Java Program to Create custom exception

Example 1: Java program to create custom checked exception

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

// create a checked exception class
class CustomException extends Exception {
  public CustomException(String message) {
    // call the constructor of Exception class
    super(message);
  }
}

class Main {

  ArrayList<String> languages = new ArrayList<>(Arrays.asList("Java", "Python", "JavaScript"));

  // check the exception condition
  public void checkLanguage(String language) throws CustomException {

    // throw exception if language already present in ArrayList
    if(languages.contains(language)) {
      throw new CustomException(language + " already exists");
    }
    else {
      // insert language to ArrayList
      languages.add(language);
      System.out.println(language + " 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.checkLanguage("Swift");
      obj.checkLanguage("Java");
    }

    catch(CustomException e) {
      System.out.println("[" + e + "] Exception Occured");
    }
  }
}

Output

Swift is added to the ArrayList
[CustomException: Java already exists] Exception Occured

In the above example, we have extended the Exception class to create a custom exception named CustomException. Here, we call the constructor of Exception class from the CustomException class using super keyword.

Inside the method checkLanguage(), we have checked the exception condition, and if the exception occurs, the try..catch block handles the exception.

Here, this is the checked exception. We can also create unchecked exception class in Java.

Example 2: Create custom unchecked exception class

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);
  }
}

class Main {

  ArrayList<String> languages = new ArrayList<>(Arrays.asList("Java", "Python", "JavaScript"));

  // check the exception condition
  public void checkLanguage(String language) {

    // throw exception if language already present in ArrayList
    if(languages.contains(language)) {
      throw new CustomException(language + " already exists");
    }
    else {
      // insert language to ArrayList
      languages.add(language);
      System.out.println(language + " is added to the ArrayList");
    }
  }

  public static void main(String[] args) {

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

    // check if language already present
    obj.checkLanguage("Swift");
    obj.checkLanguage("Java");
  }
}

Output

Swift is added to the ArrayList
Exception in thread "main" CustomException: Java already exists
        at Main.checkLanguage(Main.java:21)
        at Main.main(Main.java:37)

In the above example, we have extended the RuntimeException class to create an unchecked custom exception class.

Here, you can notice that, we haven't declared any try...catch block. It is because the unchecked exception is checked at runtime.

Besides that, other functionality of unchecked exception is similar to the above mentioned program.