Question

所以每次点击任何按钮时都会出现NullPointerException。出于某种原因，当我在switch语句中时，我收到错误。我的DB帮助程序类只返回一个游标，所以我在我的特定活动中使用该Cursor。我只是希望能够根据用户偏好来回切换数据库条目。而且我还想显示行的不同元素而不是整行。任何建议将不胜感激。我认为这种方法将提供我需要的东西，但我无法让它发挥作用。

import android.app.Activity;
import android.database.Cursor;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class SQLView extends Activity implements OnClickListener{

    /* these 3 variables are the same as in the Database class */
    public static final String KEY_ROWID = "_id";
    public static final String KEY_FRONT = "card_front";
    public static final String KEY_BACK = "card_back";

    /* variables are made global so we can use them in all of our functions here */
    private Cursor myCursor;
    private TextView tv;
    private int iRow;
    private int iFront;
    private int iBack;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.sqlview);

        TextView tv = (TextView)findViewById(R.id.tvSQLinfo);
        String result = null;
        Flashcards info = new Flashcards(this);
        info.open();


        myCursor = info.getCursor();
        //info.close();

        iRow = myCursor.getColumnIndex(KEY_ROWID);
        iFront = myCursor.getColumnIndex(KEY_FRONT);
        iBack = myCursor.getColumnIndex(KEY_BACK);

        Button front = (Button) findViewById(R.id.front);
        Button back = (Button) findViewById(R.id.back);
        Button next = (Button) findViewById(R.id.next);
        Button prev = (Button) findViewById(R.id.prev);

        front.setOnClickListener(this);
        back.setOnClickListener(this);
        next.setOnClickListener(this);
        prev.setOnClickListener(this);


        myCursor.moveToFirst();
        //Set the TextView to be the first card in our database 
        tv.setText(myCursor.getString(iFront));
    }

    @Override
    public void onClick(View v) {

        /* check if we are at our last entry in the DB */
        if(myCursor.isAfterLast()){
            /* move our cursor back to the first entry */
            myCursor.moveToFirst();
        }

        // TODO Auto-generated method stub
        switch (v.getId()){
            case R.id.front:
                tv.setText(myCursor.getString(iFront));
                break;

            case R.id.back:
                tv.setText(myCursor.getString(iBack));
                break;

            case R.id.next:
                if(myCursor.moveToNext()){
                    tv.setText(myCursor.getString(iFront));
                }else{
                    myCursor.moveToFirst();
                }
                break;

            case R.id.prev:
                myCursor.moveToPrevious();
                tv.setText(myCursor.getString(iFront));
                break;
        }
    }
}

Answer 1

tv中的局部变量onCreate实际上并未分配给同名的类成员变量，导致NPE语句中的switch。取代

TextView tv = (TextView)findViewById(R.id.tvSQLinfo);

与

tv = (TextView)findViewById(R.id.tvSQLinfo);

在switch语句中使用游标（SQLite）移动时出现NullPointerException

1 个答案: