我正在为大学的一门课开发一个简单的观鸟日志应用程序，但我似乎无法发现到底出了什么问题，所以在花了几天时间把头撞到墙上后，我想，让有一双新眼睛的人来看看会更有意义。
该应用程序非常简单 - 我有两个 Activity ，一个带有 ListView，由数据库填充(MainActivity)，另一个则采用参数来创建新条目(AddActivity)。除了这些之外，我还有一个 DBHelper (MySQLiteHelper) 来处理我的数据库调用，一个简单的类来描述我的对象应该是什么样子 (Sighting)，一个数据源类 (SightingDataSource) 来处理促进对帮助程序的调用以及一堆不相关的资源(布局等)。AddActivity 在调用 addSighting 方法时会因空指针异常而崩溃。无论出于何种原因，它也不喜欢 bCount 字段的 int 类型转换，因此出于测试目的，我设置了一个静态 int，同时我找出了问题的异常部分。
这是我的来源: 添加 Activity :
package com.vladislavtachev.cscb763;
import android.app.ListActivity;
import android.os.Bundle;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
public class AddActivity extends ListActivity {
public SightingsDataSource datasource;
private EditText locFld = null;
private EditText bNameFld = null;
private EditText bCountFld = null;
private Button addSightBtn = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_add);
locFld = (EditText) findViewById(R.id.locFld);
bNameFld = (EditText) findViewById(R.id.bNameFld);
bCountFld = (EditText) findViewById(R.id.bCountFld);
setContentView(R.layout.activity_add);
addSightBtn = (Button) findViewById(R.id.addSightBtn);
addSightBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
addSight(v);
}
});
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if (id == R.id.action_settings) { return true; }
return super.onOptionsItemSelected(item);
}
public void addSight(View view){
long time = System.currentTimeMillis()/1000L;
datasource.addSighting(locFld.getText().toString(),
time,
bNameFld.getText().toString(),3
// Integer.parseInt(bCountFld.getText().toString())
);
// datasource.addSighting(sighting);
setContentView(R.layout.activity_main);
}
}
SightingDataSource
package com.vladislavtachev.cscb763;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
public class SightingsDataSource {
//DB info
private SQLiteDatabase database;
private MySQLiteHelper dbHelper;
private String[] allColumns = { MySQLiteHelper.COL_ID, MySQLiteHelper.COL_LOCATION,
MySQLiteHelper.COL_TIME, MySQLiteHelper.COL_BNAME, MySQLiteHelper.COL_BCOUNT };
public SightingsDataSource(Context context){
dbHelper = new MySQLiteHelper(context);
}
public void open(){
database = dbHelper.getWritableDatabase();
}
public void close(){ dbHelper.close(); }
public void addSighting(String l, long time, String bName, int bCount){
ContentValues values = new ContentValues();
values.put(MySQLiteHelper.COL_LOCATION, l);
values.put(MySQLiteHelper.COL_TIME, time);
values.put(MySQLiteHelper.COL_BNAME, bName);
values.put(MySQLiteHelper.COL_BCOUNT, bCount);
long insertId = database.insert(MySQLiteHelper.TABLE_SIGHTINGS, null, values);
Cursor cursor = database.query(MySQLiteHelper.TABLE_SIGHTINGS, allColumns, MySQLiteHelper.COL_ID + " = " + insertId, null, null, null, null);
cursor.moveToFirst();
Sighting newSighting = cursorToSighting(cursor);
cursor.close();
}
private Sighting cursorToSighting(Cursor cursor){
Sighting sighting = new Sighting();
sighting.setId(cursor.getLong(0));
sighting.setLocation(cursor.getString(1));
sighting.setTime(cursor.getLong(2));
sighting.setbName(cursor.getString(3));
sighting.setbCount(cursor.getInt(4));
return sighting;
}
public List<Sighting> getAllSightings(){
List<Sighting> sightings = new ArrayList<Sighting>();
Cursor cursor = database.query(MySQLiteHelper.TABLE_SIGHTINGS, allColumns, null, null, null, null, null);
cursor.moveToFirst();
while (!cursor.isAfterLast()) {
Sighting sighting = cursorToSighting(cursor);
sightings.add(sighting);
cursor.moveToNext();
}
cursor.close();
return sightings;
}
}
MySQLiteHelper
package com.vladislavtachev.cscb763;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.sqlite.SQLiteDatabase;
import android.content.Context;
public class MySQLiteHelper extends SQLiteOpenHelper{
public final static String DB_NAME = "sightings.db";
public final static int DB_VER = 1;
public final static String TABLE_SIGHTINGS = "sightings";
public final static String COL_ID = "_id";
public final static String COL_LOCATION = "_loc";
public final static String COL_TIME = "_time";
public final static String COL_BNAME = "_bname";
public final static String COL_BCOUNT = "_bcount";
private final static String DB_CREATE = "create table if not exists"
+ TABLE_SIGHTINGS + "("
+ COL_ID + " integer primary key autoincrement, "
+ COL_LOCATION + " text not null, "
+ COL_TIME + " integer not null, "
+ COL_BNAME + " text not null, "
+ COL_BCOUNT + " integer not null);";
public MySQLiteHelper(Context context){
super(context, DB_NAME, null, DB_VER);
}
@Override
public void onCreate(SQLiteDatabase database){
database.execSQL(DB_CREATE);
}
@Override
public void onUpgrade(SQLiteDatabase database, int oldVer, int newVer){
database.execSQL("DROP TABLE IF EXISTS " + TABLE_SIGHTINGS);
onCreate(database);
}
}
activity_add.xml布局
<RelativeLayout 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"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context="com.vladislavtachev.cscb763.AddActivity"
style="@android:style/Theme.Material">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceMedium"
android:text="@string/lblBName"
android:id="@+id/lblBName"
android:layout_alignParentTop="true"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true" />
<EditText
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/bNameFld"
android:layout_below="@+id/lblBName"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceMedium"
android:text="@string/lblBCount"
android:id="@+id/lblBCount"
android:layout_alignBottom="@+id/bCountFld"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true" />
<EditText
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:inputType="number"
android:ems="10"
android:id="@+id/bCountFld"
android:layout_below="@+id/bNameFld"
android:layout_alignRight="@+id/bNameFld"
android:layout_alignEnd="@+id/bNameFld"
android:layout_toEndOf="@+id/lblBCount"
android:layout_toRightOf="@+id/lblBCount" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceMedium"
android:text="@string/lblLocation"
android:id="@+id/lblLocation"
android:layout_alignBottom="@+id/locFld"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true" />
<EditText
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/locFld"
android:layout_below="@+id/bCountFld"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true"
android:layout_toEndOf="@+id/lblLocation"
android:layout_toRightOf="@+id/lblLocation" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/addSightBtn"
android:id="@+id/addSightBtn"
android:layout_centerVertical="true"
android:layout_centerHorizontal="true"/>
<ListView
android:layout_width="10px"
android:layout_height="10px"
android:id="@android:id/list"
android:layout_alignParentBottom="true"
android:layout_alignRight="@+id/locFld"
android:layout_alignEnd="@+id/locFld" />
</RelativeLayout>
错误堆栈
07-01 18:08:49.131 2384-2384/com.vladislavtachev.cscb763 E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.vladislavtachev.cscb763, PID: 2384
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.vladislavtachev.cscb763.SightingsDataSource.addSighting(java.lang.String, long, java.lang.String, int)' on a null object reference
at com.vladislavtachev.cscb763.AddActivity.addSight(AddActivity.java:75)
at com.vladislavtachev.cscb763.AddActivity$1.onClick(AddActivity.java:38)
at android.view.View.performClick(View.java:4780)
at android.view.View$PerformClick.run(View.java:19866)
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.ActivityThread.main(ActivityThread.java:5254)
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:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
感谢所有帮助!
干杯， 五、/
最佳答案
datasource 从未在 AddActivity 中初始化。您还调用 setContentView 两次。删除第二个。
关于java - 添加到数据库时出现空指针异常，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/31172551/