Creating a class "RoomCounter.java":

• Import required package.
• Define the class "RoomCounter".
  • Declare a private variable "countPeople".
  • Define a default constructor and initialize the value.
  • Give function definition to "addPerson ()".
    • Increment the value of "countPeople".
  • Give function definition to "removePerson ()".
    • Check of the value of "countPeople" is less than or equal to 0.
      • Throw "NegativeCounterException" with a message.
        • Decrement the value of "countPeople".
  • Give function definition to "getcount ()".
    • Return the value of "countPeople".
  • Define the "main ()" method.
    • Create an object "rc" for the class "RoomCounter".
    • Get and print the value of count by using the function "rc.getCount ()".
    • Add three persons by calling the function "addPerson ()" three times.
    • Get and print the value of count by using the function "rc.getCount ()".
    • Inside "try" block,
      • Remove two persons by calling the function "removePerson ()" two times.
      • Get and print the value of count by using the function "rc.getCount ()".
      • Remove one person by calling the function "removePerson ()" one time.
      • Get and print the value of count by using the function "rc.getCount ()".
      • Again remove one person by calling the function "removePerson ()" one time.
      • Get and print the value of count by using the function "rc.getCount ()".
        • Catch "NegativeCounterException".
        • • Get and print the message by using the function "getMessage ()".

Creating a class "NegativeCounterException.java":

• Define the class "NegativeCounterException" that extends "Exception".
  • Define parameterized constructor.
    • Call the parent class’s method by using "super" keyword.

Program:

RoomCounter.java:

//Define a class

public class RoomCounter

{

//Declare a private variable

private int countPeople;

//Define a default constructor

public RoomCounter()

{

//Assign 0

countPeople = 0;

}

//Give function definition to add person

public void addPerson()

{

//Increment the variable

countPeople++;

}

//Function definition to remove person

public void removePerson()throws NegativeCounterException

{

//Check if count is less than or equal to 0

if(countPeople <= 0 )

//Throw an exception

throw new NegativeCounterException("Can not remove a person");

//Decrement the counter variable

countPeople--;

}

//Function to get count

public int getCount()

{

//Return count

return countPeople;

}

//Define the main method

public static void main(String[] args)

{

//Create an object for the class

RoomCounter rc = new RoomCounter();

//Get the count after invoking default constructor

System...