Random integer declared in onCreate

I am new in this community and sorry if my question does not meet the policy, I will try my best to improve for my next questions.

I am currently learning Android, I have good bases in Java.

I try to use the following code in onCreate to get a random integer (1-20):

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

    int randomNumber = (int)(Math.random()*20)+1;

}

And on my MainActivity class:

public class MainActivity extends AppCompatActivity {
    int randomNumber;

It does not work, I get only 0 as random number.

However if I do not declare my random integer on onCreat but only in Mainactivity, it works:

public class MainActivity extends AppCompatActivity {
    int randomNumber=(int)(Math.random()*20)+1;

Finally if I use another way to get a random number declared on the onCreate, it Works as shown below:

public class MainActivity extends AppCompatActivity {
    int randomNumber;

....

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Random rand = new Random();
        randomNumber=rand.nextInt(20)+1;
    }
}

I do not understand why I could not declare my random in the onCreate method. with Math.random but worked with rand.nextInt Anyway, I was able to make it using other code but I would like to understand why.

onClick method code were not shown.

Thanks a lot for your help!