When to use throws in a Java method declaration

Question

So I thought I had a good basic understanding of exception-handling in Java  but I was recently reading some code that gave me some confusion and doubts  My main doubt that I want to address here is when should a person use throws in a Java method declaration like the following       public void method   throws SomeException                   method body here         From reading some similar posts I gather that throws is used as a sort of declaration that SomeException could be thrown during the execution of the method   My confusion comes from some code that looked like this        public void method   throws IOException                  try                            BufferedReader br   new BufferedReader new FileReader  file txt                           catch IOException e                             System out println e getMessage                         Is there any reason that you would want to use a throws in this example  It seems that if you are just doing basic exception-handling of something like an IOException that you would simply need the try catch block and that s it

User · Answer

You only need to include a throws clause on a method if the method throws a checked exception. If the method throws a runtime exception then there is no need to do so.

See here for some background on checked vs unchecked exceptions: http://download.oracle.com/javase/tutorial/essential/exceptions/runtime.html

If the method catches the exception and deals with it internally (as in your second example) then there is no need to include a throws clause.

User · Answer

You re correct  in that example the throws is superfluous   It s possible that it was left there from some previous implementation - perhaps the exception was originally thrown instead of caught in the catch block

User · Answer

The code that you looked at is not ideal  You should either    Catch the exception and handle it  in which case the throws is unnecesary  Remove the try catch  in which case the Exception will be handled by a calling method  Catch the exception  possibly perform some action and then rethrow the exception  not just the message

User · Answer

This is not an answer  but a comment  but I could not write a comment with a formatted code  so here is the comment   Lets say there is  public static void main String   args      try          do nothing or throw a RuntimeException     throw new RuntimeException  test        catch  Exception e        System out println e getMessage         throw e          The output is   test Exception in thread  main  java lang RuntimeException  test     at MyClass main MyClass java 10    That method does not declare any  throws  Exceptions  but throws them  The trick is that the thrown exceptions are RuntimeExceptions  unchecked  that are not needed to be declared on the method  It is a bit misleading for the reader of the method  since all she sees is a  throw e   statement but no declaration of the throws exception  Now  if we have  public static void main String   args  throws Exception     try       throw new Exception  test        catch  Exception e        System out println e getMessage         throw e          We MUST declare the  throws  exceptions in the method otherwise we get a compiler error

User · Answer

The code you posted is wrong  it should throw an Exception if is catching a specific exception in order to handler IOException but throwing not catched exceptions   Something like   public void method   throws Exception     try             BufferedReader br   new BufferedReader new FileReader  file txt         catch IOException e              System out println e getMessage              or  public void method       try             BufferedReader br   new BufferedReader new FileReader  file txt         catch IOException e              System out println  Catching IOException               System out println e getMessage         catch Exception e              System out println  Catching any other Exceptions like NullPontException  FileNotFoundExceptioon  etc                System out println e getMessage

User · Answer

In the example you gave  the method will never throw an IOException  therefore the declaration is wrong  but valid    My guess is that the original method threw the IOException  but it was then updated to handle the exception within but the declaration was not changed

User · Answer

If you are catching an exception type  you do not need to throw it  unless you are going to rethrow it   In the example you post  the developer should have done one or another  not both   Typically  if you are not going to do anything with the exception  you should not catch it     The most dangerous thing you can do is catch an exception and not do anything with it   A good discussion of when it is appropriate to throw exceptions is here  When to throw an exception

[java] When to use throws in a Java method declaration?

Examples related to java

Examples related to exception-handling

Examples related to throws