android - 尝试在空对象引用上调用虚拟方法 'java.lang.String java.lang.Object.toString()'

Question

我是 Android 新手，这是我的第一个项目。我正在尝试制作一个从用户那里获取输入并对 mysql 数据库执行一些操作的 android 程序，我运行了该程序但出现了这些错误:

    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
        at com.example.nada.mysqldemo.MainActivity.viewResultMessage(MainActivity.java:89)
        at com.example.nada.mysqldemo.UserServiceAPI.onPostExecute(UserServiceAPI.java:83)
        at android.os.AsyncTask.finish(AsyncTask.java:632)
        at android.os.AsyncTask.access$600(AsyncTask.java:177)
        at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5221)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

谁能告诉我错误在哪里以及如何解决？我在这里附上我的代码:主 Activity .java

    import android.app.Activity;
    import android.app.ProgressDialog;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.EditText;
    import android.widget.Toast;

    public class MainActivity extends Activity {
    private EditText idEditText, nameEditText;
    private ProgressDialog loadingDialog;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    idEditText = (EditText) findViewById(R.id.et_id);
    nameEditText = (EditText) findViewById(R.id.et_name);

    loadingDialog = new ProgressDialog(this);
    loadingDialog.setCancelable(false);
    loadingDialog.setCanceledOnTouchOutside(false);
    loadingDialog.setMessage("Please wait...");
}

public void selectOneButtonAction(View v) {
    checkId();
    loadingDialog.show();
    new UserServiceAPI(this, UserServiceAPI.SELECT_ONE).execute(idEditText
            .getText().toString().trim(), "");
}

public void selectAllButtonAction(View v) {
    loadingDialog.show();
    new UserServiceAPI(this, UserServiceAPI.SELECT_ONE).execute("", "");
}

public void insertButtonAction(View v) {
    checkName();
    loadingDialog.show();
    new UserServiceAPI(this, UserServiceAPI.SELECT_ONE).execute("",
            nameEditText.getText().toString().trim());
}

public void deleteButtonAction(View v) {
    checkId();
    loadingDialog.show();
    new UserServiceAPI(this, UserServiceAPI.SELECT_ONE).execute(idEditText
            .getText().toString().trim(), "");
}

public void updateButtonAction(View v) {
    checkId();
    checkName();
    loadingDialog.show();
    new UserServiceAPI(this, UserServiceAPI.SELECT_ONE).execute(idEditText
            .getText().toString().trim(), nameEditText.getText().toString()
            .trim());
}

private void checkId() {
    if (idEditText.getText().toString().trim().length() == 0) {
        showToast("Missing id");
        return;
    }
}

private void checkName() {
    if (nameEditText.getText().toString().trim().length() == 0) {
        showToast("Missing name");
        return;
    }
}

private void showToast(String text) {
    Toast.makeText(this, text, Toast.LENGTH_LONG).show();
}

public void viewResultMessage(Object result) {
    if (loadingDialog.isShowing())
        loadingDialog.dismiss();
    showToast(result.toString());
}

    }

用户服务接口(interface)

    import java.net.SocketException;
    import java.net.SocketTimeoutException;
    import java.util.ArrayList;
    import org.apache.http.HttpEntity;
    import org.apache.http.HttpResponse;
    import org.apache.http.NameValuePair;
    import org.apache.http.client.entity.UrlEncodedFormEntity;
    import org.apache.http.client.methods.HttpPost;
    import org.apache.http.impl.client.DefaultHttpClient;
    import org.apache.http.message.BasicNameValuePair;
    import org.apache.http.util.EntityUtils;
    import org.json.JSONArray;
    import org.json.JSONObject;
    import android.os.AsyncTask;
    import android.util.Log;
    public class UserServiceAPI extends AsyncTask<String, Integer, Object> {
    public static final String INSERT = "insert";
    public static final String UPDATE = "update";
    public static final String DELETE = "delete";
    public static final String SELECT_ONE = "selectone";
    public static final String SELECT_ALL = "selectall";
    private String action;
    private MainActivity activity;
    //private String url = "http://192.168.1.100/mysql_android/user_operations.php";
private String url = "http://10.0.2.2/mysql_android/user_operations.php";

public UserServiceAPI(MainActivity activity, String action) {
    this.action = action;
    this.activity = activity;
}

@Override
protected Object doInBackground(String... params) {
    ArrayList<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>();
    nameValuePairs.add(new BasicNameValuePair("action", action));
    nameValuePairs.add(new BasicNameValuePair("id", params[0]));
    nameValuePairs.add(new BasicNameValuePair("name", params[1]));
    String connRs = connect(url, nameValuePairs);
    try {
        if (action.equals(INSERT) || action.equals(UPDATE)
                || action.equals(DELETE)) {
            JSONObject jsonObject = new JSONObject(connRs);
            return jsonObject.getString("message");
        } else if (action.equals(SELECT_ALL)) {
            JSONArray jsonArray = new JSONArray(connRs);
            if (jsonArray.length() == 0) {
                return "No data found";
            }
            String rs = "";
            for (int i = 0; i < jsonArray.length(); i++) {
                JSONObject jsonObject = jsonArray.getJSONObject(i);
                String name = jsonObject.getString("name");
                rs += name;
            }
            return rs;
        } else if (action.equals(SELECT_ONE)) {
            JSONObject jsonObject = new JSONObject(connRs);
            String user = jsonObject.getString("user");
            if (user == null || user.equals("null")) {
                return "No date for this id";
            } else {
                return jsonObject.getString("name");
            }
        }
    } catch (Exception e) {
    }
    return null;
}

@Override
protected void onPostExecute(Object result) {
    super.onPostExecute(result);
    activity.viewResultMessage(result);
}

/**
 * connect to server with url and parameters
 *
 * @param url
 * @param params
 * @return
 */
public static String connect(String url, ArrayList<NameValuePair> params) {
    DefaultHttpClient httpClient = new DefaultHttpClient();
    try {
        HttpPost httpPost = new HttpPost(url);
        httpPost.setEntity(new UrlEncodedFormEntity(params));
        HttpResponse httpResponse = httpClient.execute(httpPost);

        int status = httpResponse.getStatusLine().getStatusCode();
        if (status == 200) {
            HttpEntity httpEntity = httpResponse.getEntity();
            return EntityUtils.toString(httpEntity);
        }
    } catch (SocketTimeoutException ex) {
        Log.d("RS", "SocketTimeoutException: " + ex.toString());
    } catch (SocketException ex) {
        Log.d("RS", "SocketException: " + ex.toString());
    } catch (Exception e) {
        Log.d("RS", "Connect EX: " + e.toString());
        e.printStackTrace();
    }
    return null;
}
}

如果有人能帮上忙，我将不胜感激

Answer 1

日志信息:

    at com.example.nada.mysqldemo.MainActivity.viewResultMessage(MainActivity.java:89)

指向 viewResultMessage 中的以下行:

showToast(result.toString());

这意味着result 为null。 result 从哪里来？它来自 onPostExecute 中的这个调用:

activity.viewResultMessage(result);

结果 从哪里来？根据AsyncTask documentation :

The specified result is the value returned by doInBackground(Params...).

因此，在您的 doInBackground 方法中，您将返回 null。问题可能出在这里:

} catch (Exception e) {
}
return null;

在这些行中，您将捕获可能发生的任何可能的异常，忽略异常并返回null。这会导致您观察到的行为。

要解决此问题，找出此处捕获的异常并解决该问题。在这种情况下，您还可以考虑返回 null 以外的内容。