Java Custom Exception Examples

By mkyong | Updated: June 13, 2016

Viewed: 2,448 (+5 pv/w)

Tags: exception java

In Java, there are two types of exceptions – checked and unchecked exception. Here’s the summary :

1. Checked – Extends java.lang.Exception, for recoverable condition, try-catch the exception explicitly, compile error.
2. Unchecked – Extends java.lang.RuntimeException, for unrecoverable condition, like programming errors, no need try-catch, runtime error.

1. Custom Checked Exception

1.1 If the client is able to recover from the exception, make it a checked exception. To create a custom checked exception, extends java.lang.Exception

package com.mkyong.examples.exception;

public class NameNotFoundException extends Exception {

    public NameNotFoundException(String message) {
        super(message);
    }

}

1.2 For checked exception, you need to try and catch the exception.

package com.mkyong.examples;

import com.mkyong.examples.exception.NameNotFoundException;

public class CustomerService {

    public Customer findByName(String name) throws NameNotFoundException {

        if ("".equals(name)) {
            throw new NameNotFoundException("Name is empty!");
        }

        return new Customer(name);

    }

    public static void main(String[] args) {

        CustomerService obj = new CustomerService();

        try {

            Customer cus = obj.findByName("");

        } catch (NameNotFoundException e) {
            e.printStackTrace();
        }

    }
}

Output

com.mkyong.examples.exception.NameNotFoundException: Name is empty!
	at com.mkyong.examples.CustomerService.findByName(CustomerService.java:10)
	at com.mkyong.examples.CustomerService.main(CustomerService.java:39)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

2. Custom Unchecked Exception

2.1 If the client cannot do anything to recover from the exception, make it an unchecked exception. To create a custom unchecked exception, extends java.lang.RuntimeException

package com.mkyong.examples.exception;

public class ListTooLargeException extends RuntimeException{

    public ListTooLargeException(String message) {
        super(message);
    }

}

2.3 For unchecked exception, try and catch the exception is optional.

package com.mkyong.examples;

import com.mkyong.examples.exception.ListTooLargeException;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class CustomerService {

    public void analyze(List<String> data) {

        if (data.size() > 50) {
            //runtime exception
            throw new ListTooLargeException("List can't exceed 50 items!");
        }

		//...
    }

    public static void main(String[] args) {

        CustomerService obj = new CustomerService();

		//create 100 size
        List<String> data = new ArrayList<>(Collections.nCopies(100, "mkyong"));

        obj.analyze(data);


    }
}

Output

Exception in thread "main" com.mkyong.examples.exception.ListTooLargeException: List can't exceed 50 items!
	at com.mkyong.examples.CustomerService.analyze(CustomerService.java:25)
	at com.mkyong.examples.CustomerService.main(CustomerService.java:38)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

References

1. Unchecked Exceptions — The Controversy
2. difference between java.lang.RuntimeException and java.lang.Exception