android - NullPointerException 错误

Question

我的日志

08-06 01:27:34.874: ERROR/AndroidRuntime(721): Uncaught handler: thread main exiting due to uncaught exception
08-06 01:27:34.893: ERROR/AndroidRuntime(721): java.lang.NullPointerException
08-06 01:27:34.893: ERROR/AndroidRuntime(721):     at one.two.Booking$MyOnItemSelectedListener.onItemSelected(Booking.java:53)
08-06 01:27:34.893: ERROR/AndroidRuntime(721):     at android.widget.AdapterView.fireOnSelected(AdapterView.java:856)
08-06 01:27:34.893: ERROR/AndroidRuntime(721):     at android.widget.AdapterView.access$200(AdapterView.java:41)
08-06 01:27:34.893: ERROR/AndroidRuntime(721):     at android.widget.AdapterView$SelectionNotifier.run(AdapterView.java:827)
08-06 01:27:34.893: ERROR/AndroidRuntime(721):     at android.os.Handler.handleCallback(Handler.java:587)
08-06 01:27:34.893: ERROR/AndroidRuntime(721):     at android.os.Handler.dispatchMessage(Handler.java:92)
08-06 01:27:34.893: ERROR/AndroidRuntime(721):     at android.os.Looper.loop(Looper.java:123)
08-06 01:27:34.893: ERROR/AndroidRuntime(721):     at android.app.ActivityThread.main(ActivityThread.java:3948)
08-06 01:27:34.893: ERROR/AndroidRuntime(721):     at java.lang.reflect.Method.invokeNative(Native Method)
08-06 01:27:34.893: ERROR/AndroidRuntime(721):     at java.lang.reflect.Method.invoke(Method.java:521)
08-06 01:27:34.893: ERROR/AndroidRuntime(721):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:782)
08-06 01:27:34.893: ERROR/AndroidRuntime(721):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:540)
08-06 01:27:34.893: ERROR/AndroidRuntime(721):     at dalvik.system.NativeStart.main(Native Method)

预订.java

package one.two;

import android.app.Activity;
import android.database.Cursor;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.SimpleCursorAdapter;
import android.widget.Spinner;
import android.widget.TextView;
import android.widget.AdapterView.OnItemSelectedListener;


public class Booking extends Activity
{
    private DBAdapter db; 
    private Spinner colourSpinner;
    public Cursor c;
    public TextView txtArrival;
    /** Called when the activity is first created. */
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        db = new DBAdapter(this);
        this.setContentView(R.layout.booking);
        db.open();
        fillData();
        db.close();
        TextView txtArrival = (TextView) findViewById(R.id.txtArrival);
        Spinner colourSpinner = (Spinner) findViewById(R.id.myspinner);
        colourSpinner.setOnItemSelectedListener(new MyOnItemSelectedListener());
    }

    private void fillData()
    {
        c = db.getSpinnerData();
        startManagingCursor(c);
        String[] from = new String[]{DBAdapter.KEY_ARRIVAL};
        int[] to = new int[]{android.R.id.text1};
        SimpleCursorAdapter adapter =
        new SimpleCursorAdapter(this,R.layout.booking, c, from, to );
        adapter.setDropDownViewResource( android.R.layout.simple_spinner_dropdown_item );
        Spinner colourSpinner = (Spinner) findViewById(R.id.myspinner);
        colourSpinner.setAdapter(adapter);
    }
    public class MyOnItemSelectedListener implements OnItemSelectedListener
    {
        public void onItemSelected(AdapterView<?> arg0, View v,int position, long id)
        {
            boolean result = Booking.this.c.moveToPosition(position);
            if (result) {
                String title=Booking.this.c.getString(1);
                Booking.this.txtArrival.setText(title);
            }

        }
        @Override
        public void onNothingSelected(AdapterView<?> arg0)
        {
            // TODO Auto-generated method stub

        }
    };
}

我想知道如何解决这个错误。谢谢。

Answer 1

您正在创建与您的类成员同名的局部变量。这导致成员被本地成员隐藏，这就是您获得 NullPointerExceptions 的原因。您根本没有为成员提供任何值，这就是它们为空的原因。

你要做的是:

package one.two;

import android.app.Activity;
import android.database.Cursor;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.SimpleCursorAdapter;
import android.widget.Spinner;
import android.widget.TextView;
import android.widget.AdapterView.OnItemSelectedListener;


public class Booking extends Activity
{
    private DBAdapter db; 
    private Spinner colourSpinner;
    public Cursor c;
    public TextView txtArrival;

    /** Called when the activity is first created. */
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        db = new DBAdapter(this);
        this.setContentView(R.layout.booking);
        txtArrival = (TextView) findViewById(R.id.txtArrival); // Member assignment
        colourSpinner = (Spinner) findViewById(R.id.myspinner); // Member assignment
        db.open();
        fillData();
        db.close();
        colourSpinner.setOnItemSelectedListener(new MyOnItemSelectedListener());
    }

    private void fillData()
    {
        c = db.getSpinnerData();
        startManagingCursor(c);
        String[] from = new String[]{DBAdapter.KEY_ARRIVAL};
        int[] to = new int[]{android.R.id.text1};
        SimpleCursorAdapter adapter =
        new SimpleCursorAdapter(this,R.layout.booking, c, from, to );
        adapter.setDropDownViewResource( android.R.layout.simple_spinner_dropdown_item );
        // Removed this line, since members is set in onCreate
        colourSpinner.setAdapter(adapter);
    }
    public class MyOnItemSelectedListener implements OnItemSelectedListener
    {
        public void onItemSelected(AdapterView<?> arg0, View v,int position, long id)
        {
            boolean result = Booking.this.c.moveToPosition(position);
            if (result) {
                String title=Booking.this.c.getString(1);
                Booking.this.txtArrival.setText(title);
            }

        }
        @Override
        public void onNothingSelected(AdapterView<?> arg0)
        {
            // TODO Auto-generated method stub

        }
    };
}