android - 开始一个使用 asynctask 的新 Activity

Question

当我尝试启动将使用 AsyncTask 的新 Activity 时，我的应用程序崩溃了，我的 LoginActivity。当我开始任何其他不使用 AsyncTask 的 Activity 时，它工作正常。这是我的 MainActivity:

public class MainActivity extends ActionBarActivity implements OnClickListener {

    private Button main_redeem;
    private Button main_check;
    private Button main_nearby;
    private Button main_login;
    private Button main_scan;


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

        main_redeem = (Button)findViewById(R.id.mainRedeem);
        main_redeem.setOnClickListener(this);

        main_check = (Button)findViewById(R.id.mainCheckPunches);
        main_check.setOnClickListener(this);

        main_nearby = (Button)findViewById(R.id.mainNearbyBusiness);
        main_nearby.setOnClickListener(this);

        main_scan = (Button)findViewById(R.id.mainLogin);
        main_scan.setOnClickListener(this);

        main_scan = (Button)findViewById(R.id.mainScan);
        main_scan.setOnClickListener(this);

    }


    @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;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }


    @Override
    public void onClick(View v) {
        // responds to button clicks

        if(v.getId() == main_redeem.getId()){
            Intent redeem_intent = new Intent(MainActivity.this, RedeemPunches.class);
            startActivity(redeem_intent);

        }

        else if(v.getId() == main_check.getId()){
            Intent check_intent = new Intent(MainActivity.this, CheckPunches.class);
            startActivity(check_intent);

        }

        else if(v.getId() == main_nearby.getId()){
            Intent nearby_intent = new Intent(MainActivity.this, NearbyBusinesses.class);
            startActivity(nearby_intent);

        }
        //This is the activity that crashes
        else if(v.getId() == main_login.getId()){
            Intent login_intent = new Intent(MainActivity.this, LoginActivity.class);
            startActivity(login_intent);

        }

        else if(v.getId() == main_scan.getId()){
            Intent scan_intent = new Intent(MainActivity.this, ScanCode.class);
            startActivity(scan_intent);

        }
    }


}


03-01 18:44:24.841: E/AndroidRuntime(1278): FATAL EXCEPTION: main
03-01 18:44:24.841: E/AndroidRuntime(1278): Process: com.nb.punchcard, PID: 1278
03-01 18:44:24.841: E/AndroidRuntime(1278): java.lang.NullPointerException: Attempt to invoke virtual method 'int android.widget.Button.getId()' on a null object reference
03-01 18:44:24.841: E/AndroidRuntime(1278):     at com.nb.punchcard.MainActivity.onClick(MainActivity.java:94)
03-01 18:44:24.841: E/AndroidRuntime(1278):     at android.view.View.performClick(View.java:4756)
03-01 18:44:24.841: E/AndroidRuntime(1278):     at android.view.View$PerformClick.run(View.java:19749)
03-01 18:44:24.841: E/AndroidRuntime(1278):     at android.os.Handler.handleCallback(Handler.java:739)
03-01 18:44:24.841: E/AndroidRuntime(1278):     at android.os.Handler.dispatchMessage(Handler.java:95)
03-01 18:44:24.841: E/AndroidRuntime(1278):     at android.os.Looper.loop(Looper.java:135)
03-01 18:44:24.841: E/AndroidRuntime(1278):     at android.app.ActivityThread.main(ActivityThread.java:5221)
03-01 18:44:24.841: E/AndroidRuntime(1278):     at java.lang.reflect.Method.invoke(Native Method)
03-01 18:44:24.841: E/AndroidRuntime(1278):     at java.lang.reflect.Method.invoke(Method.java:372)
03-01 18:44:24.841: E/AndroidRuntime(1278):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
03-01 18:44:24.841: E/AndroidRuntime(1278):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

这是我的 LoginActivity 的代码:

public class LoginActivity extends Activity {

    private EditText txtName;
    private EditText txtPass;
    private Button btnSubmit;
    TextView strError;
    TextView txtView;
    String serverResponse = null;
    private static final String TAG_STATUS = "strStatus";
    JSONObject user = null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);
        txtName = (EditText) findViewById(R.id.login_name_entry);
        txtPass = (EditText) findViewById(R.id.login_pass_entry);
        btnSubmit = (Button) findViewById(R.id.login_submit_btn);
        strError = (TextView) findViewById(R.id.login_textView_error);
        txtView = (TextView) findViewById(R.id.textView1);

        btnSubmit.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String strName = txtName.getText().toString();
                String strPassword = txtPass.getText().toString();
                validateVendor task = new validateVendor();
                task.execute(new String[]{strName, strPassword});

            }
        });
    }


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

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
    private class validateVendor extends AsyncTask<String, String, JSONObject>{

        @Override
        protected JSONObject doInBackground(String... params) {
            ArrayList<NameValuePair> postParam = new ArrayList<NameValuePair>();
            postParam.add(new BasicNameValuePair("strName", params[0]));
            postParam.add(new BasicNameValuePair("strPassword", params[1]));

            //JSONObject serverRes = null;
            JSONParser jParser = new JSONParser();
            JSONObject serverRes = null;
            try {
                serverRes = jParser.getJSONFromUrl(//myURL);
                //serverRes = 
            } catch (UnsupportedEncodingException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

            return serverRes;
        }
        @Override
         protected void onPostExecute(JSONObject serverRes){
             String strRes = "";

             try { 
                strRes = serverRes.getString(TAG_STATUS);
                txtView.setText(strRes);
            } catch (JSONException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

            if(strRes.equals("accepted")){
                strError.setText("Correct");
                Intent i = new Intent(LoginActivity.this,MainActivity.class);
                startActivity(i);               

            }
            else{
             strError.setText("Sorry!! Incorrect Username or Password"); 
            }  
        }
    }
}

Answer 1

如日志中所示:

NullPointerException: Attempt to invoke virtual method 'int android.widget.Button.getId()' on a null object reference

因为 main_login Button 对象是 null。

在调用 getId 方法之前初始化 main_login 按钮对象:

main_login = (Button)findViewById(R.id.mainLogin);
main_login.setOnClickListener(this);