How do I fix The expression of type List needs unchecked conversion

Question

In the Java snippet   SyndFeedInput fr   new SyndFeedInput    SyndFeed sf   fr build new XmlReader myInputStream    List lt SyndEntry gt  entries   sf getEntries      the last line generates the warning    The expression of type List needs unchecked conversion to conform to List lt SyndEntry gt    What s an appropriate way to fix this

User · Accepted Answer

Since getEntries returns a raw List  it could hold anything   The warning-free approach is to create a new List lt SyndEntry gt   then cast each element of the sf getEntries   result to SyndEntry before adding it to your new list  Collections checkedList does not do this checking for you mdash although it would have been possible to implement it to do so    By doing your own cast up front  you re  complying with the warranty terms  of Java generics  if a ClassCastException is raised  it will be associated with a cast in the source code  not an invisible cast inserted by the compiler

User · Answer

SyndFeedInput fr   new SyndFeedInput    SyndFeed sf   fr build new XmlReader myInputStream    List lt   gt  entries   sf getEntries

User · Answer

It looks like SyndFeed is not using generics   You could either have an unsafe cast and a warning suppression    SuppressWarnings  unchecked   List lt SyndEntry gt  entries    List lt SyndEntry gt   sf getEntries      or call Collections checkedList - although you ll still need to suppress the warning    SuppressWarnings  unchecked   List lt SyndEntry gt  entries   Collections checkedList sf getEntries    SyndEntry class

User · Answer

If you look at the javadoc for the class SyndFeed  I guess you are referring to the class com sun syndication feed synd SyndFeed   the method getEntries   doesn t return java util List lt SyndEntry gt   but returns just java util List   So you need an explicit cast for this

User · Answer

This is a common problem when dealing with pre-Java 5 APIs  To automate the solution from erickson  you can create the following generic method   public static  lt T gt  List lt T gt  castList Class lt   extends T gt  clazz  Collection lt   gt  c        List lt T gt  r   new ArrayList lt T gt  c size         for Object o  c        r add clazz cast o        return r      This allows you to do   List lt SyndEntry gt  entries   castList SyndEntry class  sf getEntries       Because this solution checks that the elements indeed have the correct element type by means of a cast  it is safe  and does not require SuppressWarnings

User · Answer

If you don t want to put  SuppressWarning  unchecked   on each sf getEntries   call  you can always make a wrapper that will return List   See this other question

User · Answer

Did you write the SyndFeed   Does sf getEntries return List or List lt SyndEntry gt    My guess is it returns List and changing it to return List lt SyndEntry gt  will fix the problem   If SyndFeed is part of a library  I don t think you can remove the warning without adding the  SuppressWarning  unchecked   annotation to your method

User · Answer

It looks like SyndFeed is not using generics   You could either have an unsafe cast and a warning suppression    SuppressWarnings  unchecked   List lt SyndEntry gt  entries    List lt SyndEntry gt   sf getEntries      or call Collections checkedList - although you ll still need to suppress the warning    SuppressWarnings  unchecked   List lt SyndEntry gt  entries   Collections checkedList sf getEntries    SyndEntry class

User · Answer

If you don t want to put  SuppressWarning  unchecked   on each sf getEntries   call  you can always make a wrapper that will return List   See this other question

User · Answer

It looks like SyndFeed is not using generics   You could either have an unsafe cast and a warning suppression    SuppressWarnings  unchecked   List lt SyndEntry gt  entries    List lt SyndEntry gt   sf getEntries      or call Collections checkedList - although you ll still need to suppress the warning    SuppressWarnings  unchecked   List lt SyndEntry gt  entries   Collections checkedList sf getEntries    SyndEntry class

User · Answer

SyndFeedInput fr   new SyndFeedInput    SyndFeed sf   fr build new XmlReader myInputStream    List lt   gt  entries   sf getEntries

User · Answer

If you are using Guava and all you want to do is iterate through your values    for SyndEntry entry  Iterables filter sf getEntries    SyndEntry class             If you need an actual List you can use   List lt SyndEntry gt  list   Lists newArrayList      Iterables filter sf getEntries    SyndEntry class      or  List lt SyndEntry gt  list   ImmutableList copyOf      Iterables filter sf getEntries    SyndEntry class

User · Answer

Even easier  return new ArrayList lt   gt  getResultOfHibernateCallback

User · Answer

If you are using Guava and all you want to do is iterate through your values    for SyndEntry entry  Iterables filter sf getEntries    SyndEntry class             If you need an actual List you can use   List lt SyndEntry gt  list   Lists newArrayList      Iterables filter sf getEntries    SyndEntry class      or  List lt SyndEntry gt  list   ImmutableList copyOf      Iterables filter sf getEntries    SyndEntry class

User · Answer

If you look at the javadoc for the class SyndFeed  I guess you are referring to the class com sun syndication feed synd SyndFeed   the method getEntries   doesn t return java util List lt SyndEntry gt   but returns just java util List   So you need an explicit cast for this

User · Answer

If you don t want to put  SuppressWarning  unchecked   on each sf getEntries   call  you can always make a wrapper that will return List   See this other question

User · Answer

If you look at the javadoc for the class SyndFeed  I guess you are referring to the class com sun syndication feed synd SyndFeed   the method getEntries   doesn t return java util List lt SyndEntry gt   but returns just java util List   So you need an explicit cast for this

User · Answer

Did you write the SyndFeed   Does sf getEntries return List or List lt SyndEntry gt    My guess is it returns List and changing it to return List lt SyndEntry gt  will fix the problem   If SyndFeed is part of a library  I don t think you can remove the warning without adding the  SuppressWarning  unchecked   annotation to your method

User · Answer

If you look at the javadoc for the class SyndFeed  I guess you are referring to the class com sun syndication feed synd SyndFeed   the method getEntries   doesn t return java util List lt SyndEntry gt   but returns just java util List   So you need an explicit cast for this

User · Answer

Did you write the SyndFeed   Does sf getEntries return List or List lt SyndEntry gt    My guess is it returns List and changing it to return List lt SyndEntry gt  will fix the problem   If SyndFeed is part of a library  I don t think you can remove the warning without adding the  SuppressWarning  unchecked   annotation to your method

User · Answer

Did you write the SyndFeed   Does sf getEntries return List or List lt SyndEntry gt    My guess is it returns List and changing it to return List lt SyndEntry gt  will fix the problem   If SyndFeed is part of a library  I don t think you can remove the warning without adding the  SuppressWarning  unchecked   annotation to your method

User · Answer

If you don t want to put  SuppressWarning  unchecked   on each sf getEntries   call  you can always make a wrapper that will return List   See this other question

User · Answer

Even easier  return new ArrayList lt   gt  getResultOfHibernateCallback

User · Answer

This is a common problem when dealing with pre-Java 5 APIs  To automate the solution from erickson  you can create the following generic method   public static  lt T gt  List lt T gt  castList Class lt   extends T gt  clazz  Collection lt   gt  c        List lt T gt  r   new ArrayList lt T gt  c size         for Object o  c        r add clazz cast o        return r      This allows you to do   List lt SyndEntry gt  entries   castList SyndEntry class  sf getEntries       Because this solution checks that the elements indeed have the correct element type by means of a cast  it is safe  and does not require SuppressWarnings

User · Answer

It looks like SyndFeed is not using generics   You could either have an unsafe cast and a warning suppression    SuppressWarnings  unchecked   List lt SyndEntry gt  entries    List lt SyndEntry gt   sf getEntries      or call Collections checkedList - although you ll still need to suppress the warning    SuppressWarnings  unchecked   List lt SyndEntry gt  entries   Collections checkedList sf getEntries    SyndEntry class

[java] How do I fix "The expression of type List needs unchecked conversion...'?

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