Question

我是android新手。我在我的应用程序中创建了一个列表视图。我想在操作栏上添加一个后退按钮。我已经尝试了很多但是无法完成它。以下是我使用过的代码片段：

public class Home extends ListActivity {

    private ProgressDialog pDialog;

    // URL to get contacts JSON
    private static String url = "http://api-11hr.anovatesoft.com/v1/list";

    // JSON Node names
    private static final String TAG_CONTACTS = "contacts";
    private static final String TAG_USERNAME = "username";
    private static final String TAG_NAME = "name";
    private static final String TAG_EMAIL = "email";
    private static final String TAG_ADDRESS = "address";
    private static final String TAG_CONTACT_NUMBER = "contactnumber";
    private static final String TAG_POSTAL_CODE = "postalcode";
    private static final String TAG_IMAGE = "image";
    SessionManager session;
    private double latitude;
    private double longitude;
    private String apikey;
    private String status;
    GPSTracker gps;


    // contacts JSONArray
    JSONArray contacts = null;

    // Hashmap for ListView
    ArrayList<HashMap<String, String>> contactList;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.home);
        ActionBar actionBar = getActionBar();
        actionBar.setDisplayHomeAsUpEnabled(true);

        AlertDialog.Builder alertDialog2 = new AlertDialog.Builder(
                Home.this);

// Setting Dialog Title
        alertDialog2.setTitle("Too quiet?");

// Setting Dialog Message
        alertDialog2.setMessage("Follow your favourite merchants and see what deals they're launching at the 11th Hour!");



// Setting Positive "Yes" Btn
        alertDialog2.setPositiveButton("YES",
                new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        // Write your code here to execute after dialog

                        dialog.cancel();

                    }
                });


        alertDialog2  .show();






        Gps_func();


        // Session class instance
        session = new SessionManager(getApplicationContext());

        // get user data from session
        HashMap<String, String> user = session.getUserDetails();


        // apikey
        // Calling Application class (see application tag in AndroidManifest.xml)
        final GlobalClass globalVariable = (GlobalClass) getApplicationContext();

        //Set name and email in global/application context
        apikey = globalVariable.getApikey();


        latitude = globalVariable.getLatitude();
        longitude = globalVariable.getLongitude();
        status = globalVariable.getStatus();



        contactList = new ArrayList<HashMap<String, String>>();

        ListView lv = getListView();

        // Listview on item click listener
        lv.setOnItemClickListener(new OnItemClickListener() {

            @Override
            public void onItemClick(AdapterView<?> parent, View view,
                                    int position, long id) {
                // getting values from selected ListItem

                String cost = ((TextView) view.findViewById(R.id.email))
                        .getText().toString();


                // Starting single contact activity
                Intent in = new Intent(getApplicationContext(),
                        Adds.class);

                in.putExtra(TAG_EMAIL, cost);

                startActivity(in);

            }
        });

        // Calling async task to get json
        new GetContacts().execute();
    }

    private void Gps_func() {

        gps = new GPSTracker(Home.this);

        // check if GPS enabled
        if (gps.canGetLocation()) {

            double latitude = gps.getLatitude();
            double longitude = gps.getLongitude();

            // \n is for new line
            //  Toast.makeText(getApplicationContext(), "Your Location is - \nLat: " + latitude + "\nLong: " + longitude, Toast.LENGTH_LONG).show();
            Log.d("tag", "Longitude:\n" + longitude + "\n Latitude: \n" + latitude);

        } else {
            // can't get location
            // GPS or Network is not enabled
            // Ask user to enable GPS/network in settings
            gps.showSettingsAlert();

            if (gps.canGetLocation()) {
                double latitude = gps.getLatitude();
                double longitude = gps.getLongitude();

                // \n is for new line
                // Toast.makeText(getApplicationContext(), "Your Location is - \nLat: " + latitude + "\nLong: " + longitude, Toast.LENGTH_LONG).show();
                Log.d("tag", "Longitude:\n" + longitude + "\n Latitude: \n" + latitude);

            }
        }

    }


    @Override
    protected void onRestart() {
        super.onRestart();
        Gps_func();
    }

    /**
     * Async task class to get json by making HTTP call
     */
    private class GetContacts extends AsyncTask<Void, Void, Void> {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            // Showing progress dialog
            pDialog = new ProgressDialog(Home.this);
            pDialog.setMessage("Please wait....");
            pDialog.setCancelable(false);
            pDialog.show();

        }

        @Override
        protected Void doInBackground(Void... arg0) {
            // Creating service handler class instance
            ServiceHandler1 sh = new ServiceHandler1(apikey, latitude, longitude);


            // Making a request to url and getting response
            String jsonStr = sh.makeServiceCall(url, ServiceHandler1.POST);

            Log.d("Response: ", "> " + jsonStr);


            if (jsonStr != null) {
                try {
                    JSONObject jsonObj = new JSONObject(jsonStr);

                    // Getting JSON Array node
                    contacts = jsonObj.getJSONArray(TAG_CONTACTS);

                    // looping through All Contacts
                    for (int i = 0; i < contacts.length(); i++) {
                        JSONObject c = contacts.getJSONObject(i);

                        String username = c.getString(TAG_USERNAME);
                        String name = c.getString(TAG_NAME);
                        String email = c.getString(TAG_EMAIL);
                        String address = c.getString(TAG_ADDRESS);
                        String gender = c.getString(TAG_CONTACT_NUMBER);
                        String postalcode = c.getString(TAG_POSTAL_CODE);


                        // tmp hashmap for single contact
                        HashMap<String, String> contact = new HashMap<String, String>();

                        // adding each child node to HashMap key => value

                        contact.put(TAG_EMAIL, email);


                        // adding contact to contact list
                        contactList.add(contact);
                    }
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            } else {
                Log.e("ServiceHandler", "Couldn't get any data from the url");
            }

            return null;
        }


        @Override
        protected void onPostExecute(Void result) {
            super.onPostExecute(result);
            // Dismiss the progress dialog
            if (pDialog.isShowing())
                pDialog.dismiss();
            /**
             * Updating parsed JSON data into ListView
             * */
            ListAdapter adapter = new SimpleAdapter(
                    Home.this, contactList,
                    R.layout.list_item1, new String[]{TAG_EMAIL,
            }, new int[]{
                    R.id.email,});

            setListAdapter(adapter);
        }
        public boolean onOptionsItemSelected(MenuItem item){
            Intent myIntent = new Intent(getApplicationContext(),Home.class);
            startActivityForResult(myIntent, 0);
            return true;
        }

    }


}

抛出NullPointerException ... 请给我一个完美解决方案。这是显示的日志：

无法开始活动 ComponentInfo {com.example.kmi_dev.fbloginsample / com.example.kmi_dev.fbloginsample.Home}：显示java.lang.NullPointerException

Answer 1

首先，如果您的活动延长getSupportActionBar()，则只会使用ActionBarActivity，所以在您的情况下，您必须像使用getActionBar()一样使用Theme。问题出在您的AndroidManifest.xml。如果您没有使用android:theme="@android:style/Theme.Holo.Light.NoActionBar"之类的内容，请务必使用nullexception，因为这可能会导致getActionBar().setHomeButtonEnabled(true);。除了你所做的，还需要添加

getActionBar().setDisplayHomeAsUpEnabled(true);

前

{{ hostvars[host]['ansible_eth1']['ipv4']['address'] }}

Answer 2

如果您使用的是 支持库 ，则需要使用getSupportActionBar()。

试试这个：

@Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.home);
        ActionBar actionBar = getActionBar();
        if(actionBar == null)
                  actionBar = getActionBarSupport();
        actionBar.setDisplayHomeAsUpEnabled(true);
}

另外要确保这两件事：
1.在声明活动时，您在清单上没有这个：

 android:theme="@android:style/Theme.NoTitleBar.Fullscreen"

2。您的活动不是孩子（不是对话框），如果您在声明活动时添加清单，请将其删除：

android:theme="@android:style/Theme.Dialog"

如何获得行动吧？

2 个答案: