Question

我有这两个班：

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        DatabaseHelper helper = new DatabaseHelper(this);
        String[] temp = new String[] { "40", "35", "28" };
        String[] hum = new String[] { "50%", "30%", "80%" };
        for (int i = 0; i < temp.length; i++) {
            helper.insertRecord(temp[i], hum[i]);
        }
        Cursor c = helper.getData();
        String res = "";
        if(c.moveToFirst()) {
            while(!c.isAfterLast()) {
                res+=c.getString(0)+" "+c.getString(1)+"\n";
                c.moveToNext();
            }
            c.close();
        }
        TextView tv = (TextView) findViewById(R.id.tv);
        tv.setText(res);
    }    

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

}


public class DatabaseHelper extends SQLiteOpenHelper {
    private static final String DB_NAME = "database_name";
    private static final String TABLE_NAME = "my_table";
    private static final String COL_TEMPERATURA = "temperatura";
    private static final String COL_UMIDITA = "umidita";

    public DatabaseHelper(Context context) {
        super(context, DB_NAME, null, 33);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
        db.execSQL("CREATE TABLE " + TABLE_NAME + " (" + COL_TEMPERATURA
                + " TEXT, " + COL_UMIDITA + " TEXT)");
    }

    @Override
    public void onUpgrade(SQLiteDatabase arg0, int arg1, int arg2) {
        // TODO Auto-generated method stub

    }

    public void insertRecord(String temperatura, String umidita) {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues cv = new ContentValues();
        cv.put(COL_TEMPERATURA, temperatura);
        cv.put(COL_UMIDITA, umidita);
        db.insert(TABLE_NAME, null, cv);
        db.close();

    }

    public Cursor getData() {
        SQLiteDatabase db = this.getReadableDatabase();
        String[] cols = new String[]{COL_TEMPERATURA, COL_UMIDITA};
        Cursor res = db.query(TABLE_NAME, cols, null, null, null, null, null);
        return res;
    }

}

但是当我执行我的代码时，我在调用时在MainActivity上有一个nullpointer异常 tv.setText（RES）;

为什么？

日志：

09-08 19:25:16.249: E/AndroidRuntime(5911): FATAL EXCEPTION: main
09-08 19:25:16.249: E/AndroidRuntime(5911): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.database/com.example.database.MainActivity}: java.lang.NullPointerException
09-08 19:25:16.249: E/AndroidRuntime(5911):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
09-08 19:25:16.249: E/AndroidRuntime(5911):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
09-08 19:25:16.249: E/AndroidRuntime(5911):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
09-08 19:25:16.249: E/AndroidRuntime(5911):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
09-08 19:25:16.249: E/AndroidRuntime(5911):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-08 19:25:16.249: E/AndroidRuntime(5911):     at android.os.Looper.loop(Looper.java:137)
09-08 19:25:16.249: E/AndroidRuntime(5911):     at android.app.ActivityThread.main(ActivityThread.java:5103)
09-08 19:25:16.249: E/AndroidRuntime(5911):     at java.lang.reflect.Method.invokeNative(Native Method)
09-08 19:25:16.249: E/AndroidRuntime(5911):     at java.lang.reflect.Method.invoke(Method.java:525)
09-08 19:25:16.249: E/AndroidRuntime(5911):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
09-08 19:25:16.249: E/AndroidRuntime(5911):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
09-08 19:25:16.249: E/AndroidRuntime(5911):     at dalvik.system.NativeStart.main(Native Method)
09-08 19:25:16.249: E/AndroidRuntime(5911): Caused by: java.lang.NullPointerException
09-08 19:25:16.249: E/AndroidRuntime(5911):     at com.example.database.MainActivity.onCreate(MainActivity.java:31)
09-08 19:25:16.249: E/AndroidRuntime(5911):     at android.app.Activity.performCreate(Activity.java:5133)
09-08 19:25:16.249: E/AndroidRuntime(5911):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
09-08 19:25:16.249: E/AndroidRuntime(5911):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
09-08 19:25:16.249: E/AndroidRuntime(5911):     ... 11 more

Answer 1

你必须测试光标c是否为空：

 Cursor c = helper.getData();
    String res = "";
    if(c.moveToFirst() && c.getCount() != 0) {
        while(!c.isAfterLast()) {
            res+=c.getString(0)+" "+c.getString(1)+"\n";
            c.moveToNext();
        }
        c.close();
    }

Answer 2

我建议你在线设置断点：

if(c.moveToFirst()) {

看一下步骤结束后变量“c”和“res”会发生什么。这应该为您提供足够的信息来确定发生了什么。

要完全确定“tv”已初始化，请尝试设置：

tv.setText("test");

只是为了确保“tv”不为空。

从Cursor：null指针异常中获取数据

2 个答案: