My if statement will never equal true - Why?

Question

I've been working on an app, and in a certain activity, a if statement seems to never equal true but I can't figure out why. I suppose it has something to do with the SharedPreferences. Could someone take a look at my code and see why this is happening so I could prevent it later on while typing more complex code?

This is the activity I'm having trouble with:

public class SignInActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.content_main3);

        SharedPreferences example = getSharedPreferences("label", 0);
        final String username = example.getString("username", "nothing found");

        final Button LogInButton = (Button) findViewById(R.id.LoginButton);



        LogInButton.setOnClickListener(
                new Button.OnClickListener(){
                    public void onClick(View v){
                        EditText userNameInput = (EditText) findViewById(R.id.UsernameLogIn);
                        String userString = userNameInput.getText().toString();


                        if (username == userString){
                            Intent nextScreen = new Intent(getApplicationContext(), MainActivity.class);
                            startActivity(nextScreen);
                        }
                    }
                }
        );
    }
}