How To Utilisation Custom Exception Inwards Coffee - Tutorial Example

Sometimes nosotros necessitate to hit custom Exception inward Java, i.e. Exceptions which are non defined inward JDK or whatsoever tertiary political party library your application is using. Though it’s widely recommended on several Exception best practices article, fifty-fifty Joshua Bloch has recommended inward Effective Java to prefer measure exception over custom exception, sometimes you lot truly necessitate it. There are for certain guidelines to assist to abide by whether you lot truly necessitate a custom exception or not. You should write your ain exception classes if you lot answer yeah to whatsoever of the next questions; otherwise, you lot tin give the sack in all probability purpose soul else's.

•     Do you lot necessitate an exception type that isn't represented yesteryear those inward the Java platform?
•     Would it assist users if they could differentiate your exceptions from those thrown yesteryear classes written yesteryear other vendors?
•     Does your code throw to a greater extent than than i related exception?
•     If you lot purpose soul else's exceptions, volition users receive got access to those exceptions? The similar query is, should your bundle last independent too self-contained?

Custom Exception or Custom Message amongst Standard Exception?

For instance if you lot declare an Exception that doesn't render whatsoever useful information other than a custom mention too then it in all probability uses generic Exception flat amongst a custom message every bit shown inward below example:

public class DuplicateIDException extends Exception {}

This custom exception doesn't render whatsoever extra information e.g. choice ids, too that's why tin give the sack last easily replaced yesteryear a custom message too measure Exception class, every bit shown below:

throw new Exception("ID already taken");

Even better, if you lot intend the customer code is non going to convey whatsoever activeness other than logging if the id is already taken, throw an unchecked exception:

throw new RuntimeException("ID already taken");

Checked or Unchecked?

Once you lot hit the conclusion to hit custom Exception, the adjacent affair is to determine on checked vs unchecked exception. As I said before, yesteryear default hit your exception unchecked too you lot volition abide by it whether it should last checked spell writing customer code. General guideline is to hit an exception unchecked if the customer code is non going to convey whatsoever activeness other than logging.

How to hit Custom Exception inward Java - The Code

Here is our consummate code instance of creating custom or user defined Exception inward Java.  In our example, nosotros receive got created NoSuchProductException, which is thrown yesteryear methods returning products. This is an unchecked Exception every bit nosotros made it inherit from RuntimeException. It inherit getMessage() method from Throwable too likewise has a method getProductId() which returns production id for which this exception has caused. Don't hit an Exception flat every bit nested class fifty-fifty if it's used solely yesteryear i class, ever declare Exceptions inward their ain class.

import java.util.HashMap; import java.util.Map;  /**  * Java Program to hit custom exception too examples to present how to purpose  * custom exception inward Java.  *  * @author Javin Paul  */ public class CustomExceptionDemo {      private static final Map&lt;Integer, String&gt; products = new HashMap<>();      static {         products.put(100, "Coke");         products.put(101, "KitKat");         products.put(102, "Bisuits");         products.put(103, "Toast");     }      public static void main(String args[]) {         CustomExceptionDemo t = new CustomExceptionDemo();         t.getProduct(1000);     }      public String getProduct(int id) {         if (products.get(id) == null) {             throw new NoSuchProductException("No such production exists", id);         }         return products.get(id);     } }  class NoSuchProductException extends RuntimeException {      private int productId;      public NoSuchProductException() {         super();     }      public NoSuchProductException(String message, int productId) {         super(message);         this.productId = productId;     }      public NoSuchProductException(String message, int productId, Throwable cause) {         super(message, cause);         this.productId = productId;     }      @Override     public String toString() {         return super.toString();     }      @Override     public String getMessage() {         return super.getMessage() + " for productId :" + productId;     }      public int getProductId() {         return productId;     } }  Output: Exception inward thread "main" NoSuchProductException: No such production exists for productId :1000     at CustomExceptionDemo.getProduct(CustomExceptionDemo.java:26)     at CustomExceptionDemo.main(CustomExceptionDemo.java:21)

Things to recollect spell creating Custom Exception inward Java

Though creating a custom, exception is every bit slow every bit subclassing java.lang.Exception class, at that spot are few best practices you lot tin give the sack follow to hit most of it. There is thence much criticism of checked exception due to boilerplate require to grip it, you lot volition hardly hit your custom exception every bit checked.

1) Don’t' purpose Exception to command application behavior. Exception treatment is really expensive every bit it requires native calls to re-create stack trace, each fourth dimension exception is created.

2) While creating a custom exception, prefer to hit an unchecked, Runtime exception than a checked exception, peculiarly if you lot know that customer is non going to convey whatsoever reactive activeness other than logging.

3) If your custom exception is created yesteryear passing approximately other exception, too then ever incorporate master copy Exception every bit a source; purpose constructor which takes Exception rather than solely message String.

4) Apart from providing default no declaration constructor on your custom Exception class, reckon providing at to the lowest degree 2 to a greater extent than constructors, i which should convey a failure message too other which tin give the sack convey approximately other Throwable every bit the cause.

5) If possible, avoid creating custom Exception too re-use existing, measure Exception classes from JDK itself. Most of the fourth dimension you lot volition realize that all you lot necessitate is a shape of IllegalArgumentException or ParseException or something similar.

6) While defining custom Exception, i of the most mutual error programmer hit is to intend that constructor is inherited from java.lang.Exception class, for example, they intend that their Exception flat volition automatically inherit default no declaration constructor too the i which takes a String message. This is non true. The constructor is non inherited inward Java, non fifty-fifty default constructor. It's truly added yesteryear the compiler rather than inherited from rear class. That's why I receive got declared 2 constructors, i amongst String parameter too other every bit Throwable parameter:

public NoSuchProductException(String message, int productId) {         super(message);         this.productId = productId;     }      public NoSuchProductException(String message, int productId, Throwable cause) {         super(message, cause);         this.productId = productId;     }
 

This is truly the measure means of creating custom Exception inward Java. In guild to relieve time, you lot tin give the sack fifty-fifty hit a template of to a higher house flat inward Eclipse IDE.

7) For readable code, it's practiced practise to append the string Exception to the names of all classes that inherits (directly or indirectly) from the Exception flat e.g. instead of naming your flat IncorrectPassword, mention it IncorrectPasswordException.


That's all virtually How to hit custom Exception classes inward Java. As I said, showtime seek out to avoid the temptation of creating a hit novel Exception class, intend if you lot tin give the sack reuse existing ones. If you lot absolutely need, too then hit for certain to follow best practices. At the bare minimum, hit an endeavor to hit unchecked exception rather than a checked one.

Further Learning
Complete Java Masterclass
Java Fundamentals: The Java Language
Java In-Depth: Become a Complete Java Engineer!

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