Android check null or empty string in Android

问题:

In my trying AsyncTask I get email address from my server. In onPostExecute() I have to check is email address empty or null. I used following code to check it:

if (userEmail != null && !userEmail.isEmpty()) {
    Toast.makeText(getActivity(), userEmail, Toast.LENGTH_LONG).show();
    UserEmailLabel.setText(userEmail);
}

But in my Toast I see null is printed. My full code:

private class LoadPersonalData extends AsyncTask<String, Void, Void> {
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }

    protected Void doInBackground(String... res) {
        List<NameValuePair> params = new ArrayList<NameValuePair>();
        params.add(new BasicNameValuePair("user_id", PrefUserName));
        params.add(new BasicNameValuePair("type", type_data));
        JSONObject json = jsonParser.makeHttpRequest(Url, "POST", params);
        String result = "";
        try {
            result = json.getString("message");
        } catch (JSONException e) {
            e.printStackTrace();
        }
        if (result.equals("success")) {
            try {
                userEmail = json.getString("email");
            } catch (JSONException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        return null;
    }

    @Override
    protected void onPostExecute(Void result) {
        // TODO Auto-generated method stub
        super.onPostExecute(result);            
        if (userEmail != null && !userEmail.isEmpty()) {
            Toast.makeText(getActivity(), userEmail, Toast.LENGTH_LONG).show();
            UserEmailLabel.setText(userEmail);
        }
    }

How can I check for null and empty string? Thanks in advance.

回答1:

Use TextUtils.isEmpty( someString )

String myString = null;

if (TextUtils.isEmpty(myString)) {
    return; // or break, continue, throw
}

// myString is neither null nor empty if this point is reached
Log.i("TAG", myString);

Notes

• The documentation states that both null and zero length are checked for. No need to reinvent the wheel here.
• A good practice to follow is early return.

回答2:

From @Jon Skeet comment, really the String value is "null". Following code solved it

if (userEmail != null && !userEmail.isEmpty() && !userEmail.equals("null")) 

回答3:

Checking for empty string if it is equal to null, length is zero or containing "null" string

public static boolean isEmptyString(String text) {
        return (text == null || text.trim().equals("null") || text.trim()
                .length() <= 0);
    }

回答4:

Yo can check it with this:

if(userEmail != null && !userEmail .isEmpty())

And remember you must use from exact above code with that order. Because that ensuring you will not get a null pointer exception from userEmail.isEmpty() if userEmail is null.

Above description, it's only available since Java SE 1.6. Check userEmail.length() == 0 on previous versions.

回答5:

You can check it with utility method "isEmpty" from TextUtils,

isEmpty(CharSequence str) method check both condition, for null and length.

public static boolean isEmpty(CharSequence str) {
     if (str == null || str.length() == 0)
        return true;
     else
        return false;
}

回答6:

if you check null or empty String so you can try this

if (createReminderRequest.getDate() == null && createReminderRequest.getDate().trim().equals("")){
    DialogUtility.showToast(this, ProjectUtils.getString(R.string.please_select_date_n_time));
}

回答7:

All you can do is to call equals() method on empty String literal and pass the object you are testing as shown below :

 String nullString = null;
 String empty = new String();
 boolean test = "".equals(empty); // true 
 System.out.println(test); 
 boolean check = "".equals(nullString); // false 
 System.out.println(check);

回答8:

This worked for me

EditText   etname = (EditText) findViewById(R.id.etname);
 if(etname.length() == 0){
                    etname.setError("Required field");
                }else{
                    Toast.makeText(getApplicationContext(),"Saved",Toast.LENGTH_LONG).show();
                }

or Use this for strings

String DEP_DATE;
if (DEP_DATE.equals("") || DEP_DATE.length() == 0 || DEP_DATE.isEmpty() || DEP_DATE == null){
}