Create Custom Exception in Java

import java.util.ArrayList;
import java.util.Arrays;

    // create a checked exception class
    class CustomException extends Exception {
    public CustomException(String message) {
        super(message);
    }
}
public class Main {
    ArrayList<String> cities = new ArrayList<>(Arrays.asList("London", "New York", "Washington"));

    // check the exception condition
    public void checkCity(String city) throws CustomException {

        // throw exception if city already present in ArrayList
        if(cities.contains(city)) {
            throw new CustomException(city + " already exists");
        }
        else {

            // insert city to ArrayList
            cities.add(city);
            System.out.println(city + " is added to the ArrayList");
        }
    }
    public static void main(String[] args) {

        // create object of Main class
        Main obj = new Main();

        // exception is handled using try...catch
        try {
            obj.checkCity("Delhi");
            obj.checkCity("London");
        }
        catch(CustomException e) {
            System.out.println("[" + e + "] Exception Occured");
        }
    }
}

Delhi is added to the ArrayList
[CustomException: London already exists] Exception Occured

• This program deals with creating customized case of checked exceptions.
• These types of exceptions are ought to be dealt with either a try-catch block in the program, or utilize throws keyword.
• In other words compiler will throw an error.
• To create custom exception, we create a class CustomException that extends its parent Exception class.
• In its constructor, we call super class constructor i.e. Exception.
• After that, we create a Main class which at first declares a new String type ArrayList by the name of cities.
• Main class contains a method checkCity(), having a string type parameter, that throws an exception if the value in the argument is already present in the city list, otherwise that value is appended to the list.
• The try-catch block in the main method, try calls the checkCity() method, and catch handles the exception that occurs during the second call with Java in the argument.   

import java.util.ArrayList;
import java.util.Arrays;

// create a unchecked exception class
class CustomException extends RuntimeException {
    public CustomException(String message) {
        // call the constructor of RuntimeException
        super(message);
    }
}
public class Main {
    ArrayList<String> cities = new ArrayList<>(Arrays.asList("London", "New York", "Washington"));

    // check the exception condition
    public void checkCity(String city) {

        // throw exception if city already present in ArrayList
        if(cities.contains(city)) {
            throw new CustomException(city + " already exists");
        }
        else {
            // insert city to ArrayList
            cities.add(city);
            System.out.println(city + " is added to the ArrayList");
        }
   }
    public static void main(String[] args) {
    
        // create object of Main class
        Main obj = new Main();

        // check if city already present
        obj.checkCity("Delhi");
        obj.checkCity("London");
    }
}

Delhi is added to the ArrayList
Exception in thread "main" CustomException: London already exists
     at Main.checkCity(Main.java:21)
     at Main.main(Main.java:37)

• This program deals with creating customized case of unchecked exceptions.
• These types of exceptions are thrown when the program is executed, and are not checked during compile time.
• Therefore no try-catch block is required.
• The procedure for custom unchecked exception is similar to the previous example with slight changes.
• The foremost step is the definition of CustomException class which extends RunTimeException class.
• Similar to the previous case, we call the constructor of super class.
• Again we define a Main class, with a function that’ll throw an exception.
• If the value in the argument does not exist in the ArrayList, then it is appended to the list.
• Otherwise, a message (exception) is displayed indicating that it already exists.