How To Practise Custom Exception Inwards Coffee - Tutorial Example

Sometimes nosotros ask to produce custom Exception inwards Java, i.e. Exceptions which are non defined inwards JDK or whatever 3rd political party library your application is using. Though it’s widely recommended on several Exception best practices article, fifty-fifty Joshua Bloch has recommended inwards Effective Java to prefer measure exception over custom exception, sometimes yous genuinely ask it. There are sure enough guidelines to assist to uncovering whether yous genuinely ask a custom exception or not. You should write your ain exception classes if yous respond yep to whatever of the next questions; otherwise, yous tin likely role soul else's.

•     Do yous ask an exception type that isn't represented yesteryear those inwards 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 yous role soul else's exceptions, volition users conduct maintain access to those exceptions? The like inquiry is, should your packet hold upward independent together with self-contained?

Custom Exception or Custom Message alongside Standard Exception?

For event if yous declare an Exception that doesn't render whatever useful information other than a custom advert thence it likely uses generic Exception shape alongside a custom message every bit shown inwards below example:

public class DuplicateIDException extends Exception {}

This custom exception doesn't render whatever extra information e.g. choice ids, together with that's why tin hold upward easily replaced yesteryear a custom message together with measure Exception class, every bit shown below:

throw new Exception("ID already taken");

Even better, if yous intend the customer code is non going to convey whatever activity other than logging if the id is already taken, throw an unchecked exception:

throw new RuntimeException("ID already taken");

Checked or Unchecked?

Once yous produce the determination to produce custom Exception, the side yesteryear side matter is to determine on checked vs unchecked exception. As I said before, yesteryear default produce your exception unchecked together with yous volition uncovering it whether it should hold upward checked piece writing customer code. General guideline is to produce an exception unchecked if the customer code is non going to convey whatever activity other than logging.

How to produce Custom Exception inwards Java - The Code

Here is our consummate code event of creating custom or user defined Exception inwards Java.  In our example, nosotros conduct maintain 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 together with besides has a method getProductId() which returns production id for which this exception has caused. Don't produce an Exception shape every bit nested class fifty-fifty if it's used alone yesteryear i class, ever declare Exceptions inwards their ain class.

import java.util.HashMap; import java.util.Map;  /**  * Java Program to produce custom exception together with examples to present how to role  * custom exception inwards 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 inwards thread "main" NoSuchProductException: No such production exists for productId :1000     at CustomExceptionDemo.getProduct(CustomExceptionDemo.java:26)     at CustomExceptionDemo.main(CustomExceptionDemo.java:21)

Things to think piece creating Custom Exception inwards Java

Though creating a custom, exception is every bit slow every bit subclassing java.lang.Exception class, at that topographic point are few best practices yous tin follow to produce most of it. There is thence much criticism of checked exception due to boilerplate require to conduct maintain it, yous volition hardly produce your custom exception every bit checked.

1) Don’t' role Exception to command application behavior. Exception treatment is real 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 produce an unchecked, Runtime exception than a checked exception, peculiarly if yous know that customer is non going to convey whatever reactive activity other than logging.

3) If your custom exception is created yesteryear passing or thence other exception, thence ever incorporate master copy Exception every bit a source; role constructor which takes Exception rather than alone message String.

4) Apart from providing default no declaration constructor on your custom Exception class, see providing at to the lowest degree 2 to a greater extent than constructors, i which should conduct maintain a failure message together with other which tin conduct maintain or thence other Throwable every bit the cause.

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

6) While defining custom Exception, i of the most mutual error programmer produce is to intend that constructor is inherited from java.lang.Exception class, for example, they intend that their Exception shape volition automatically inherit default no declaration constructor together with the i which takes a String message. This is non true. The constructor is non inherited inwards Java, non fifty-fifty default constructor. It's genuinely added yesteryear the compiler rather than inherited from bring upward class. That's why I conduct maintain declared 2 constructors, i alongside String parameter together with 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 genuinely the measure agency of creating custom Exception inwards Java. In companionship to salve time, yous tin fifty-fifty produce a template of higher upward shape inwards Eclipse IDE.

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


That's all nigh How to produce custom Exception classes inwards Java. As I said, commencement seek out to avoid the temptation of creating a produce novel Exception class, intend if yous tin reuse existing ones. If yous absolutely need, thence produce sure enough to follow best practices. At the bare minimum, produce an endeavor to produce unchecked exception rather than a checked one.

Further Learning
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