Factorial

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" tools:context=".MainActivity">

<EditText
    android:id="@+id/etnum"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_margin="20dp"
    android:layout_marginStart="20dp"
    android:layout_marginLeft="20dp"
    android:layout_marginTop="20dp"
    android:layout_marginEnd="20dp"
    android:layout_marginRight="20dp"
    android:layout_marginBottom="20dp"
    android:ems="10"
    android:hint="Enter number"
    android:inputType="number" />

<Button
    android:id="@+id/btn"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_margin="20dp"
    android:layout_marginStart="20px"
    android:layout_marginLeft="20px"
    android:layout_marginTop="20px"
    android:layout_marginEnd="20px"
    android:layout_marginRight="20px"
    android:layout_marginBottom="20px"
    android:text="Factorial" />

<TextView
    android:id="@+id/tv"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="20dp"
    android:layout_marginStart="20px"
    android:layout_marginLeft="20px"
    android:layout_marginTop="20px"
    android:layout_marginEnd="20px"
    android:layout_marginRight="20px"
    android:layout_marginBottom="20px"
    android:text="Answer" />

</LinearLayout>

Java:
package com.example.factorial;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
private EditText et;
private TextView tv;
private Button bt;

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

    et = (EditText)findViewById(R.id.etnum);
    tv = (TextView)findViewById(R.id.tv);
    bt=(Button)findViewById(R.id.btn);

    bt.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            int num=Integer.parseInt(et.getText().toString());
            int fact=1;
            for(int i=1;i<=num;i++){
                fact=fact*i;

            }
            tv.setText(String.valueOf(fact));
        }
    });
}

}