我正在将一些数据输入此数据库，然后尝试显示该数据，但我得到了Null Pointer Exception at getReadableDatabase()。我输入数据的主要活动如下：
public class TabListData extends Activity implements OnClickListener {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
Button b = (Button) findViewById(R.id.enter);
b.setOnClickListener(this);
Button b1 = (Button) findViewById(R.id.employeeslist);
b1.setOnClickListener(this);
}
@Override
public void onClick(View v) {
EditText name = (EditText) findViewById(R.id.name);
EditText mobile = (EditText) findViewById(R.id.mobile);
String nam = name.getText().toString();
String mob = mobile.getText().toString();
Button b = (Button) v;
Button b1 = (Button) v;
Database d = new Database(getBaseContext());
switch (b.getId()) {
case R.id.enter:
d.input(nam, mob);
case R.id.employeeslist:
/* ArrayList<EmployeesResult> results = d.selectEmployees();
Toast t = Toast.makeText(getBaseContext(), "# records: "
+ results.size(), 1000);
t.show();*/
Intent i = new Intent(this, EmployeesList.class);
// i.putExtra("Name", "Name");
startActivity(i);
}
}
}
这是我的助手类
public class BaseHelper extends SQLiteOpenHelper {
public BaseHelper(Context context) {
super(context, "Employees.db", null, 1);
}
@Override
public void onCreate(SQLiteDatabase db) {
String createtable = "create table Employee " + "(Name TEXT, " + "Mobile TEXT)";
db.execSQL(createtable);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// TODO Auto-generated method stub
}
}
这是我的数据库类。我正在使用它进行输入和数据检索。当selectEmployees()方法不存在但现在显示Null Pointer Exception时，输入正常。
public class Database {
private BaseHelper helper;
SQLiteDatabase db;
public Database(Context c){
helper = new BaseHelper(c);
}
public void input(String Name, String Mobile){
db = helper.getWritableDatabase();
try{
ContentValues values = new ContentValues();
values.put("Mobile", Mobile);
values.put("Name", Name);
db.insert("Employee", "", values);
Log.e("ENTERED", "ENTERED "+values);
}finally {
if (db != null)
db.close();
}
}
public ArrayList<EmployeesResult> selectEmployees() {
db = helper.getReadableDatabase();
try{
ArrayList<EmployeesResult> results = new ArrayList<EmployeesResult>();
Cursor c = db.rawQuery("select * from Employee", null);
if (c.getCount() > 0) {
c.moveToFirst();
do {
results.add(new EmployeesResult(
c.getString(c.getColumnIndex("Name"))));
//c.getInt(c.getColumnIndex("Mobile"))));
}while(c.moveToNext());
}
return results;
}finally {
if (db != null)
db.close();
}
}
}
EmployeeResult类：
public class EmployeesResult {
public EmployeesResult(String Name){
name = Name;
//mobile = Mobile;
}
public String name;
public int mobile;
}
这是Logcat
07-15 23:44:11.008: ERROR/AndroidRuntime(292): FATAL EXCEPTION: main
07-15 23:44:11.008: ERROR/AndroidRuntime(292): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.TabListData/com.TabListData.EmployeesList}: java.lang.NullPointerException
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at android.app.ActivityThread.access$2300(ActivityThread.java:125)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at android.os.Handler.dispatchMessage(Handler.java:99)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at android.os.Looper.loop(Looper.java:123)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at android.app.ActivityThread.main(ActivityThread.java:4627)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at java.lang.reflect.Method.invokeNative(Native Method)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at java.lang.reflect.Method.invoke(Method.java:521)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at dalvik.system.NativeStart.main(Native Method)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): Caused by: java.lang.NullPointerException
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:98)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:158)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at com.TabListData.Database.selectEmployees(Database.java:35)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at com.TabListData.EmployeesList.onCreate(EmployeesList.java:30)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
07-15 23:44:11.008: ERROR/AndroidRuntime(292): ... 11 more
答案 0 :(得分：1)
我的问题解决了我发现问题恰好在ListActivity和EmployeeResult类中的问题
对于ListActivity，只使用文本的XML布局在ListView中显示文本 喜欢这个
<?xml version="1.0" encoding="UTF-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:padding="10dp"
android:textSize="16sp" >
</TextView>
在ListActivity中你只需要设置一个ListAdapter
在onCreate（）方法
中是这样的List<String> data = dh.SelectStudent();
setListAdapter(new ArrayAdapter<String>(ListofStudents.this, R.layout.text, data));
并且在EmployeesResult类中输入了ArrayAdapter中的String和List
DataBase类中的方法应该是
public ArrayList<String> SelectStudent() {
SQLiteDatabase db = _dbHelper.getReadableDatabase();
try {
ArrayList<String> results = new ArrayList<String>();
Cursor c = db.rawQuery("select * from Student", null);
if (c.getCount() > 0) {
c.moveToFirst();
do {
results.add(c.getString(1));
// results.add(c.getString(4));
} while (c.moveToNext());
}
return results;
} finally {
if (db != null)
db.close();
}
}
我已经在List和ArrayAdepter中传递了String，如果你想在列表视图中的项目中想要多于一个字符串，如Name和Mobile Number，或者String和Image的组合，那么你需要创建一个单独的类有图像和字符串，你需要在List和ArrayAdepter中传递该类而不是String。
THANX
答案 1 :(得分：1)
错误可能是由Context
中的空指针引起的public class SqLite extends SQLiteOpenHelper {
public SqLite(Context context, String table, boolean readOnly) {
super(context, DB_NAME, null, 1);
getReadableDatabase();