Type safety Unchecked cast

Question

In my spring application context file  I have something like    lt util map id  someMap  map-class  java util HashMap  key-type  java lang String  value-type  java lang String  gt       lt entry key  some key  value  some value    gt       lt entry key  some key 2  value  some value    gt      lt  util map gt    In java class  the implementation looks like   private Map lt String  String gt  someMap   new HashMap lt String  String gt     someMap    HashMap lt String  String gt  getApplicationContext   getBean  someMap      In Eclipse  I see a warning that says   Type safety  Unchecked cast from Object to HashMap  What did I do wrong  How do I resolve the issue

User · Answer

As the messages above indicate  the List cannot be differentiated between a List lt Object gt  and a List lt String gt  or List lt Integer gt    I ve solved this error message for a similar problem   List lt String gt  strList    List lt String gt   someFunction    String s   strList get 0     with the following   List lt   gt  strList    List lt   gt   someFunction    String s    String  strList get 0     Explanation  The first type conversion verifies that the object is a List without caring about the types held within  since we cannot verify the internal types at the List level    The second conversion is now required because the compiler only knows the List contains some sort of objects   This verifies the type of each object in the List as it is accessed

User · Answer

Another solution  if you find yourself casting the same object a lot and you don t want to litter your code with  SupressWarnings  unchecked    would be to create a method with the annotation  This way you re centralizing the cast  and hopefully reducing the possibility for error    SuppressWarnings  unchecked   public static List lt String gt  getFooStrings Map lt String  List lt String gt  gt  ctx        return  List lt String gt   ctx get  foos

User · Answer

The solution to avoid the unchecked warning   class MyMap extends HashMap lt String  String gt      someMap    MyMap getApplicationContext   getBean  someMap

User · Answer

The problem is that a cast is a runtime check - but due to type erasure  at runtime there s actually no difference between a HashMap lt String String gt  and HashMap lt Foo Bar gt  for any other Foo and Bar   Use  SuppressWarnings  unchecked   and hold your nose  Oh  and campaign for reified generics in Java

User · Answer

Another solution  if you find yourself casting the same object a lot and you don t want to litter your code with  SupressWarnings  unchecked    would be to create a method with the annotation  This way you re centralizing the cast  and hopefully reducing the possibility for error    SuppressWarnings  unchecked   public static List lt String gt  getFooStrings Map lt String  List lt String gt  gt  ctx        return  List lt String gt   ctx get  foos

User · Answer

If you really want to get rid of the warnings  one thing you can do is create a class that extends from the generic class   For example  if you re trying to use  private Map lt String  String gt  someMap   new HashMap lt String  String gt       You can create a new class like such  public class StringMap extends HashMap lt String  String gt             Override constructors     Then when you use  someMap    StringMap  getApplicationContext   getBean  someMap      The compiler DOES know what the  no longer generic  types are  and there will be no warning  This may not always be the perfect solution  some might argue this kind of defeats the purpose of generic classes  but you re still re-using all of the same code from the generic class  you re just declaring at compile time what type you want to use

User · Answer

Well  first of all  you re wasting memory with the new HashMap creation call  Your second line completely disregards the reference to this created hashmap  making it then available to the garbage collector  So  don t do that  use   private Map lt String  String gt  someMap    HashMap lt String  String gt  getApplicationContext   getBean  someMap      Secondly  the compiler is complaining that you cast the object to a HashMap without checking if it is a HashMap  But  even if you were to do   if getApplicationContext   getBean  someMap   instanceof HashMap        private Map lt String  String gt  someMap    HashMap lt String  String gt  getApplicationContext   getBean  someMap        You would probably still get this warning  The problem is  getBean returns Object  so it is unknown what the type is  Converting it to HashMap directly would not cause the problem with the second case  and perhaps there would not be a warning in the first case  I m not sure how pedantic the Java compiler is with warnings for Java 5   However  you are converting it to a HashMap lt String  String gt    HashMaps are really maps that take an object as a key and have an object as a value  HashMap lt Object  Object gt  if you will  Thus  there is no guarantee that when you get your bean that it can be represented as a HashMap lt String  String gt  because you could have HashMap lt Date  Calendar gt  because the non-generic representation that is returned can have any objects   If the code compiles  and you can execute String value   map get  thisString    without any errors  don t worry about this warning  But if the map isn t completely of string keys to string values  you will get a ClassCastException at runtime  because the generics cannot block this from happening in this case

User · Answer

A warning is just that   A warning   Sometimes warnings are irrelevant  sometimes they re not   They re used to call your attention to something that the compiler thinks could be a problem  but may not be   In the case of casts  it s always going to give a warning in this case   If you are absolutely certain that a particular cast will be safe  then you should consider adding an annotation like this  I m not sure of the syntax  just before the line    SuppressWarnings  value  unchecked

User · Answer

You are getting this message because getBean returns an Object reference and you are casting it to the correct type   Java 1 5 gives you a warning   That s the nature of using Java 1 5 or better with code that works like this   Spring has the typesafe version  someMap getApplicationContext   getBean lt HashMap lt String  String gt  gt   someMap      on its todo list

User · Answer

A warning is just that   A warning   Sometimes warnings are irrelevant  sometimes they re not   They re used to call your attention to something that the compiler thinks could be a problem  but may not be   In the case of casts  it s always going to give a warning in this case   If you are absolutely certain that a particular cast will be safe  then you should consider adding an annotation like this  I m not sure of the syntax  just before the line    SuppressWarnings  value  unchecked

User · Answer

The problem is that a cast is a runtime check - but due to type erasure  at runtime there s actually no difference between a HashMap lt String String gt  and HashMap lt Foo Bar gt  for any other Foo and Bar   Use  SuppressWarnings  unchecked   and hold your nose  Oh  and campaign for reified generics in Java

User · Answer

If you really want to get rid of the warnings  one thing you can do is create a class that extends from the generic class   For example  if you re trying to use  private Map lt String  String gt  someMap   new HashMap lt String  String gt       You can create a new class like such  public class StringMap extends HashMap lt String  String gt             Override constructors     Then when you use  someMap    StringMap  getApplicationContext   getBean  someMap      The compiler DOES know what the  no longer generic  types are  and there will be no warning  This may not always be the perfect solution  some might argue this kind of defeats the purpose of generic classes  but you re still re-using all of the same code from the generic class  you re just declaring at compile time what type you want to use

User · Answer

Below code causes Type safety Warning  Map lt String  Object gt  myInput    Map lt String  Object gt   myRequest get         Workaround   Create a new Map Object without mentioning the parameters because the type of object held within the list is not verified   Step 1  Create a new temporary Map  Map lt      gt  tempMap    Map lt      gt   myRequest get     Step 2  Instantiate the main Map  Map lt String  Object gt  myInput new HashMap lt  gt  myInputObj size       Step 3  Iterate the temporary Map and set the values into the main Map   for Map Entry lt      gt  entry  myInputObj entrySet             myInput put  String entry getKey   entry getValue

User · Answer

The solution to avoid the unchecked warning   class MyMap extends HashMap lt String  String gt      someMap    MyMap getApplicationContext   getBean  someMap

User · Answer

You are getting this message because getBean returns an Object reference and you are casting it to the correct type   Java 1 5 gives you a warning   That s the nature of using Java 1 5 or better with code that works like this   Spring has the typesafe version  someMap getApplicationContext   getBean lt HashMap lt String  String gt  gt   someMap      on its todo list

User · Answer

What did I do wrong  How do I resolve the issue    Here    Map lt String String gt  someMap    Map lt String String gt  getApplicationContext   getBean  someMap     You use a legacy method that we generally don t want to use since that returns Object    Object getBean String name  throws BeansException    The method to favor to get  for singleton    create  for prototype  a bean from a bean factory is     lt T gt  T getBean String name  Class lt T gt  requiredType  throws BeansException    Using it such as      Map lt String String gt  someMap   app getBean Map class  someMap      will compile but still with a unchecked conversion warning since all Map objects are not necessarily Map lt String  String gt  objects      But  lt T gt  T getBean String name  Class lt T gt  requiredType  throws BeansException  is not enough in bean generic classes such as generic collections since that requires to specify more than one class as parameter   the collection type and its generic type s         In this kind of scenario and in general  a better approach is not to use directly BeanFactory methods but let the framework to inject the bean      The bean declaration      Configuration public class MyConfiguration        Bean     public Map lt String  String gt  someMap             Map lt String  String gt  someMap   new HashMap            someMap put  some key    some value            someMap put  some key 2    some value            return someMap            The bean injection      Autowired  Qualifier  someMap   Map lt String  String gt  someMap

User · Answer

Below code causes Type safety Warning  Map lt String  Object gt  myInput    Map lt String  Object gt   myRequest get         Workaround   Create a new Map Object without mentioning the parameters because the type of object held within the list is not verified   Step 1  Create a new temporary Map  Map lt      gt  tempMap    Map lt      gt   myRequest get     Step 2  Instantiate the main Map  Map lt String  Object gt  myInput new HashMap lt  gt  myInputObj size       Step 3  Iterate the temporary Map and set the values into the main Map   for Map Entry lt      gt  entry  myInputObj entrySet             myInput put  String entry getKey   entry getValue

User · Answer

Well  first of all  you re wasting memory with the new HashMap creation call  Your second line completely disregards the reference to this created hashmap  making it then available to the garbage collector  So  don t do that  use   private Map lt String  String gt  someMap    HashMap lt String  String gt  getApplicationContext   getBean  someMap      Secondly  the compiler is complaining that you cast the object to a HashMap without checking if it is a HashMap  But  even if you were to do   if getApplicationContext   getBean  someMap   instanceof HashMap        private Map lt String  String gt  someMap    HashMap lt String  String gt  getApplicationContext   getBean  someMap        You would probably still get this warning  The problem is  getBean returns Object  so it is unknown what the type is  Converting it to HashMap directly would not cause the problem with the second case  and perhaps there would not be a warning in the first case  I m not sure how pedantic the Java compiler is with warnings for Java 5   However  you are converting it to a HashMap lt String  String gt    HashMaps are really maps that take an object as a key and have an object as a value  HashMap lt Object  Object gt  if you will  Thus  there is no guarantee that when you get your bean that it can be represented as a HashMap lt String  String gt  because you could have HashMap lt Date  Calendar gt  because the non-generic representation that is returned can have any objects   If the code compiles  and you can execute String value   map get  thisString    without any errors  don t worry about this warning  But if the map isn t completely of string keys to string values  you will get a ClassCastException at runtime  because the generics cannot block this from happening in this case

User · Answer

The problem is that a cast is a runtime check - but due to type erasure  at runtime there s actually no difference between a HashMap lt String String gt  and HashMap lt Foo Bar gt  for any other Foo and Bar   Use  SuppressWarnings  unchecked   and hold your nose  Oh  and campaign for reified generics in Java

User · Answer

Well  first of all  you re wasting memory with the new HashMap creation call  Your second line completely disregards the reference to this created hashmap  making it then available to the garbage collector  So  don t do that  use   private Map lt String  String gt  someMap    HashMap lt String  String gt  getApplicationContext   getBean  someMap      Secondly  the compiler is complaining that you cast the object to a HashMap without checking if it is a HashMap  But  even if you were to do   if getApplicationContext   getBean  someMap   instanceof HashMap        private Map lt String  String gt  someMap    HashMap lt String  String gt  getApplicationContext   getBean  someMap        You would probably still get this warning  The problem is  getBean returns Object  so it is unknown what the type is  Converting it to HashMap directly would not cause the problem with the second case  and perhaps there would not be a warning in the first case  I m not sure how pedantic the Java compiler is with warnings for Java 5   However  you are converting it to a HashMap lt String  String gt    HashMaps are really maps that take an object as a key and have an object as a value  HashMap lt Object  Object gt  if you will  Thus  there is no guarantee that when you get your bean that it can be represented as a HashMap lt String  String gt  because you could have HashMap lt Date  Calendar gt  because the non-generic representation that is returned can have any objects   If the code compiles  and you can execute String value   map get  thisString    without any errors  don t worry about this warning  But if the map isn t completely of string keys to string values  you will get a ClassCastException at runtime  because the generics cannot block this from happening in this case

User · Answer

As the messages above indicate  the List cannot be differentiated between a List lt Object gt  and a List lt String gt  or List lt Integer gt    I ve solved this error message for a similar problem   List lt String gt  strList    List lt String gt   someFunction    String s   strList get 0     with the following   List lt   gt  strList    List lt   gt   someFunction    String s    String  strList get 0     Explanation  The first type conversion verifies that the object is a List without caring about the types held within  since we cannot verify the internal types at the List level    The second conversion is now required because the compiler only knows the List contains some sort of objects   This verifies the type of each object in the List as it is accessed

User · Answer

Well  first of all  you re wasting memory with the new HashMap creation call  Your second line completely disregards the reference to this created hashmap  making it then available to the garbage collector  So  don t do that  use   private Map lt String  String gt  someMap    HashMap lt String  String gt  getApplicationContext   getBean  someMap      Secondly  the compiler is complaining that you cast the object to a HashMap without checking if it is a HashMap  But  even if you were to do   if getApplicationContext   getBean  someMap   instanceof HashMap        private Map lt String  String gt  someMap    HashMap lt String  String gt  getApplicationContext   getBean  someMap        You would probably still get this warning  The problem is  getBean returns Object  so it is unknown what the type is  Converting it to HashMap directly would not cause the problem with the second case  and perhaps there would not be a warning in the first case  I m not sure how pedantic the Java compiler is with warnings for Java 5   However  you are converting it to a HashMap lt String  String gt    HashMaps are really maps that take an object as a key and have an object as a value  HashMap lt Object  Object gt  if you will  Thus  there is no guarantee that when you get your bean that it can be represented as a HashMap lt String  String gt  because you could have HashMap lt Date  Calendar gt  because the non-generic representation that is returned can have any objects   If the code compiles  and you can execute String value   map get  thisString    without any errors  don t worry about this warning  But if the map isn t completely of string keys to string values  you will get a ClassCastException at runtime  because the generics cannot block this from happening in this case

User · Answer

A warning is just that   A warning   Sometimes warnings are irrelevant  sometimes they re not   They re used to call your attention to something that the compiler thinks could be a problem  but may not be   In the case of casts  it s always going to give a warning in this case   If you are absolutely certain that a particular cast will be safe  then you should consider adding an annotation like this  I m not sure of the syntax  just before the line    SuppressWarnings  value  unchecked

User · Answer

A warning is just that   A warning   Sometimes warnings are irrelevant  sometimes they re not   They re used to call your attention to something that the compiler thinks could be a problem  but may not be   In the case of casts  it s always going to give a warning in this case   If you are absolutely certain that a particular cast will be safe  then you should consider adding an annotation like this  I m not sure of the syntax  just before the line    SuppressWarnings  value  unchecked

User · Answer

You are getting this message because getBean returns an Object reference and you are casting it to the correct type   Java 1 5 gives you a warning   That s the nature of using Java 1 5 or better with code that works like this   Spring has the typesafe version  someMap getApplicationContext   getBean lt HashMap lt String  String gt  gt   someMap      on its todo list

User · Answer

You are getting this message because getBean returns an Object reference and you are casting it to the correct type   Java 1 5 gives you a warning   That s the nature of using Java 1 5 or better with code that works like this   Spring has the typesafe version  someMap getApplicationContext   getBean lt HashMap lt String  String gt  gt   someMap      on its todo list

User · Answer

What did I do wrong  How do I resolve the issue    Here    Map lt String String gt  someMap    Map lt String String gt  getApplicationContext   getBean  someMap     You use a legacy method that we generally don t want to use since that returns Object    Object getBean String name  throws BeansException    The method to favor to get  for singleton    create  for prototype  a bean from a bean factory is     lt T gt  T getBean String name  Class lt T gt  requiredType  throws BeansException    Using it such as      Map lt String String gt  someMap   app getBean Map class  someMap      will compile but still with a unchecked conversion warning since all Map objects are not necessarily Map lt String  String gt  objects      But  lt T gt  T getBean String name  Class lt T gt  requiredType  throws BeansException  is not enough in bean generic classes such as generic collections since that requires to specify more than one class as parameter   the collection type and its generic type s         In this kind of scenario and in general  a better approach is not to use directly BeanFactory methods but let the framework to inject the bean      The bean declaration      Configuration public class MyConfiguration        Bean     public Map lt String  String gt  someMap             Map lt String  String gt  someMap   new HashMap            someMap put  some key    some value            someMap put  some key 2    some value            return someMap            The bean injection      Autowired  Qualifier  someMap   Map lt String  String gt  someMap

User · Answer

The problem is that a cast is a runtime check - but due to type erasure  at runtime there s actually no difference between a HashMap lt String String gt  and HashMap lt Foo Bar gt  for any other Foo and Bar   Use  SuppressWarnings  unchecked   and hold your nose  Oh  and campaign for reified generics in Java

[java] Type safety: Unchecked cast

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