How to create custom exceptions in Java

Question

How do we create custom exceptions in Java

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public class MyException extends Exception              special exception code goes here     Throw it as    throw new MyException   Something happened     Catch as   catch  MyException e          something

User · Answer

For a checked exception   public class MyCustomException extends Exception       Technically  anything that extends Throwable can be an thrown  but exceptions are generally extensions of the Exception class so that they re checked exceptions  except RuntimeException or classes based on it  which are not checked   as opposed to the other common type of throwable  Errors which usually are not something designed to be gracefully handled beyond the JVM internals   You can also make exceptions non-public  but then you can only use them in the package that defines them  as opposed to across packages   As far as throwing catching custom exceptions  it works just like the built-in ones - throw via  throw new MyCustomException     and catch via  catch  MyCustomException e

User · Answer

To define a checked exception you create a subclass  or hierarchy of subclasses  of java lang Exception   For example   public class FooException extends Exception     public FooException     super        public FooException String message    super message       public FooException String message  Throwable cause    super message  cause       public FooException Throwable cause    super cause         Methods that can potentially throw or propagate this exception must declare it   public void calculate int i  throws FooException  IOException        and code calling this method must either handle or propagate this exception  or both    try     int i   5    myObject calculate 5     catch FooException ex         Print error and terminate application    ex printStackTrace      System exit 1     catch IOException ex         Rethrow as FooException    throw new FooException ex       You ll notice in the above example that IOException is caught and rethrown as FooException   This is a common technique used to encapsulate exceptions  typically when implementing an API    Sometimes there will be situations where you don t want to force every method to declare your exception implementation in its throws clause   In this case you can create an unchecked exception   An unchecked exception is any exception that extends java lang RuntimeException  which itself is a subclass of java lang Exception    public class FooRuntimeException extends RuntimeException             Methods can throw or propagate FooRuntimeException exception without declaring it  e g   public void calculate int i      if  i  lt  0        throw new FooRuntimeException  i  lt  0      i           Unchecked exceptions are typically used to denote a programmer error  for example passing an invalid argument to a method or attempting to breach an array index bounds   The java lang Throwable class is the root of all errors and exceptions that can be thrown within Java   java lang Exception and java lang Error are both subclasses of Throwable   Anything that subclasses Throwable may be thrown or caught   However  it is typically bad practice to catch or throw Error as this is used to denote errors internal to the JVM that cannot usually be  handled  by the programmer  e g  OutOfMemoryError    Likewise you should avoid catching Throwable  which could result in you catching Errors in addition to Exceptions

[java] How to create custom exceptions in Java?

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