java.lang.NullPointerException- Android 中的错误答案

【问题标题】：java.lang.NullPointerException- error in Androidjava.lang.NullPointerException- Android 中的错误
【发布时间】：2014-04-08 05:28:43
【问题描述】：

我正在为我的 Android 应用程序使用电话号码和密码创建一个简单的登录活动。我想将用户输入的电话号码传递到我的欢迎屏幕，但问题是当用户第一次没有注销然后运行应用程序后，我收到了上述错误。请帮我解决这个错误。

这是我的 LoginActivity.java 文件，

public class LoginActivity extends Activity {
Button btnLogin;
Button btnLinkToRegister;
EditText inputphone_number;
EditText inputPassword;
TextView loginErrorMsg;

// JSON Response node names
private static String KEY_SUCCESS = "success";
private static String KEY_ERROR = "error";
private static String KEY_ERROR_MSG = "error_msg";
private static String KEY_UID = "uid";
private static String KEY_NAME = "name";
private static String KEY_PHONE_NUMBER = "phone_number";
private static String KEY_CREATED_AT = "created_at";

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

    // Importing all assets like buttons, text fields
    inputphone_number = (EditText) findViewById(R.id.loginEmail);
    inputPassword = (EditText) findViewById(R.id.loginPassword);
    btnLogin = (Button) findViewById(R.id.btnLogin);
    btnLinkToRegister = (Button) findViewById(R.id.btnLinkToRegisterScreen);
    loginErrorMsg = (TextView) findViewById(R.id.login_error);

    // Login button Click Event
    btnLogin.setOnClickListener(new View.OnClickListener() {

        public void onClick(View view) {
            String phone_number = inputphone_number.getText().toString();
            String password = inputPassword.getText().toString();
            UserFunctions userFunction = new UserFunctions();
            Log.d("Button", "Login");
            JSONObject json = userFunction.loginUser(phone_number, password);

            // check for login response
            try {
                if (json.getString(KEY_SUCCESS) != null) {
                    loginErrorMsg.setText("");
                    String res = json.getString(KEY_SUCCESS); 
                    if(Integer.parseInt(res) == 1){
                        // user successfully logged in
                        // Store user details in SQLite Database
                        DatabaseHandler db = new DatabaseHandler(getApplicationContext());
                        JSONObject json_user = json.getJSONObject("user");

                        // Clear all previous data in database
                        userFunction.logoutUser(getApplicationContext());
                        db.addUser(json_user.getString(KEY_NAME), json_user.getString(KEY_PHONE_NUMBER), json.getString(KEY_UID), json_user.getString(KEY_CREATED_AT));                     

                        // Launch Dashboard Screen
                        Intent dashboard = new Intent(getApplicationContext(), MainActivity.class);

                        // this is how i sent phone number to next activity
                        dashboard.putExtra("phone_number", phone_number);


                        // Close all views before launching Dashboard
                        dashboard.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                        startActivity(dashboard);

                        // Close Login Screen
                        finish();
                    }else{
                        // Error in login
                        loginErrorMsg.setText("Incorrect username/password");
                    }
                }
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
    });

    // Link to Register Screen
    btnLinkToRegister.setOnClickListener(new View.OnClickListener() {

        public void onClick(View view) {
            Intent i = new Intent(getApplicationContext(),
                    RegisterActivity.class);
            startActivity(i);
            finish();
        }
    });
}
}

这是 MainActivity.java 文件，

public class MainActivity extends Activity {

UserFunctions userFunctions;
Button btnLogout;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);



       /**
     * Dashboard Screen for the application
     * */        
    // Check login status in database
    userFunctions = new UserFunctions();
    if(userFunctions.isUserLoggedIn(getApplicationContext())){
        setContentView(R.layout.dashboard);

        TextView phone_number = (TextView) findViewById(R.id.userNameView);

        Intent i = getIntent();

        String phoneNumber = i.getExtras().getString("phone_number");
        phone_number.setText(phoneNumber);

        btnLogout = (Button) findViewById(R.id.btnLogout);

        btnLogout.setOnClickListener(new View.OnClickListener() {

            public void onClick(View arg0) {
                // TODO Auto-generated method stub
                userFunctions.logoutUser(getApplicationContext());
                Intent login = new Intent(getApplicationContext(), LoginActivity.class);
                login.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                startActivity(login);
                // Closing dashboard screen
                finish();
            }
        });

    }else{
        // user is not logged in show login screen
        Intent login = new Intent(getApplicationContext(), LoginActivity.class);
        login.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        startActivity(login);
        // Closing dashboard screen
        finish();
    }
//  setContentView(R.layout.main);

//  MainDBAdapter mainDBAdapter = new MainDBAdapter(this);
    DatabaseHandler mainDBHandler = new DatabaseHandler(this);


}

public void onClickedJobs(View view){

    Intent intent = new Intent(this,TourNumberList.class);
    startActivity(intent);

}
 }

UserFunctions.java 文件，

public class UserFunctions {

private JSONParser jsonParser;

private static String loginURL = "http://10.0.2.2/android_login_api/";
private static String registerURL = "http://10.0.2.2/android_login_api/";

private static String login_tag = "login";
private static String register_tag = "register";

// constructor
public UserFunctions(){
    jsonParser = new JSONParser();
}

/**
 * function make Login Request
 * @param phone_number
 * @param password
 * */
public JSONObject loginUser(String phone_number, String password){
    // Building Parameters
    List<NameValuePair> params = new ArrayList<NameValuePair>();
    params.add(new BasicNameValuePair("tag", login_tag));
    params.add(new BasicNameValuePair("phone_number", phone_number));
    params.add(new BasicNameValuePair("password", password));
    JSONObject json = jsonParser.getJSONFromUrl(loginURL, params);
    // return json
    // Log.e("JSON", json.toString());
    return json;
}

/**
 * function make Login Request
 * @param name
 * @param phone_number
 * @param password
 * */
public JSONObject registerUser(String name, String phone_number, String password){
    // Building Parameters
    List<NameValuePair> params = new ArrayList<NameValuePair>();
    params.add(new BasicNameValuePair("tag", register_tag));
    params.add(new BasicNameValuePair("name", name));
    params.add(new BasicNameValuePair("phone_number", phone_number));
    params.add(new BasicNameValuePair("password", password));

    // getting JSON Object
    JSONObject json = jsonParser.getJSONFromUrl(registerURL, params);
    // return json
    return json;
}

/**
 * Function get Login status
 * */
public boolean isUserLoggedIn(Context context){
    DatabaseHandler db = new DatabaseHandler(context);
    int count = db.getRowCount();
    if(count > 0){
        // user logged in
        return true;
    }
    return false;
}

/**
 * Function to logout user
 * Reset Database
 * */
public boolean logoutUser(Context context){
    DatabaseHandler db = new DatabaseHandler(context);
    db.resetTables();
    return true;
}

}

这是 LogCat 错误，

04-08 10:27:56.763: E/AndroidRuntime(342): FATAL EXCEPTION: main
04-08 10:27:56.763: E/AndroidRuntime(342): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.tourinfo/com.example.tourinfo.MainActivity}: java.lang.NullPointerException
04-08 10:27:56.763: E/AndroidRuntime(342):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
04-08 10:27:56.763: E/AndroidRuntime(342):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
04-08 10:27:56.763: E/AndroidRuntime(342):  at android.app.ActivityThread.access$1500(ActivityThread.java:117)
04-08 10:27:56.763: E/AndroidRuntime(342):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
04-08 10:27:56.763: E/AndroidRuntime(342):  at android.os.Handler.dispatchMessage(Handler.java:99)
04-08 10:27:56.763: E/AndroidRuntime(342):  at android.os.Looper.loop(Looper.java:123)
04-08 10:27:56.763: E/AndroidRuntime(342):  at android.app.ActivityThread.main(ActivityThread.java:3683)
04-08 10:27:56.763: E/AndroidRuntime(342):  at java.lang.reflect.Method.invokeNative(Native Method)
04-08 10:27:56.763: E/AndroidRuntime(342):  at java.lang.reflect.Method.invoke(Method.java:507)
04-08 10:27:56.763: E/AndroidRuntime(342):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
04-08 10:27:56.763: E/AndroidRuntime(342):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
04-08 10:27:56.763: E/AndroidRuntime(342):  at dalvik.system.NativeStart.main(Native Method)
04-08 10:27:56.763: E/AndroidRuntime(342): Caused by: java.lang.NullPointerException
04-08 10:27:56.763: E/AndroidRuntime(342):  at com.example.tourinfo.MainActivity.onCreate(MainActivity.java:37)
04-08 10:27:56.763: E/AndroidRuntime(342):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
04-08 10:27:56.763: E/AndroidRuntime(342):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
04-08 10:27:56.763: E/AndroidRuntime(342):  ... 11 more

【问题讨论】：

网络操作不应在主线程上执行。
第 37 行是什么？
您是否在您的清单中声明了 mainactivity..？
这是第 37 行- String phoneNumber = i.getExtras().getString("phone_number");
登录活动未正确传递电话号码..

标签： java android json nullpointerexception

【解决方案1】：

在第 37 行替换

i.getExtras().getString("phone_number");

与

i.getStringExtra("phone_number");

使用共享偏好：- 为了节省

SharedPreferences sharedPref = context.getSharedPreferences("text1",
                Context.MODE_PRIVATE);
        Editor editor = sharedPref.edit();
        if (phone!= null) {
            editor.putString("phone", phone);
        }
editor.commit();

用于取回：-

SharedPreferences sharedPref = context.getSharedPreferences("text1",
                Context.MODE_PRIVATE);
        phone= sharedPref.getString("phone", null);

【讨论】：

但是第一次在loginActivity中输入手机号和密码，通过手机号没有任何问题。当用户未注销并尝试运行应用程序后发生此错误。
使用 sharedpreference 存储电话号码并每次都从中获取
如何使用sharedpreferences，我不知道如何使用它。
它在上下文中显示错误下划线，其中 context.getSharedPreferences.
你可以使用 ActivityName.this.getSHAredPreferences 代替上下文