我尝试过寻找答案，但我对这个编程领域和使用Android Studio相对较新。首先，这是我在模拟器崩溃时收到的错误。请耐心等待很多代码，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中
<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>
我再次为大量的代码道歉，但我希望对于那些经验丰富的人来说，这可能是显而易见的。提前谢谢。
答案 0 :(得分：1)
关闭错误日志后，您的listviewitem.xml方法看起来好像在getView()，但您引用的小部件（TextView）位于{{1} }}。
修改：嗯，至少有一个是activity_main.xml id
就解决方案而言，请确保您在txtOfficer布局中分配的所有id属性都是唯一的，并确保仅使用xml来查找包含的小部件在调用findViewById()内：
View答案 1 :(得分：1)
@PPartisan 指出的问题是真实存在的问题，但请注意，在activity_main.xml中，您将EditText元素引用为java中的TextView元素代码。