);
        if (cursor != null)
            cursor.moveToFirst();
        assert cursor != null;
        Notes notes = new Notes(Integer.parseInt(cursor.getString(0)),cursor.getString(1),cursor.getString(2),cursor.getString(3), cursor.getString(4), cursor.getString(5));
        db.close();
        cursor.close();
        return notes;
    }


    public void deleteNote(Notes notes) {
        SQLiteDatabase db = getWritableDatabase();
        db.delete(TABLE_NOTES, KEY_ID + "=?", new String[] {String.valueOf(notes.getId())});
        db.close();
    }

    public int getNotesCount () {
        SQLiteDatabase db = getReadableDatabase();
        Cursor cursor = db.rawQuery("SELECT * FROM " + TABLE_NOTES, null);
        int count = cursor.getCount();
        cursor.close();
        db.close();

        return count;
    }

    public int updateNotes(Notes notes) {
        SQLiteDatabase db = getWritableDatabase();

        ContentValues values = new ContentValues();

        values.put(KEY_LOCATION, notes.getLocation());
        values.put(KEY_TIME, notes.getTime());
        values.put(KEY_DATE, notes.getDate());
        values.put(KEY_OFFICER, notes.getOfficer());
        values.put(KEY_DETAILS, notes.getDetails());

        int rowsAffected = db.update(TABLE_NOTES, values, KEY_ID + "=?", new String[]{String.valueOf(notes.getId())});
db.close();

        return rowsAffected;

    }


    public List<Notes> getAllNotes() {
        List<Notes> notes = new ArrayList<Notes>();

        SQLiteDatabase db = getWritableDatabase();
        Cursor cursor = db.rawQuery("SELECT * FROM " + TABLE_NOTES, null);

        if (cursor.moveToFirst()) {

            do {
                notes.add(new Notes(Integer.parseInt(cursor.getString(0)),cursor.getString(1),cursor.getString(2),cursor.getString(3), cursor.getString(4), cursor.getString(5)));

            }
            while (cursor.moveToNext());
        }
        cursor.close();
        db.close();
        return notes;
    }

}

Notes.Java

package com.example.richard.stopandsearch;

public class Notes {

    private String _location, _time, _date, _officer, _details;
    private int _id;

    public Notes (int id, String location, String time, String date, String officer, String details) {
        _id = id;
        _location = location;
        _time = time;
        _date = date;
        _officer = officer;
        _details = details;
    }

    public int getId() {return _id; }
    public String getLocation() {
        return _location;
    }
    public String getTime() {
        return _time;
    }
    public String getDate() {
        return _date;
    }
    public String getOfficer() {
        return _officer;
    }
    public String getDetails() {
        return _details;
    }


}

Activity_Main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:orientation="vertical"
    android:background="#f9f9f9">

    <TabHost
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/tabHost">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">

            <TabWidget
                android:id="@android:id/tabs"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"></TabWidget>

            <FrameLayout
                android:id="@android:id/tabcontent"
                android:layout_width="match_parent"
                android:layout_height="match_parent">

                <LinearLayout
                    android:id="@+id/tabInfo"
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:orientation="vertical">

                    <ImageView
                        android:layout_width="fill_parent"
                        android:layout_height="wrap_content"
                        android:id="@+id/imgLogo3"
                        android:layout_gravity="center_horizontal"
                        android:src="@drawable/policelogo" />

                    <TextView
                        android:layout_width="wrap_content"
                        android:layout_height="wrap_content"
                        android:textAppearance="?android:attr/textAppearanceLarge"
                        android:text="Information"
                        android:id="@+id/txtInfo"
                        android:textColor="#1b0f3e"
                        android:layout_gravity="center_horizontal"
                        android:layout_marginTop="16dp" />
                </LinearLayout>

                <LinearLayout
                    android:id="@+id/tabNotes"
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:orientation="vertical">

                    <ImageView
                        android:layout_width="wrap_content"
                        android:layout_height="wrap_content"
                        android:id="@+id/imgLogo"
                        android:layout_gravity="center_horizontal"
                        android:src="@drawable/policelogo" />

                    <TextView
                        android:layout_width="wrap_content"
                        android:layout_height="wrap_content"
                        android:textAppearance="?android:attr/textAppearanceLarge"
                        android:text="Notes"
                        android:id="@+id/txtNotes"
                        android:layout_marginTop="16dp"
                        android:layout_gravity="center_horizontal"
                        android:singleLine="false"
                        android:textColor="#1b0f3e" />

                    <ListView
                        android:layout_width="match_parent"
                        android:layout_height="wrap_content"
                        android:id="@+id/listView"
                        android:layout_gravity="center_horizontal" />
                </LinearLayout>

                <LinearLayout
                    android:id="@+id/tabAddNote"
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:orientation="vertical">

                    <ImageView
                        android:layout_width="fill_parent"
                        android:layout_height="wrap_content"
                        android:id="@+id/imgLogo2"
                        android:layout_gravity="center_horizontal"
                        android:src="@drawable/policelogo" />

                    <TextView
                        android:layout_width="wrap_content"
                        android:layout_height="wrap_content"
                        android:textAppearance="?android:attr/textAppearanceLarge"
                        android:text="Add Note"
                        android:id="@+id/lblNotesTitle"
                        android:layout_gravity="center_horizontal"
                        android:layout_marginTop="10dp"
                        android:textColor="#1b0f3e" />

                    <EditText
                        android:layout_width="fill_parent"
                        android:layout_height="wrap_content"
                        android:inputType="textPersonName"
                        android:ems="10"
                        android:id="@+id/txtLocation"
                        android:layout_marginTop="16dp"
                        android:hint="Location"
                        android:textColor="#1b0f3e" />

                    <EditText
                        android:layout_width="fill_parent"
                        android:layout_height="wrap_content"
                        android:inputType="time"
                        android:ems="10"
                        android:id="@+id/txtTime"
                        android:hint="Time"
                        android:layout_marginTop="16dp"
                        android:textColor="#1b0f3e" />

                    <EditText
                        android:layout_width="fill_parent"
                        android:layout_height="wrap_content"
                        android:inputType="date"
                        android:ems="10"
                        android:id="@+id/txtDate"
                        android:hint="Date"
                        android:layout_marginTop="16dp"
                        android:textColor="#1b0f3e" />

                    <EditText
                        android:layout_width="fill_parent"
                        android:layout_height="wrap_content"
                        android:id="@+id/txtOfficer"
                        android:hint="Officer name/number"
                        android:layout_marginTop="16dp"
                        android:textColor="#1b0f3e" />

                    <EditText
                        android:layout_width="fill_parent"
                        android:layout_height="wrap_content"
                        android:id="@+id/txtDetails"
                        android:hint="Event details"
                        android:layout_marginTop="16dp"
                        android:lines="4"
                        android:textColor="#1b0f3e" />

                    <Button
                        android:layout_width="200dp"
                        android:layout_height="wrap_content"
                        android:text="Add Note"
                        android:id="@+id/btnAdd"
                        android:layout_gravity="center_horizontal"
                        android:layout_marginTop="16dp"
                        android:enabled="false"
                        android:textColor="#1b0f3e" />
                </LinearLayout>

            </FrameLayout>
        </LinearLayout>
    </TabHost>

</LinearLayout>

我再次对大量代码表示歉意，但我希望有一些东西对于有经验的人来说可能是显而易见的，但我却无法理解。提前致谢。

Answer 1

从错误日志来看，看起来好像您在 getView() 方法中膨胀了 listviewitem.xml，但是小部件 (TextViews) 您引用的内容位于 activity_main.xml 中。

编辑:嗯，至少有一个是，带有 id txtOfficer

就分辨率而言，请确保您在 xml 布局中分配的所有 id 属性都是唯一的，并确保您仅使用 findViewById() 定位调用 View 中包含的小部件:

view = getLayoutInflater().inflate(R.layout.listviewitem, parent, false);
view.findViewById(*id of a widget inside listviewitem.xml*)