Why does my Android app crash with a NullPointerException when initializing a variable with findViewById(R.id.******) at the beginning of the class?

Question

This code, with the block at the top commented, runs successfully:

public class MainActivity extends AppCompatActivity {

    /*
    EditText username = (EditText)findViewById(R.id.editText_Username);
    EditText password = (EditText)findViewById(R.id.editText_Password);
    TextView inputdata = (TextView)findViewById(R.id.textView_InputData);
    TextView welcome = (TextView)findViewById(R.id.textView_Welcome);
    Button login=(Button)findViewById(R.id.button_Login);
    Button anotherLogin=(Button)findViewById(R.id.button_Login_Another);
    */

    public void doLoginOnClick(View v)
    {
        String s1=username.getText().toString();
        inputdata.setText(s1);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton)findViewById(R.id.fab); 
        fab.setOnClickListener(new View.OnClickListener()
        {
            @Override
            public void onClick(View view)
            {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    } 
 
}

I am trying to capture id of component using

findViewById(R.id.***)

as you can see, I put this code in comment at the very beginning of the code.

EditText username = (EditText)findViewById(R.id.editText_Username);
EditText password = (EditText)findViewById(R.id.editText_Password);
TextView inputdata = (TextView)findViewById(R.id.textView_InputData);
TextView welcome = (TextView)findViewById(R.id.textView_Welcome);
Button login=(Button)findViewById(R.id.button_Login);
Button anotherLogin=(Button)findViewById(R.id.button_Login_Another);

If I remove the comment above and run it (both in emulator and real device), the program crashes immediately, I am surprised what's wrong here?

even I have tried to initialize the same thing using constructor.

But if I put it inside onCreate() there's no crash? Why?

I was trying to fetch info of username and password and display it to the textview in textView_Inputdata using

EditText username = (EditText)findViewById(R.id.editText_Username);
EditText password = (EditText)findViewById(R.id.editText_Password);
TextView inputdata = (TextView)findViewById(R.id.textView_InputData);
inputdata.setText(username.getText.toString()+" "+password.getText.toString());

Is there better or easier way to do that?

Without any stacktrace i bet its a nullPointerException caused by findViewById returning null caused by no layout loaded. Call it after setContentView(). — Nanoc, Apr 06 '16 at 08:56

score 21 · Accepted Answer · answered Apr 06 '16 at 08:55

Instance member variables are initialized when the instance itself is initialized. It's too early for findViewById()

Before onCreate() your activity does not yet have a Window that findViewById() needs internally. Before setContentView() (that you should be calling in onCreate()) there are no views to be found either.

Therefore init your view references in onCreate() and after setContentView().

score 4 · Answer 2 · edited Aug 13 '18 at 07:34

Add this code

 EditText username =  findViewById(R.id.editText_Username);
 EditText password = findViewById(R.id.editText_Password);
 TextView inputdata = findViewById(R.id.textView_InputData);
 TextView welcome = findViewById(R.id.textView_Welcome);
 Button login = findViewById(R.id.button_Login);
 Button anotherLogin = findViewById(R.id.button_Login_Another);

in

onCreate();

method after

setContentView(R.layout.activity_main);

score 1 · Answer 3 · edited Aug 15 '17 at 00:24

1

Hey I see that you are defining and initialising the instance variables. What I do is I define the the instance variables - EditText username and then in the onCreate method I initialise them - username = (EditText) findViewById(R.id.editText_Username);

The reason you don't initialize the instance variables is because the elements are not ready until after the setContentView in onCreate method - I could be wrong with this, but my best practice is define the instance variable and then initialize them in the onCreate method

edited Aug 15 '17 at 00:24

OneCricketeer

179,855
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answered Apr 06 '16 at 09:00

Dan

2,020
5
32
55

Yes I would suggest that, I do it ad find it very effective – Dan Apr 06 '16 at 09:12

score 1 · Answer 4 · edited Aug 15 '17 at 00:24

1

Before setting setContentView() you are not able to initialize the view items.Because view will not be exists for activity at that time.

Keep the initializations in one separate method and call that method after setting the view to the activity.

edited Aug 15 '17 at 00:24

OneCricketeer

179,855
19
132
245

answered Apr 06 '16 at 09:02

chaithu

23
5

score 1 · Answer 5 · answered Oct 14 '21 at 12:11

You must call findViewById() after setContentView() in onCreate(), before that there isn't UI/Layout initialize thats why your findViewById() unable to find ids and throws NullPointerException.

public class MainActivity extends AppCompatActivity {

    public void doLoginOnClick(View v){
        String s1 = username . getText ().toString();
        inputdata.setText(s1);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar =(Toolbar) findViewById (R.id.toolbar);
        setSupportActionBar(toolbar);

        EditText username =(EditText) findViewById (R.id.editText_Username);
        EditText password =(EditText) findViewById (R.id.editText_Password);
        TextView inputdata =(TextView) findViewById (R.id.textView_InputData);
        TextView welcome =(TextView) findViewById (R.id.textView_Welcome);
        Button login =(Button) findViewById (R.id.button_Login);
        Button anotherLogin =(Button) findViewById (R.id.button_Login_Another);

        FloatingActionButton fab =(FloatingActionButton) findViewById (R.id.fab);
        fab.setOnClickListener(new View . OnClickListener ()
        {
            @Override
            public void onClick(View view)
            {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
            }
        });
    }
}

score 1 · Answer 6 · answered Oct 18 '21 at 13:49

1

Initialize all your views after onCreate() Method
Read More about activity lifecycle here

answered Oct 18 '21 at 13:49

Manjeet deswal

692
2
5
18

score 1 · Answer 7 · answered Oct 18 '21 at 23:44

Think of it this way - until you tell the activity which user interface layout(xml) to use, it will not know where to get those views from.
You would do setContentView(R.layout.your_xml) to tell the activity which layout to use as the user interface for the activity. You would do this in the onCreate callback because this is the callback to do 'one-time activity level set up' tasks. setContentView() is one such task. Once you do this, you can begin to access the views that are present within the layout file R.layout.your_xml.

onCreate(...) 
{
    ...
    setContentView(R.layout.your_xml) 
    // Ready to use the views from above xml. 
    findViewById(R.id.your_view) 
    ... 
}

score 1 · Answer 8 · edited Oct 20 '21 at 02:42

setContentView() is responsible for inflating the layout of the activity. If any of the components are used before inflation of the layout, it will crash with a NullPointerException.

 @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        setContentView(R.layout.activity_layout);
        username =  findViewById(R.id.editText_Username);
        password = findViewById(R.id.editText_Password);
        inputdata = findViewById(R.id.textView_InputData);
        welcome = findViewById(R.id.textView_Welcome);
        login = findViewById(R.id.button_Login);
        anotherLogin = findViewById(R.id.button_Login_Another);
            
    }

You can still declare the variable in the beginning of the class, but you have to initialize it in onCreate.

I'm awarding the bounty to this answer because I think it's the best of the new answers. In particular, it includes the fact that the variables can be declared at the beginning of the class, but initialized in `onCreate`. — Ryan M, Oct 20 '21 at 02:45

score 1 · Answer 9 · answered Oct 19 '21 at 06:47

View Binding will help you prevent from these issues:

https://developer.android.com/topic/libraries/view-binding

Or declare the views on top, and initialize them after setContentView() in onCreate().

 public class MainActivity extends AppCompatActivity {

//Declare view on top
EditText username, password;
TextView inputdata,welcome;
Button login,notherLogin;


public void doLoginOnClick(View v)
{
    String s1=username.getText().toString();
    inputdata.setText(s1);
}

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

    //set view
    username =  findViewById(R.id.editText_Username);
    password = findViewById(R.id.editText_Password);
    inputdata = findViewById(R.id.textView_InputData);
    welcome = findViewById(R.id.textView_Welcome);
    login = findViewById(R.id.button_Login);
    anotherLogin = findViewById(R.id.button_Login_Another);

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    FloatingActionButton fab = (FloatingActionButton)findViewById(R.id.fab); 
    fab.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View view)
        {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });
} }

Why does my Android app crash with a NullPointerException when initializing a variable with findViewById(R.id.******) at the beginning of the class?

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