Cannot execute Synchronised thread

I tried a code from Textbook about synchronized thread.
Tried writing the code as it is. But getting the error:

cannot find symbol f.start(); and t.display();

The textbook code was supposed to be tried without the synchronized keyword.
But it seems the compiler is unable to recognize the object.
Please help..

class First 
{
  	synchronized void display (String s)
   	{
   		System.out.println(s);
    
   		try
   		{
  			Thread.sleep(1000);
   		}
   		catch(InterruptedException e)
   		{
   			System.out.println("Interrupted");	
   		}
		System.out.println("***");
   	}
}
    
class Second  implements Runnable 
{
  	String s;
   	First f;	
   	Thread t;
    
   	public Second(First f1,String s1)
   	{
   		f=f1;
   		s=s1;
   		t=new Thread(this);
   		f.start();
   	}
    	
   	public void run()
   	{
   		t.display(s);
   	}
}
    
class SyncThread
{
   	public static void main(String args[])
   	{
   		First f=new First();
   		Second ob1=new Second(f,"First");
   		Second ob2=new Second(f,"Second");
   		Second ob3=new Second(f,"Third");
   
   		try
   		{
   			ob1.t.join();
   			ob2.t.join();
   			ob3.t.join();
   		}
   		catch(InterruptedException e)
   		{
   			System.out.println("Interrupted");
   		}
   	}
}

Instead of doing

t = new Thread(this);
f.start();

try:

t = new Thread(this);
t.start();

and in the run method, try f.display() instead of t.display()

Looks like you're messing up some variables. Try using some meaningful names instead of f and t. It's more difficult to mess up thread.start() and first.display() than t.start() and f.display().

Thread class does not have display method and your class First has the display method. I think you have wrongly typed t.display() and f.start(). Try to swith to f.display() and t.start().

It is giving you an error possibly because you have not used the @Override annotation above 'run()' function.
Here is the revised code:

 @Override
public void run()
{
f.display(s);
}

And Please do not run a thread in the constructor as it is not safe also you have made a mistake as it should be t.start() and not f.start(), similar changes in run()
should f.display(s) not t.display(s).

Changes:

@Override
public void run()
{
f.display(s);
}

And:

t=new Thread(this);
t.start();

Remember your variables because it would cause problems like these!
Now the program should run fine without any errors!