java multithreading basic console program

i'm new to multithreading. So here is the problem I'm working on: Store the last four digits of your student ID in two separate classes. For example, if your ID is SE123456789 then store 67 in first Thread1 Class and 89 in Thread2 Class that will be passed to Factorial class later.
Thread1 class will pass 67 to the Factorial class and printfactorial() will print factorial of all the numbers from 1 to 67.
Thread2 class will pass 89to the Factorial class and printfactorial() will print factorial of all the numbers from 1 to 89.
You should use thread.sleep(10) after each calculation inside the loop.
As you know that you cannot store and print factorial of large numbers in simple integer or long type variables, so you are required to use BigInteger to store and print very long numbers.
printfactorial() method must be synchronized so that the results of Thread1 are printed first then the result of Thread2 is calculated and printed.
and here's what I have done so far.
I have four different classes

Main
Factorial
Thread1
Thread2

Thread1 and Thread2 both extend Thread class.
here's the code I wrote so far:

> Main

public class Main {
public static void main(String args[]){
Factorial factorial = new Factorial();  
Thread1 t1 = new Thread1(factorial);
Thread1 t2 = new Thread1(factorial);
t1.start();
t2.start();
}
}

> Factorial Class

import java.math.BigInteger;
public class Factorial {
public void printFactorial(int number){
BigInteger bigInteger = new BigInteger("1");
try{
for(int i=1; i<=number; i++){
bigInteger = bigInteger.multiply(BigInteger.valueOf(i));
Thread.sleep(10); 
System.out.println(bigInteger);
}
}catch(InterruptedException ex){
System.out.println("the interruption has occurred in the thread");
}
}
}

> Thread1

package com.mycompany.factorial;
public class Thread1 extends Thread {
Factorial factorial;
Thread1(Factorial fact){
factorial = fact;
}
@Override
public void start(){
synchronized(factorial){
try{
/*my ID is: SE170400080
so the second last two digits are 00.
**/
factorial.printFactorial(00); //here's the problem
}catch(Exception e){
System.out.println("the interruption has occurred in the thread");
}
}
}
}

> Thread2

package com.mycompany.factorial;
public class Thread2 extends Thread {
Factorial factorial;
Thread2(Factorial fact){
factorial = fact;
}
@Override
public void start(){
synchronized(factorial){
try{
factorial.printFactorial(80);
}catch(Exception e){
System.out.println("the interruption has occurred in the thread");
}
}
}
}

after running the main it builds successfully but does not show the required output.

help would be really appreciated, I tried best to keep it on point.

1:

Not a complete answer but you need to know this:

A Thread instance is not a thread. A thread is an object in the operating system that runs your code. A Java Thread instance is an object in your program that you can use to create and manage an operating system thread.

The Thread instance creates the OS thread when you call t.start(). Don't override the start() method because if you do that, then it will never create the thread. You'll just be calling the method that you wrote if you override start().

When you call the normal t.start() method, and it creates a new thread, then the new thread will call the t.run(). The run() method is the one you want to override because it's code is what will be executed in the new thread.

There are a few errors in your code:

1. Override run method, rather than start: start only starts your thread, but the actual code being run belongs to the run method. A nice little tutorial is on the Oracle's website.

2. In main, you create two instances of Thread1, a likely copy-pasting error