当我尝试将我的意图加载到一个处理我数据库中数据列表视图的类时，我得到一个'NullPointerException'。对于listviews来说，我是个新手，所以希望有人可以告诉我哪里出了问题！
我原以为空指针是因为没有数据在光标所在的第一行位置找到，但我认为正在查看logcat，这是一个问题我的XML布局'条目'？
继承我的ListView：
package com.example.sqliteexample;
import android.app.Activity;
import android.app.ListActivity;
import android.database.Cursor;
import android.os.Bundle;
import android.support.v4.widget.SimpleCursorAdapter;
import android.widget.ListView;
public class SQLView extends Activity {
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
HotOrNot H = new HotOrNot(this, null, null);
setContentView(R.layout.layout);
ListView listContent = (ListView)findViewById(R.id.contentList);
HotOrNot Content = new HotOrNot(this, null, null);
Cursor cursor = Content.getData();
startManagingCursor(cursor);
@SuppressWarnings("static-access")
String [] from = new String [] {H.KEY_NAME, H.KEY_HOTNESS};
int [] to = new int [] {R.id.txtName, R.id.txtAge};
@SuppressWarnings("deprecation")
SimpleCursorAdapter cursorAdapter = new SimpleCursorAdapter(this, R.layout.entries, cursor, from, to);
listContent.setAdapter(cursorAdapter);
我的数据库处理程序类中的'getData'方法：
public Cursor getData() {
String [] columns = new String[]{KEY_ROWID, KEY_NAME, KEY_HOTNESS};
Cursor c = ourDatabase.query(DATABASE_TABLE, columns, null, null, null, null, null);
return c;
HotOrNot数据库处理程序类：
package com.example.sqliteexample;
import android.app.ListActivity;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.widget.ListView;
public class HotOrNot {
public static final String KEY_ROWID = "_id";
public static final String KEY_NAME = "persons_name";
public static final String KEY_HOTNESS = "persons_hotness";
private static String DATABASE_NAME = "HotOrNotdb";
private static String DATABASE_TABLE = "peopleTable";
private static int DATABASE_VERSION = 30;
private static DBHelper ourHelper;
private static Context ourContext;
private static SQLiteDatabase ourDatabase;
ListView listview;
public void DB_NAME(String DBName)
{
DATABASE_NAME = DBName;
}
public String returnDB_NAME()
{
return DATABASE_NAME;
}
public void DB_tableNAME(String DBtName)
{
DATABASE_TABLE = DBtName;
}
public String returnDB_tNAME()
{
return DATABASE_TABLE;
}
public void DB_NAME(int DBVersion)
{
DATABASE_VERSION = DBVersion;
}
public int returnDB_VERSION()
{
return DATABASE_VERSION;
}
// class constructor for context. when the object is constructed in the main programme
// the context of that class i.e 'this' is sent to this constructor to set the object context.
// null values set on SQLiteDatabase and DBhelper as there is nothing to pass from the called
// objects.
public HotOrNot (Context c, SQLiteDatabase newSQL, DBHelper BD)
{
ourContext = c;
ourDatabase = newSQL;
ourHelper = BD;
}
private static class DBHelper extends SQLiteOpenHelper
{
public DBHelper(Context context) {
super(context, KEY_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("CREATE TABLE " + DATABASE_TABLE + " (" +
KEY_ROWID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
KEY_NAME + " TEXT NOT NULL, " +
KEY_HOTNESS + " TEXT NOT NULL);"
);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE);
onCreate(db);
}
}
public HotOrNot open() throws SQLException
{
ourHelper = new DBHelper(ourContext);
ourDatabase = ourHelper.getWritableDatabase();
return this;
}
public void close()
{
ourHelper.close();
}
public long createEntry(String name, String hotness) {
ContentValues cv = new ContentValues();
cv.put(KEY_NAME, name);
cv.put(KEY_HOTNESS, hotness);
return ourDatabase.insert(DATABASE_TABLE, null, cv);
}
public Cursor getData() {
String [] columns = new String[]{KEY_ROWID, KEY_NAME, KEY_HOTNESS};
Cursor c = ourDatabase.query(DATABASE_TABLE, columns, null, null, null, null, null);
//String result = "";
//int iRow = c.getColumnIndex(KEY_ROWID);
//int iName = c.getColumnIndex(KEY_NAME);
//int ihotness = c.getColumnIndex(KEY_HOTNESS);
//for(c.moveToFirst(); !c.isAfterLast(); c.moveToNext())
//{
//result = result + c.getString(iRow) + "" + c.getString(iName) + "" + c.getString(ihotness);
//}
//c.close();
return c;
}
public String getName(long l) {
String [] columns = new String[]{KEY_ROWID, KEY_NAME, KEY_HOTNESS};
Cursor c = ourDatabase.query(DATABASE_TABLE, columns, KEY_ROWID + "=" + l, null, null, null, null);
if(c != null)
{
// move to the selected row
c.moveToFirst();
String name = c.getString(1);
return name;
}
return null;
}
public String getHotness(long l) {
String [] columns = new String[]{KEY_ROWID, KEY_NAME, KEY_HOTNESS};
Cursor c = ourDatabase.query(DATABASE_TABLE, columns, KEY_ROWID + "=" + l, null, null, null, null);
if(c != null)
{
// move to the selected row
c.moveToFirst();
String hotness = c.getString(2);
return hotness;
}
return null;
}
public void updateEntry(long newl, String nameEdit, String hotnessEdit) {
ContentValues cvUpdate = new ContentValues();
cvUpdate.put(KEY_NAME, nameEdit);
cvUpdate.put(KEY_HOTNESS, hotnessEdit);
ourDatabase.update(DATABASE_TABLE, cvUpdate, KEY_ROWID + "=" + newl, null);
}
public void delID(long l) {
ourDatabase.delete(DATABASE_TABLE, KEY_ROWID + "=" + l, null);
}
}
listview的XML布局：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<TextView
android:id="@+id/txtName"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Name:" />
<TextView
android:id="@+id/txtAge"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Age:" />
</LinearLayout>
listview XML布局：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<ListView
android:id="@+id/contentList"
android:layout_width="match_parent"
android:layout_height="wrap_content" >
</ListView>
</LinearLayout>
logcat的：
01-11 00:23:26.655: D/AndroidRuntime(272): Shutting down VM
01-11 00:23:26.655: W/dalvikvm(272): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
01-11 00:23:26.675: E/AndroidRuntime(272): FATAL EXCEPTION: main
01-11 00:23:26.675: E/AndroidRuntime(272): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.sqliteexample/com.example.sqliteexample.SQLView}: java.lang.NullPointerException
01-11 00:23:26.675: E/AndroidRuntime(272): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
01-11 00:23:26.675: E/AndroidRuntime(272): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
01-11 00:23:26.675: E/AndroidRuntime(272): at android.app.ActivityThread.access$2300(ActivityThread.java:125)
01-11 00:23:26.675: E/AndroidRuntime(272): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
01-11 00:23:26.675: E/AndroidRuntime(272): at android.os.Handler.dispatchMessage(Handler.java:99)
01-11 00:23:26.675: E/AndroidRuntime(272): at android.os.Looper.loop(Looper.java:123)
01-11 00:23:26.675: E/AndroidRuntime(272): at android.app.ActivityThread.main(ActivityThread.java:4627)
01-11 00:23:26.675: E/AndroidRuntime(272): at java.lang.reflect.Method.invokeNative(Native Method)
01-11 00:23:26.675: E/AndroidRuntime(272): at java.lang.reflect.Method.invoke(Method.java:521)
01-11 00:23:26.675: E/AndroidRuntime(272): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
01-11 00:23:26.675: E/AndroidRuntime(272): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
01-11 00:23:26.675: E/AndroidRuntime(272): at dalvik.system.NativeStart.main(Native Method)
01-11 00:23:26.675: E/AndroidRuntime(272): Caused by: java.lang.NullPointerException
01-11 00:23:26.675: E/AndroidRuntime(272): at com.example.sqliteexample.HotOrNot.getData(HotOrNot.java:143)
01-11 00:23:26.675: E/AndroidRuntime(272): at com.example.sqliteexample.SQLView.onCreate(SQLView.java:24)
01-11 00:23:26.675: E/AndroidRuntime(272): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
01-11 00:23:26.675: E/AndroidRuntime(272): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
答案 0 :(得分：3)
您根本忘记致电open()。
HotOrNot Content = new HotOrNot(this, null, null);
Content.open();
Cursor cursor = Content.getData();
（但请阅读Java命名约定，哪些状态变量应以小写字母开头。）
答案 1 :(得分：0)
public HotOrNot (Context c, SQLiteDatabase newSQL, DBHelper BD)
{
ourContext = c;
ourDatabase = newSQL;
ourHelper = BD;
}
在这里，您将onCreate中的null传递给SQLiteDatabase
HotOrNot H = new HotOrNot(this, null, null);