Why can t overriding methods throw exceptions broader than the overridden method

Question

I was going through SCJP 6 book by Kathe sierra and came across this explanations of throwing exceptions in overridden method  I quite didn t get it  Can any one explain it to me       The overriding method must NOT throw checked exceptions that are new    or broader than those declared by the overridden method  For example  a    method that declares a FileNotFoundException cannot be overridden by a    method that declares a SQLException  Exception  or any other non-runtime    exception unless it s a subclass of FileNotFoundException

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Well java lang Exception extends java lang Throwable   java io FileNotFoundException extends java lang Exception   So if a method throws java io FileNotFoundException then in the override method you cannot throw anything higher up the hierarchy than FileNotFoundException e g  you can t throw java lang Exception   You could throw a subclass of FileNotFoundException though   However you would be forced to handle the FileNotFoundException in the overriden method   Knock up some code and give it a try   The rules are there so you don t lose the original throws declaration by widening the specificity  as the polymorphism means you can invoke the overriden method on the superclass

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The overriding method must NOT throw checked exceptions that are new or broader than those declared by the overridden method   This simply means when you override an existing method  the exception that this overloaded method throws should be either the same exception which the original method throws or any of its subclasses   Note that checking whether all checked exceptions are handled is done at compile time and not at runtime  So at compile time itself  the Java compiler checks the type of exception the overridden method is throwing  Since which overridden method will be executed can be decided only at runtime  we cannot know what kind of Exception we have to catch      Example  Let s say we have class A and its subclass B  A has method m1 and class B has overridden this method  lets call it m2 to avoid confusion     Now let s say m1 throws E1  and m2 throws E2  which is E1 s superclass  Now we write the following piece of code    A myAObj   new B    myAObj m1      Note that m1 is nothing but a call to m2  again  method signatures are same in overloaded methods so do not get confuse with m1 and m2   they are just to differentiate in this example    they both have same signature   But at compile time  all java compiler does is goes to the reference type  Class A in this case  checks the method if it is present and expects the programmer to handle it  So obviously  you will throw or catch E1  Now  at runtime  if the overloaded method throws E2  which is E1 s superclass  then     well  it s very wrong  for the same reason we cannot say B myBObj   new A     Hence  Java does not allow it  Unchecked exceptions thrown by the overloaded method must be same  subclasses  or non-existent

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I provide this answer here to the old question since no answers tell the fact that the overriding method can throw nothing here s again what the overriding method can throw   1  throw the same exception  public static class A        public void m1          throws IOException               System out println  A m1              public static class B      extends A        Override     public void m1           throws IOException               System out println  B m1              2  throw subclass of the overriden method s thrown exception  public static class A        public void m2          throws Exception               System out println  A m2              public static class B      extends A        Override     public void m2           throws IOException               System out println  B m2              3  throw nothing   public static class A           public void m3          throws IOException               System out println  A m3             public static class B      extends A           Override     public void m3             throws NOTHING               System out println  B m3              4  Having RuntimeExceptions in throws is not required   There can be RuntimeExceptions in throws or not  compiler won t complain about it  RuntimeExceptions are not checked exceptions  Only checked exceptions are required to appear in throws if not catched

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What explanation do we attribute to the below  class BaseClass        public  void print             System out println  In Parent Class   Print Method               public static void display             System out println  In Parent Class  Display Method               class DerivedClass extends BaseClass        public  void print   throws Exception           System out println  In Derived Class  Print Method               public static void display             System out println  In Derived Class  Display Method              Class DerivedClass java throws a compile time exception when the print method throws a Exception   print    method of baseclass does not throw any exception   I am able to attribute this to the fact that Exception is narrower than RuntimeException   it can be either No Exception  Runtime error    RuntimeException and their child exceptions

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Rule of handling check and unchecked exceptions on overridden methods  -When parent-class method declares no exception  then child-class overriding-      method can declare    1  No exception or  2  Any number of unchecked exception  3  but strictly no checked exception   -When parent-class method declares unchecked exception  then child-class overriding-method can declare    1  No exception or  2  Any number of unchecked exception   3  but strictly no checked exception   -When parent-class method declares checked exception  then child-class overriding-method can declare    1  No exception or  2  Same checked exception or  3  Sub-type of checked exception or  4  any number of unchecked exception   All above conclusion hold true  even if combination of both checked  amp  unchecked exception is declared in parent-class    method  Ref

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In my opinion  it is a fail in the Java syntax design  Polymorphism shouldn t limit the usage of exception handling  In fact  other computer languages don t do it  C     Moreover  a method is overriden in a more specialiced subclass so that it is more complex and  for this reason  more probable to throwing new exceptions

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Let us take an interview Question  There is a method that throws NullPointerException in the superclass  Can we override it with a method that throws RuntimeException   To answer this question  let us know what is an Unchecked and Checked exception    Checked exceptions must be explicitly caught or propagated as described in Basic try-catch-finally Exception Handling  Unchecked exceptions do not have this requirement  They don t have to be caught or declared thrown  Checked exceptions in Java extend the java lang Exception class  Unchecked exceptions extend the java lang RuntimeException    public class NullPointerException extends RuntimeException  Unchecked exceptions extend the java lang RuntimeException   Thst s why NullPointerException is an Uncheked exception   Let s take an example  Example 1         public class Parent          public void name    throws NullPointerException              System out println   this is parent                public class Child  extends Parent       public  void name   throws RuntimeException               System out println   child                  public static void main String   args            Parent parent    new Child            parent name      output   gt  child           The program will compile successfully  Example 2       public class Parent          public void name    throws RuntimeException              System out println   this is parent                public class Child  extends Parent       public  void name   throws  NullPointerException                System out println   child                  public static void main String   args            Parent parent    new Child            parent name      output   gt  child           The program will also compile successfully  Therefore it is evident  that nothing happens in case of Unchecked exceptions  Now  let s take a look what happens in case of Checked exceptions   Example 3  When base class and child class both throws a checked exception      public class Parent          public void name    throws IOException              System out println   this is parent               public class Child  extends Parent       public  void name   throws IOException               System out println   child                  public static void main String   args            Parent parent    new Child             try               parent name      output  gt  child          catch  Exception e                System out println e                        The program will compile successfully  Example 4  When child class method is throwing border checked exception compared to the same method of base class   import java io IOException   public class Parent          public void name    throws IOException              System out println   this is parent               public class Child  extends Parent       public  void name   throws Exception     broader exception              System out println   child                  public static void main String   args            Parent parent    new Child             try               parent name     output  gt  Compilation failure          catch  Exception e                System out println e                        The program will fail to compile  So  we have to be careful when we are using Checked exceptions

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It means that if a method declares to throw a given exception  the overriding method in a subclass can only declare to throw that exception or its subclass  For example   class A      public void foo   throws IOException         class B extends A       Override    public void foo   throws SocketException         allowed      Override    public void foo   throws SQLException         NOT allowed     SocketException extends IOException  but SQLException does not   This is because of polymorphism   A a   new B    try       a foo      catch  IOException ex           forced to catch this by the compiler     If B had decided to throw SQLException  then the compiler could not force you to catch it  because you are referring to the instance of B by its superclass - A  On the other hand  any subclass of IOException will be handled by clauses  catch or throws  that handle IOException  The rule that you need to be able to refer to objects by their superclass is the Liskov Substitution Principle   Since unchecked exceptions can be thrown anywhere then they are not subject to this rule  You can add an unchecked exception to the throws clause as a form of documentation if you want  but the compiler doesn t enforce anything about it

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To illustrate this  consider   public interface FileOperation     void perform File file  throws FileNotFoundException     public class OpenOnly implements FileOperation     void perform File file  throws FileNotFoundException       FileReader r   new FileReader file           Suppose you then write   public class OpenClose implements FileOperation     void perform File file  throws FileNotFoundException       FileReader r   new FileReader file       r close            This will give you a compilation error  because r close   throws an IOException  which is broader than FileNotFoundException   To fix this  if you write   public class OpenClose implements FileOperation     void perform File file  throws IOException       FileReader r   new FileReader file       r close            You will get a different compilation error  because you are implementing the perform      operation  but throwing an exception not included in the interface s definition of the method   Why is this important  Well a consumer of the interface may have   FileOperation op        try     op perform file     catch  FileNotFoundException x      log           If the IOException were allowed to be thrown  the client s code is nolonger correct    Note that you can avoid this sort of issue if you use unchecked exceptions   I am not suggesting you do or don t  that is a philosophical issue

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The overriding method must NOT throw checked exceptions that are new or broader than those declared by the overridden method   Example   class Super       public void throwCheckedExceptionMethod   throws IOException           FileReader r   new FileReader new File  aFile txt             r close             class Sub extends Super            Override     public void throwCheckedExceptionMethod   throws FileNotFoundException              FileNotFoundException extends IOException         FileReader r   new FileReader new File  afile txt             try                  close   method throws IOException  that is unhandled              r close              catch  IOException e                       class Sub2 extends Sub        Override     public void throwCheckedExceptionMethod                Overriding method can throw no exception

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The overriding method CAN throw any unchecked  runtime  exception  regardless of whether the overridden method declares the exception  Example   class Super       public void test             System out println  Super test               class Sub extends Super        Override     public void test   throws IndexOutOfBoundsException              Method can throw any Unchecked Exception         System out println  Sub test               class Sub2 extends Sub        Override     public void test   throws ArrayIndexOutOfBoundsException              Any Unchecked Exception         System out println  Sub2 test               class Sub3 extends Sub2        Override     public void test                Any Unchecked Exception or no exception         System out println  Sub3 test               class Sub4 extends Sub2        Override     public void test   throws AssertionError              Unchecked Exception IS-A RuntimeException or IS-A Error         System out println  Sub4 test

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To understand this let s consider an example where we have a class Mammal which defines readAndGet method which is reading some file  doing some operation on it and returning an instance of class Mammal   class Mammal       public Mammal readAndGet   throws IOException    read file and return Mammal s object      Class Human extends class Mammal and overrides readAndGet method to return the instance of Human instead of the instance of Mammal   class Human extends Mammal        Override     public Human readAndGet   throws FileNotFoundException    read file and return Human object      To call readAndGet we will need to handle IOException because its a checked exception and mammal s readAndMethod is throwing it   Mammal mammal   new Human    try       Mammal obj   mammal readAndGet      catch  IOException ex         And we know that for compiler mammal readAndGet   is getting called from the object of class Mammal but at  runtime JVM will resolve mammal readAndGet   method call to a call from class Human because mammal is holding new Human     Method readAndMethod from Mammal is throwing IOException and because it is a checked exception compiler will force us to catch it whenever we call readAndGet on mammal  Now suppose readAndGet in Human is throwing any other checked exception e g  Exception and we know readAndGet will get called from the instance of Human because mammal is holding new Human     Because for compiler the method is getting called from Mammal  so the compiler will force us to handle only IOException but at runtime we know method will be throwing Exception exception which is not getting handled and our code will break if the method throws the exception   That s why it is prevented at the compiler level itself and we are not allowed to throw any new or broader checked exception because it will not be handled by JVM at the end   There are other rules as well which we need to follow while overriding the methods and you can read more on Why We Should Follow Method Overriding Rules to know the reasons

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The subclass s overriding method can only throw multiple checked exceptions that are subclasses of the superclass s method s checked exception  but cannot throw multiple checked exceptions that are unrelated to the superclass s method s checked exception

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Java is giving you the choice to restrict exceptions in the parent class  because it s assuming the client will restrict what is caught  IMHO you should essentially never use this  feature   because your clients may need flexibility down the road   Java is an old language that is poorly designed  Modern languages don t have such restrictions   The easiest way around this flaw is make your base class throw Exception always  Clients can throw more specific Exceptions but make your base classes really broad

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say you have super class A with method M1 throwin E1 and class B deriving from A with method M2 overriding M1  M2 can not throw anything DIFFERENT or LESS SPECIALIZED than E1   Because of polymorphism  the client using class A should be able to treat B as if it were A  Inharitance      Is-a  B is-a A   What if this code dealing with class A was handling exception E1  as M1 declares it throws this checked exception  but then different type of exception was thrown  If M1 was throwing IOException M2 could well throw FileNotFoundException  as it is-a IOException  Clients of A could handle this without a problem  If the exception thrown was wider  clients of A would not have a chance of knowing about this and therefore would not have a chance to catch it

[java] Why can't overriding methods throw exceptions broader than the overridden method?

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