Question

此时我想问一下，当我尝试从数据库中读取数据时出现错误，在验证活动中，错误是在下方签名。

Validation.java

public class Validation extends Activity{

private static DBDataSource dataSource; 


// Error Messages
private static final String REQUIRED_MSG = "required";


@Override
protected void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    dataSource = new DBDataSource(this);
    dataSource.open();
}
//this method is to check is my textbox empty or not, the i want to modified to check is my textbox empty or equals data from database
public static boolean hasText(EditText editText) {

    ArrayList<Sma> list = dataSource.getAllSma(); >> when i add this code, the apps currently stop, and the got error message java.lang.NullPointer Exception
    String text = editText.getText().toString().trim();
    editText.setError(null);

    // length 0 means there is no text
    if (text.length() == 0) {
        editText.setError(REQUIRED_MSG);
        return false;
    }

    return true;

}

这是我的DBDataSource

 public ArrayList<Sma> getAllSma() 
 {
    ArrayList<Sma> daftarSma = new ArrayList<Sma>();

    // select all SQL query
    Cursor cursor = database.query(DBHelper.TABLE_NAME, allColumns, null, null, null, null, null);


    cursor.moveToFirst();

    while (!cursor.isAfterLast()) 
    {
      Sma sekolah = cursorToSma(cursor);
      daftarSma.add(sekolah);
      cursor.moveToNext();
    }

    cursor.close();
    return daftarSma;
  }

任何人都可以帮我解决这个错误，因为我真的不知道为什么会这样。谢谢：D

这时我称之为验证活动

 public class CreateData extends Activity implements OnClickListener{
 private Validation valid;
 public void onCreate(Bundle savedInstanceState) 
 {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.create_data);
  edNama_sma        = (EditText) findViewById(R.id.nama_sma);
  registerViews();
  }

 private void registerViews() {

        edNama_sma.addTextChangedListener(new TextWatcher() {
            public void afterTextChanged(Editable s) {
                valid.hasText(edNama_sma);
            }
            public void beforeTextChanged(CharSequence s, int start, int count, int after){}

            public void onTextChanged(CharSequence s, int start, int before, int count){}
        });
    @Override
    public void onClick(View v) 
    {}

我的错误

 04-05 13:54:00.727: E/AndroidRuntime(27565): FATAL EXCEPTION: main
 04-05 13:54:00.727: E/AndroidRuntime(27565): java.lang.NullPointerException
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at      com.example.validation.search.Validation.hasText(Validation.java:76)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at com.example.search.CreateData$5.afterTextChanged(CreateData.java:230)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at android.widget.TextView.sendAfterTextChanged(TextView.java:7259)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at android.widget.TextView$ChangeWatcher.afterTextChanged(TextView.java:8915)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at android.text.SpannableStringBuilder.sendAfterTextChanged(SpannableStringBuilder.java:970)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:497)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:435)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:30)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at android.view.inputmethod.BaseInputConnection.replaceText(BaseInputConnection.java:672)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at android.view.inputmethod.BaseInputConnection.setComposingText(BaseInputConnection.java:435)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at com.android.internal.view.IInputConnectionWrapper.executeMessage(IInputConnectionWrapper.java:333)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at com.android.internal.view.IInputConnectionWrapper$MyHandler.handleMessage(IInputConnectionWrapper.java:77)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at android.os.Handler.dispatchMessage(Handler.java:99)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at android.os.Looper.loop(Looper.java:137)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at android.app.ActivityThread.main(ActivityThread.java:5099)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at java.lang.reflect.Method.invokeNative(Native Method)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at java.lang.reflect.Method.invoke(Method.java:511)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:803)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:570)
 04-05 13:54:00.727: E/AndroidRuntime(27565):   at dalvik.system.NativeStart.main(Native Method)

Answer 1

活动Validation尚未初始化且尚未调用onCreate()。因此，datasource为null，您获得NPE。

根据您发布的内容，Validation根本不应该是一项活动。您应该只为实际活动的类扩展Activity，在清单中列出并通过Intent实例化。

对于需要有效Context的方法（例如SQLiteOpenHelper构造函数），将其作为参数传递，而不是尝试使用未实例化的this的{{1}}。< / p>

数据库无法打开，错误：java.lang.NullPointerException

1 个答案: