How do I call a class in a main class

This program I have made measures the area of the room.
Now i want to create two classes. b-(
This is the original coding I made that works perfectly. But here I put everything in one class:

Code Java:

---

import javax.swing.JOptionPane;
 
public class RoomOne
{
public static void main(String[] args) throws Exception
{
//Declaration of strings
String firstNumber;
String secondNumber;
 
int numberOne=10;
int numberTwo=20;
int finalAnswer=numberOne*numberTwo;
 
JOptionPane.showMessageDialog(null, "The area of the room is: " + finalAnswer + " sqaure feet", "Room Measurement", JOptionPane.INFORMATION_MESSAGE);
{
System.exit(0);
}
}
}

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I want to have two classes, the main class, and a secondary class.
The secondary class should contain the entire coding. I want to be able to call the secondary class in the main class. How do I do that?

Your first step is to put that all in a method, then call it from the main you already have. When you have that working, then just do the same thing in another class.

Hey this is initail level of codding... please read the java book.
For your reference i am attaching the code.. But please take care....

Code Java:

---

package com.java.main;
 
import com.java.main.RoomTwo;
 
public class RoomOne {
	public static void main(String[] args) throws Exception {
		RoomTwo r2 = new RoomTwo();
		r2.roomDetail();
	}
}

---

Code Java:

---

package com.java.main;
 
import javax.swing.JOptionPane;
 
public class RoomTwo {
	public void roomDetail() throws Exception {
		// Declaration of strings
		String firstNumber;
		String secondNumber;
 
		int numberOne = 10;
		int numberTwo = 20;
		int finalAnswer = numberOne * numberTwo;
 
		JOptionPane.showMessageDialog(null, "The area of the room is: "
				+ finalAnswer + " sqaure feet", "Room Measurement",
				JOptionPane.INFORMATION_MESSAGE);
		{
			System.exit(0);
		}
	}
}

---