How do I address unchecked cast warnings

Question

Eclipse is giving me a warning of the following form      Type safety  Unchecked cast from Object to HashMap   This is from a call to an API that I have no control over which returns Object   HashMap lt String  String gt  getItems javax servlet http HttpSession session      HashMap lt String  String gt  theHash    HashMap lt String  String gt  session getAttribute  attributeKey      return theHash      I d like to avoid Eclipse warnings  if possible  since theoretically they indicate at least a potential code problem   I haven t found a good way to eliminate this one yet  though   I can extract the single line involved out to a method by itself and add  SuppressWarnings  unchecked   to that method  thus limiting the impact of having a block of code where I ignore warnings   Any better options   I don t want to turn these warnings off in Eclipse   Before I came to the code  it was simpler  but still provoked warnings   HashMap getItems javax servlet http HttpSession session      HashMap theHash    HashMap session getAttribute  attributeKey      return theHash      Problem was elsewhere when you tried to use the hash you d get warnings   HashMap items   getItems session   items put  this    that     Type safety  The method put Object  Object  belongs to the raw type HashMap   References to generic type HashMap lt K V gt  should be parameterized

User · Accepted Answer

Wow  I think I figured out the answer to my own question   I m just not sure it s worth it      The problem is the cast isn t checked   So  you have to check it yourself   You can t just check a parameterized type with instanceof  because the parameterized type information is unavailable at runtime  having been erased at compile time   But  you can perform a check on each and every item in the hash  with instanceof  and in doing so  you can construct a new hash that is type-safe   And you won t provoke any warnings   Thanks to mmyers and Esko Luontola  I ve parameterized the code I originally wrote here  so it can be wrapped up in a utility class somewhere and used for any parameterized HashMap   If you want to understand it better and aren t very familiar with generics  I encourage viewing the edit history of this answer   public static  lt K  V gt  HashMap lt K  V gt  castHash HashMap input                                              Class lt K gt  keyClass                                              Class lt V gt  valueClass      HashMap lt K  V gt  output   new HashMap lt K  V gt       if  input    null        return output    for  Object key  input keySet   toArray          if   key    null      keyClass isAssignableFrom key getClass                Object value   input get key           if   value    null      valueClass isAssignableFrom value getClass                    K k   keyClass cast key               V v   valueClass cast value               output put k  v             else               throw new AssertionError                   Cannot cast to HashMap lt    keyClass getSimpleName                          valueClass getSimpleName      gt                       value    value    is not a    valueClass getSimpleName                                  else           throw new AssertionError               Cannot cast to HashMap lt    keyClass getSimpleName                      valueClass getSimpleName      gt                   key    key    is not a     keyClass getSimpleName                          return output      That s a lot of work  possibly for very little reward     I m not sure if I ll use it or not   I d appreciate any comments as to whether people think it s worth it or not   Also  I d appreciate improvement suggestions  is there something better I can do besides throw AssertionErrors   Is there something better I could throw   Should I make it a checked Exception

User · Answer

In the HTTP Session world you can t really avoid the cast  since the API is written that way  takes and returns only Object    With a little bit of work you can easily avoid the unchecked cast   though  This means that it will turn into a traditional cast giving a ClassCastException right there in the event of an error   An unchecked exception could turn into a CCE at any point later on instead of the point of the cast  that s the reason why it s a separate warning    Replace the HashMap with a dedicated class   import java util AbstractMap  import java util Collection  import java util HashMap  import java util Map  import java util Set   public class Attributes extends AbstractMap lt String  String gt        final Map lt String  String gt  content   new HashMap lt String  String gt           Override     public Set lt Map Entry lt String  String gt  gt  entrySet             return content entrySet                Override     public Set lt String gt  keySet             return content keySet                Override     public Collection lt String gt  values             return content values                Override     public String put final String key  final String value            return content put key  value             Then cast to that class instead of Map lt String String gt  and everything will be checked at the exact place where you write your code  No unexpected ClassCastExceptions later on

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Warning suppression is not a solution  You should not be doing two level casting in one statement   HashMap lt String  String gt  getItems javax servlet http HttpSession session            first  cast the returned Object to generic HashMap lt     gt      HashMap lt      gt  theHash    HashMap lt      gt  session getAttribute  attributeKey            next  cast every entry of the HashMap to the required type  lt String  String gt      HashMap lt String  String gt  returingHash   new HashMap lt  gt         for  Entry lt      gt  entry   theHash entrySet              returingHash put  String  entry getKey     String  entry getValue               return returingHash

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I may have misunderstood the question an example and a couple of surrounding lines would be nice   but why don t you always use an appropriate interface  and Java5    I see no reason why you would ever want to cast to a HashMap instead of a Map lt KeyType ValueType gt   In fact  I can t imagine any reason to set the type of a variable to HashMap instead of Map   And why is the source an Object  Is it a parameter type of a legacy collection  If so  use generics and specify the type you want

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If you are sure that the type returned by session getAttribute   is HashMap then you can not typecast to that exact type  but rely on only checking the generic HashMap  HashMap lt     gt  getItems javax servlet http HttpSession session          HashMap lt     gt  theHash    HashMap lt     gt  session getAttribute  attributeKey        return theHash       Eclipse will then surprise warnings  but of course this can lead to runtime errors that can be hard to debug  I use this approach in not operation-critical contexts only

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Almost every problem in Computer Science can be solved by adding a level of indirection   or something   So introduce a non-generic object that is of a higher-level that a Map  With no context it isn t going to look very convincing  but anyway   public final class Items implements java io Serializable       private static final long serialVersionUID   1L      private Map lt String String gt  map      public Items Map lt String String gt  map            this map   New immutableMap map             public Map lt String String gt  getMap             return map             Override public String toString             return map toString             public final class New       public static  lt K V gt  Map lt K V gt  immutableMap          Map lt   extends K    extends V gt  original                        optimise as you wish            return Collections unmodifiableMap              new HashMap lt String String gt  original                      static Map lt String  String gt  getItems HttpSession session        Items items    Items          session getAttribute  attributeKey        return items getMap         Except too many levels of indirection

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In this particular case  I would not store Maps into the HttpSession directly  but instead an instance of my own class  which in turn contains a Map  an implementation detail of the class   Then you can be sure that the elements in the map are of the right type   But if you anyways want to check that the contents of the Map are of right type  you could use a code like this   public static void main String   args        Map lt String  Integer gt  map   new HashMap lt String  Integer gt         map put  a   1       map put  b   2       Object obj   map       Map lt String  Integer gt  ok   safeCastMap obj  String class  Integer class       Map lt String  String gt  error   safeCastMap obj  String class  String class       SuppressWarnings   unchecked    public static  lt K  V gt  Map lt K  V gt  safeCastMap Object map  Class lt K gt  keyType  Class lt V gt  valueType        checkMap map       checkMapContents keyType  valueType   Map lt      gt   map       return  Map lt K  V gt   map     private static void checkMap Object map        checkType Map class  map      private static  lt K  V gt  void checkMapContents Class lt K gt  keyType  Class lt V gt  valueType  Map lt      gt  map        for  Map Entry lt      gt  entry   map entrySet              checkType keyType  entry getKey             checkType valueType  entry getValue              private static  lt K gt  void checkType Class lt K gt  expectedType  Object obj        if   expectedType isInstance obj             throw new IllegalArgumentException  Expected     expectedType     but was     obj getClass            obj

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Here s one way I handle this when I override the equals   operation   public abstract class Section lt T extends Section gt  extends Element lt Section lt T gt  gt        Object attr1                 Compare one section object to another               param obj the object being compared with this section object        return true if this section and the other section are of the same       sub-class of section and their component fields are the same  false       otherwise                    Override     public boolean equals Object obj            if  obj    null                   this exists  but obj doesn t  so they can t be equal              return false                        prepare to cast            Section lt   gt  other           if  getClass      obj getClass                     looks like we re comparing apples to oranges             return false            else                  it must be safe to make that cast              other    Section lt   gt   obj                        and then I compare attributes between this and other         return this attr1 equals other attr1             This seems to work in Java 8  even compiled with -Xlint unchecked

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In Eclipse Preferences  Go to Java- Compiler- Errors Warnings- Generic types and check the Ignore unavoidable generic type problems check-box    This satisfies the intent of the question  i e      I d like to avoid Eclipse warnings      if not the spirit

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Unfortunately  there are no great options here  Remember  the goal of all of this is to preserve type safety    Java Generics  offers a solution for dealing with non-genericized legacy libraries  and there is one in particular called the  empty loop technique  in section 8 2  Basically  make the unsafe cast  and suppress the warning  Then loop through the map like this    SuppressWarnings  unchecked   Map lt String  Number gt  map   getMap    for  String s   map keySet     for  Number n   map values       If an unexpected type is encountered  you will get a runtime ClassCastException  but at least it will happen close to the source of the problem

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Just typecheck it before you cast it   Object someObject   session getAttribute  attributeKey    if someObject instanceof HashMap  HashMap lt String  String gt  theHash    HashMap lt String  String gt  someObject      And for anyone asking  it s quite common to receive objects where you aren t sure of the type   Plenty of legacy  SOA  implementations pass around various objects that you shouldn t always trust    The horrors    EDIT Changed the example code once to match the poster s updates  and following some comments I see that instanceof doesn t play nicely with generics   However changing the check to validate the outer object seems to play well with the commandline compiler   Revised example now posted

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A quick guess if you post your code can say for sure but you might have done something along the lines of  HashMap lt String  Object gt  test   new HashMap      which will produce the warning when you need to do  HashMap lt String  Object gt  test   new HashMap lt String  Object gt       it might be worth looking at   Generics in the Java Programming Language  if your unfamiliar with what needs to be done

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If I have to use an API that doesn t support Generics   I try and isolate those calls in wrapper routines with as few lines as possible  I then use the SuppressWarnings annotation and also add the type-safety casts at the same time   This is just a personal preference to keep things as neat as possible

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A quick guess if you post your code can say for sure but you might have done something along the lines of  HashMap lt String  Object gt  test   new HashMap      which will produce the warning when you need to do  HashMap lt String  Object gt  test   new HashMap lt String  Object gt       it might be worth looking at   Generics in the Java Programming Language  if your unfamiliar with what needs to be done

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Here is a shortened example that avoids the  unchecked cast  warning by employing two strategies mentioned in other answers    Pass down the Class of the type of interest as a parameter at runtime  Class lt T gt  inputElementClazz   Then you can use  inputElementClazz cast anyObject   For type casting of a Collection  use the wildcard   instead of a generic type T to acknowledge that you indeed do not know what kind of objects to expect from the legacy code  Collection lt   gt  unknownTypeCollection   After all  this is what the  unchecked cast  warning wants to tell us  We cannot be sure that we get a Collection lt T gt   so the honest thing to do is to use a Collection lt   gt   If absolutely needed  a collection of a known type can still be built  Collection lt T gt  knownTypeCollection     The legacy code interfaced in the example below has an attribute  input  in the StructuredViewer  StructuredViewer is a tree or table widget   input  is the data model behind it   This  input  could be any kind of Java Collection   public void dragFinished StructuredViewer structuredViewer  Class lt T gt  inputElementClazz        IStructuredSelection selection    IStructuredSelection  structuredViewer getSelection           legacy code returns an Object from getFirstElement         the developer knows hopes it is of type inputElementClazz  but the compiler cannot know     T firstElement   inputElementClazz cast selection getFirstElement             legacy code returns an object from getInput  so we deal with it as a Collection lt   gt      Collection lt   gt  unknownTypeCollection    Collection lt   gt   structuredViewer getInput            for some operations we do not even need a collection with known types     unknownTypeCollection remove firstElement           nothing prevents us from building a Collection of a known type  should we really need one     Collection lt T gt  knownTypeCollection   new ArrayList lt T gt         for  Object object   unknownTypeCollection            T aT   inputElementClazz cast object           knownTypeCollection add aT           System out println aT getClass                structuredViewer refresh        Naturally  the code above can give runtime errors if we use the legacy code with the wrong data types  e g  if we set an array as the  input  of the StructuredViewer instead of a Java Collection    Example of calling the method   dragFinishedStrategy dragFinished viewer  Product class

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Solution  Disable this warning in Eclipse  Don t  SuppressWarnings it  just disable it completely   Several of the  solutions  presented above are way out of line  making code unreadable for the sake of suppressing a silly warning

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Just typecheck it before you cast it   Object someObject   session getAttribute  attributeKey    if someObject instanceof HashMap  HashMap lt String  String gt  theHash    HashMap lt String  String gt  someObject      And for anyone asking  it s quite common to receive objects where you aren t sure of the type   Plenty of legacy  SOA  implementations pass around various objects that you shouldn t always trust    The horrors    EDIT Changed the example code once to match the poster s updates  and following some comments I see that instanceof doesn t play nicely with generics   However changing the check to validate the outer object seems to play well with the commandline compiler   Revised example now posted

User · Answer

In the HTTP Session world you can t really avoid the cast  since the API is written that way  takes and returns only Object    With a little bit of work you can easily avoid the unchecked cast   though  This means that it will turn into a traditional cast giving a ClassCastException right there in the event of an error   An unchecked exception could turn into a CCE at any point later on instead of the point of the cast  that s the reason why it s a separate warning    Replace the HashMap with a dedicated class   import java util AbstractMap  import java util Collection  import java util HashMap  import java util Map  import java util Set   public class Attributes extends AbstractMap lt String  String gt        final Map lt String  String gt  content   new HashMap lt String  String gt           Override     public Set lt Map Entry lt String  String gt  gt  entrySet             return content entrySet                Override     public Set lt String gt  keySet             return content keySet                Override     public Collection lt String gt  values             return content values                Override     public String put final String key  final String value            return content put key  value             Then cast to that class instead of Map lt String String gt  and everything will be checked at the exact place where you write your code  No unexpected ClassCastExceptions later on

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You can create a utility class like the following  and use it to suppress the unchecked warning   public class Objects                   Helps to avoid using   code  SuppressWarnings   unchecked     when casting to a generic type               SuppressWarnings   unchecked        public static  lt T gt  T uncheckedCast Object obj            return  T  obj            You can use it as follows   import static Objects uncheckedCast       HashMap lt String  String gt  getItems javax servlet http HttpSession session          return uncheckedCast session getAttribute  attributeKey         Some more discussion about this is here  http   cleveralias blogs com thought spearmints 2006 01 suppresswarning html

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Almost every problem in Computer Science can be solved by adding a level of indirection   or something   So introduce a non-generic object that is of a higher-level that a Map  With no context it isn t going to look very convincing  but anyway   public final class Items implements java io Serializable       private static final long serialVersionUID   1L      private Map lt String String gt  map      public Items Map lt String String gt  map            this map   New immutableMap map             public Map lt String String gt  getMap             return map             Override public String toString             return map toString             public final class New       public static  lt K V gt  Map lt K V gt  immutableMap          Map lt   extends K    extends V gt  original                        optimise as you wish            return Collections unmodifiableMap              new HashMap lt String String gt  original                      static Map lt String  String gt  getItems HttpSession session        Items items    Items          session getAttribute  attributeKey        return items getMap         Except too many levels of indirection

User · Answer

You can create a utility class like the following  and use it to suppress the unchecked warning   public class Objects                   Helps to avoid using   code  SuppressWarnings   unchecked     when casting to a generic type               SuppressWarnings   unchecked        public static  lt T gt  T uncheckedCast Object obj            return  T  obj            You can use it as follows   import static Objects uncheckedCast       HashMap lt String  String gt  getItems javax servlet http HttpSession session          return uncheckedCast session getAttribute  attributeKey         Some more discussion about this is here  http   cleveralias blogs com thought spearmints 2006 01 suppresswarning html

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This stuff is hard  but here are my current thoughts   If your API returns Object  then there s nothing you can do -- no matter what  you will be blindly casting the object  You let Java throw ClassCastExceptions  or you can check each element yourself and throw Assertions or IllegalArgumentExceptions or some such  but these runtime checks are all equivalent  You have to suppress the compile time unchecked cast  no matter what you do at runtime   I d just prefer to blind cast and let the JVM perform its runtime check for me since we  know  what the API should return  and are usually willing to assume that the API works  Use generics everywhere above the cast  if you need them  You aren t really buying anything there since you still have the single blind cast  but at least you can use generics from there on up so the JVM can help you avoid blind casts in other pieces of your code   In this particular case  presumably you can see the call to SetAttribute and see the type is going in  so just blind-casting the type to same on the way out is not immoral  Add a comment referencing the SetAttribute and be done with it

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Solution  Disable this warning in Eclipse  Don t  SuppressWarnings it  just disable it completely   Several of the  solutions  presented above are way out of line  making code unreadable for the sake of suppressing a silly warning

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In Android Studio if you want to disable inspection you can use      noinspection unchecked Map lt String  String gt  myMap    Map lt String  String gt   deserializeMap

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In Eclipse Preferences  Go to Java- Compiler- Errors Warnings- Generic types and check the Ignore unavoidable generic type problems check-box    This satisfies the intent of the question  i e      I d like to avoid Eclipse warnings      if not the spirit

User · Answer

A quick guess if you post your code can say for sure but you might have done something along the lines of  HashMap lt String  Object gt  test   new HashMap      which will produce the warning when you need to do  HashMap lt String  Object gt  test   new HashMap lt String  Object gt       it might be worth looking at   Generics in the Java Programming Language  if your unfamiliar with what needs to be done

User · Answer

In the HTTP Session world you can t really avoid the cast  since the API is written that way  takes and returns only Object    With a little bit of work you can easily avoid the unchecked cast   though  This means that it will turn into a traditional cast giving a ClassCastException right there in the event of an error   An unchecked exception could turn into a CCE at any point later on instead of the point of the cast  that s the reason why it s a separate warning    Replace the HashMap with a dedicated class   import java util AbstractMap  import java util Collection  import java util HashMap  import java util Map  import java util Set   public class Attributes extends AbstractMap lt String  String gt        final Map lt String  String gt  content   new HashMap lt String  String gt           Override     public Set lt Map Entry lt String  String gt  gt  entrySet             return content entrySet                Override     public Set lt String gt  keySet             return content keySet                Override     public Collection lt String gt  values             return content values                Override     public String put final String key  final String value            return content put key  value             Then cast to that class instead of Map lt String String gt  and everything will be checked at the exact place where you write your code  No unexpected ClassCastExceptions later on

User · Answer

If I have to use an API that doesn t support Generics   I try and isolate those calls in wrapper routines with as few lines as possible  I then use the SuppressWarnings annotation and also add the type-safety casts at the same time   This is just a personal preference to keep things as neat as possible

User · Answer

In this particular case  I would not store Maps into the HttpSession directly  but instead an instance of my own class  which in turn contains a Map  an implementation detail of the class   Then you can be sure that the elements in the map are of the right type   But if you anyways want to check that the contents of the Map are of right type  you could use a code like this   public static void main String   args        Map lt String  Integer gt  map   new HashMap lt String  Integer gt         map put  a   1       map put  b   2       Object obj   map       Map lt String  Integer gt  ok   safeCastMap obj  String class  Integer class       Map lt String  String gt  error   safeCastMap obj  String class  String class       SuppressWarnings   unchecked    public static  lt K  V gt  Map lt K  V gt  safeCastMap Object map  Class lt K gt  keyType  Class lt V gt  valueType        checkMap map       checkMapContents keyType  valueType   Map lt      gt   map       return  Map lt K  V gt   map     private static void checkMap Object map        checkType Map class  map      private static  lt K  V gt  void checkMapContents Class lt K gt  keyType  Class lt V gt  valueType  Map lt      gt  map        for  Map Entry lt      gt  entry   map entrySet              checkType keyType  entry getKey             checkType valueType  entry getValue              private static  lt K gt  void checkType Class lt K gt  expectedType  Object obj        if   expectedType isInstance obj             throw new IllegalArgumentException  Expected     expectedType     but was     obj getClass            obj

User · Answer

If I have to use an API that doesn t support Generics   I try and isolate those calls in wrapper routines with as few lines as possible  I then use the SuppressWarnings annotation and also add the type-safety casts at the same time   This is just a personal preference to keep things as neat as possible

User · Answer

Unfortunately  there are no great options here  Remember  the goal of all of this is to preserve type safety    Java Generics  offers a solution for dealing with non-genericized legacy libraries  and there is one in particular called the  empty loop technique  in section 8 2  Basically  make the unsafe cast  and suppress the warning  Then loop through the map like this    SuppressWarnings  unchecked   Map lt String  Number gt  map   getMap    for  String s   map keySet     for  Number n   map values       If an unexpected type is encountered  you will get a runtime ClassCastException  but at least it will happen close to the source of the problem

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I may have misunderstood the question an example and a couple of surrounding lines would be nice   but why don t you always use an appropriate interface  and Java5    I see no reason why you would ever want to cast to a HashMap instead of a Map lt KeyType ValueType gt   In fact  I can t imagine any reason to set the type of a variable to HashMap instead of Map   And why is the source an Object  Is it a parameter type of a legacy collection  If so  use generics and specify the type you want

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I may have misunderstood the question an example and a couple of surrounding lines would be nice   but why don t you always use an appropriate interface  and Java5    I see no reason why you would ever want to cast to a HashMap instead of a Map lt KeyType ValueType gt   In fact  I can t imagine any reason to set the type of a variable to HashMap instead of Map   And why is the source an Object  Is it a parameter type of a legacy collection  If so  use generics and specify the type you want

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The obvious answer  of course  is not to do the unchecked cast   If it s absolutely necessary  then at least try to limit the scope of the  SuppressWarnings annotation  According to its Javadocs  it can go on local variables  this way  it doesn t even affect the entire method   Example    SuppressWarnings  unchecked   Map lt String  String gt  myMap    Map lt String  String gt   deserializeMap      There is no way to determine whether the Map really should have the generic parameters  lt String  String gt   You must know beforehand what the parameters should be  or you ll find out when you get a ClassCastException   This is why the code generates a warning  because the compiler can t possibly know whether is safe

User · Answer

In the HTTP Session world you can t really avoid the cast  since the API is written that way  takes and returns only Object    With a little bit of work you can easily avoid the unchecked cast   though  This means that it will turn into a traditional cast giving a ClassCastException right there in the event of an error   An unchecked exception could turn into a CCE at any point later on instead of the point of the cast  that s the reason why it s a separate warning    Replace the HashMap with a dedicated class   import java util AbstractMap  import java util Collection  import java util HashMap  import java util Map  import java util Set   public class Attributes extends AbstractMap lt String  String gt        final Map lt String  String gt  content   new HashMap lt String  String gt           Override     public Set lt Map Entry lt String  String gt  gt  entrySet             return content entrySet                Override     public Set lt String gt  keySet             return content keySet                Override     public Collection lt String gt  values             return content values                Override     public String put final String key  final String value            return content put key  value             Then cast to that class instead of Map lt String String gt  and everything will be checked at the exact place where you write your code  No unexpected ClassCastExceptions later on

User · Answer

In this particular case  I would not store Maps into the HttpSession directly  but instead an instance of my own class  which in turn contains a Map  an implementation detail of the class   Then you can be sure that the elements in the map are of the right type   But if you anyways want to check that the contents of the Map are of right type  you could use a code like this   public static void main String   args        Map lt String  Integer gt  map   new HashMap lt String  Integer gt         map put  a   1       map put  b   2       Object obj   map       Map lt String  Integer gt  ok   safeCastMap obj  String class  Integer class       Map lt String  String gt  error   safeCastMap obj  String class  String class       SuppressWarnings   unchecked    public static  lt K  V gt  Map lt K  V gt  safeCastMap Object map  Class lt K gt  keyType  Class lt V gt  valueType        checkMap map       checkMapContents keyType  valueType   Map lt      gt   map       return  Map lt K  V gt   map     private static void checkMap Object map        checkType Map class  map      private static  lt K  V gt  void checkMapContents Class lt K gt  keyType  Class lt V gt  valueType  Map lt      gt  map        for  Map Entry lt      gt  entry   map entrySet              checkType keyType  entry getKey             checkType valueType  entry getValue              private static  lt K gt  void checkType Class lt K gt  expectedType  Object obj        if   expectedType isInstance obj             throw new IllegalArgumentException  Expected     expectedType     but was     obj getClass            obj

User · Answer

The obvious answer  of course  is not to do the unchecked cast   If it s absolutely necessary  then at least try to limit the scope of the  SuppressWarnings annotation  According to its Javadocs  it can go on local variables  this way  it doesn t even affect the entire method   Example    SuppressWarnings  unchecked   Map lt String  String gt  myMap    Map lt String  String gt   deserializeMap      There is no way to determine whether the Map really should have the generic parameters  lt String  String gt   You must know beforehand what the parameters should be  or you ll find out when you get a ClassCastException   This is why the code generates a warning  because the compiler can t possibly know whether is safe

User · Answer

In this particular case  I would not store Maps into the HttpSession directly  but instead an instance of my own class  which in turn contains a Map  an implementation detail of the class   Then you can be sure that the elements in the map are of the right type   But if you anyways want to check that the contents of the Map are of right type  you could use a code like this   public static void main String   args        Map lt String  Integer gt  map   new HashMap lt String  Integer gt         map put  a   1       map put  b   2       Object obj   map       Map lt String  Integer gt  ok   safeCastMap obj  String class  Integer class       Map lt String  String gt  error   safeCastMap obj  String class  String class       SuppressWarnings   unchecked    public static  lt K  V gt  Map lt K  V gt  safeCastMap Object map  Class lt K gt  keyType  Class lt V gt  valueType        checkMap map       checkMapContents keyType  valueType   Map lt      gt   map       return  Map lt K  V gt   map     private static void checkMap Object map        checkType Map class  map      private static  lt K  V gt  void checkMapContents Class lt K gt  keyType  Class lt V gt  valueType  Map lt      gt  map        for  Map Entry lt      gt  entry   map entrySet              checkType keyType  entry getKey             checkType valueType  entry getValue              private static  lt K gt  void checkType Class lt K gt  expectedType  Object obj        if   expectedType isInstance obj             throw new IllegalArgumentException  Expected     expectedType     but was     obj getClass            obj

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Take this one  it s much faster than creating a new HashMap  if it s already one  but still secure  as each element is checked against it s type      SuppressWarnings  unchecked   public static  lt K  V gt  HashMap lt K  V gt  toHashMap Object input  Class lt K gt  key  Class lt V gt  value           assert input instanceof Map   input          for  Map Entry lt      gt  e     HashMap lt      gt   input  entrySet                 assert key isAssignableFrom e getKey   getClass       Map contains invalid keys              assert value isAssignableFrom e getValue   getClass       Map contains invalid values                    if  input instanceof HashMap             return  HashMap lt K  V gt   input         return new HashMap lt K  V gt   Map lt K  V gt   input

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You can create a utility class like the following  and use it to suppress the unchecked warning   public class Objects                   Helps to avoid using   code  SuppressWarnings   unchecked     when casting to a generic type               SuppressWarnings   unchecked        public static  lt T gt  T uncheckedCast Object obj            return  T  obj            You can use it as follows   import static Objects uncheckedCast       HashMap lt String  String gt  getItems javax servlet http HttpSession session          return uncheckedCast session getAttribute  attributeKey         Some more discussion about this is here  http   cleveralias blogs com thought spearmints 2006 01 suppresswarning html

User · Answer

If I have to use an API that doesn t support Generics   I try and isolate those calls in wrapper routines with as few lines as possible  I then use the SuppressWarnings annotation and also add the type-safety casts at the same time   This is just a personal preference to keep things as neat as possible

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You can create a utility class like the following  and use it to suppress the unchecked warning   public class Objects                   Helps to avoid using   code  SuppressWarnings   unchecked     when casting to a generic type               SuppressWarnings   unchecked        public static  lt T gt  T uncheckedCast Object obj            return  T  obj            You can use it as follows   import static Objects uncheckedCast       HashMap lt String  String gt  getItems javax servlet http HttpSession session          return uncheckedCast session getAttribute  attributeKey         Some more discussion about this is here  http   cleveralias blogs com thought spearmints 2006 01 suppresswarning html

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Unfortunately  there are no great options here  Remember  the goal of all of this is to preserve type safety    Java Generics  offers a solution for dealing with non-genericized legacy libraries  and there is one in particular called the  empty loop technique  in section 8 2  Basically  make the unsafe cast  and suppress the warning  Then loop through the map like this    SuppressWarnings  unchecked   Map lt String  Number gt  map   getMap    for  String s   map keySet     for  Number n   map values       If an unexpected type is encountered  you will get a runtime ClassCastException  but at least it will happen close to the source of the problem

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This stuff is hard  but here are my current thoughts   If your API returns Object  then there s nothing you can do -- no matter what  you will be blindly casting the object  You let Java throw ClassCastExceptions  or you can check each element yourself and throw Assertions or IllegalArgumentExceptions or some such  but these runtime checks are all equivalent  You have to suppress the compile time unchecked cast  no matter what you do at runtime   I d just prefer to blind cast and let the JVM perform its runtime check for me since we  know  what the API should return  and are usually willing to assume that the API works  Use generics everywhere above the cast  if you need them  You aren t really buying anything there since you still have the single blind cast  but at least you can use generics from there on up so the JVM can help you avoid blind casts in other pieces of your code   In this particular case  presumably you can see the call to SetAttribute and see the type is going in  so just blind-casting the type to same on the way out is not immoral  Add a comment referencing the SetAttribute and be done with it

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Almost every problem in Computer Science can be solved by adding a level of indirection   or something   So introduce a non-generic object that is of a higher-level that a Map  With no context it isn t going to look very convincing  but anyway   public final class Items implements java io Serializable       private static final long serialVersionUID   1L      private Map lt String String gt  map      public Items Map lt String String gt  map            this map   New immutableMap map             public Map lt String String gt  getMap             return map             Override public String toString             return map toString             public final class New       public static  lt K V gt  Map lt K V gt  immutableMap          Map lt   extends K    extends V gt  original                        optimise as you wish            return Collections unmodifiableMap              new HashMap lt String String gt  original                      static Map lt String  String gt  getItems HttpSession session        Items items    Items          session getAttribute  attributeKey        return items getMap         Except too many levels of indirection

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Two ways  one which avoids the tag completely  the other using a naughty but nice utility method  The problem is pre-genericised Collections    I believe the rule of thumb is   cast objects one thing at a time  - what this means when trying to use raw classes in a genericised world is that because you don t know what is in this Map lt      gt   and indeed the JVM might even find that it isn t even a Map    it obvious when you think about it that you can t cast it   If you had a Map lt String    gt  map2 then HashSet lt String gt  keys    HashSet lt String gt  map2 keySet   does not give you a warning  despite this being an  act of faith  for the compiler  because it might turn out to be a TreeSet     but it is only a single act of faith   PS to the objection that iterating as in my first way  is boring  and  takes time   the answer is  no pain no gain   a genericised collection is guaranteed to contain Map Entry lt String  String gt s  and nothing else   You have to pay for this guarantee   When using generics systematically this payment  beautifully  takes the form of coding compliance  not machine time  One school of thought might say that you should set Eclipse s settings to make such unchecked casts errors  rather than warnings   In that case you would have to use my first way   package scratchpad   import java util HashMap  import java util Iterator  import java util Map  import java util Vector   public class YellowMouse           First way      Map lt String  String gt  getHashMapStudiouslyAvoidingSuppressTag HttpSession session          Map lt      gt  theHash    Map lt      gt  session getAttribute  attributeKey           Map lt String  String gt  yellowMouse   new HashMap lt String  String gt           for  Map Entry lt      gt  entry   theHash entrySet              yellowMouse put   String entry getKey     String entry getValue                     return yellowMouse                 Second way      Map lt String  String gt  getHashMapUsingNaughtyButNiceUtilityMethod HttpSession session          return uncheckedCast  session getAttribute  attributeKey                     NB this is a utility method which should be kept in your utility library  If you do that it will        be the  only  time in your entire life that you will have to use this particular tag         SuppressWarnings    unchecked         public static synchronized  lt T gt  T uncheckedCast Object obj            return  T  obj

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This makes the warnings go away      static Map lt String  String gt  getItems HttpSession session            HashMap lt      gt  theHash1    HashMap lt String String gt  session getAttribute  attributeKey            HashMap lt String String gt  theHash    HashMap lt String String gt  theHash1      return theHash

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Almost every problem in Computer Science can be solved by adding a level of indirection   or something   So introduce a non-generic object that is of a higher-level that a Map  With no context it isn t going to look very convincing  but anyway   public final class Items implements java io Serializable       private static final long serialVersionUID   1L      private Map lt String String gt  map      public Items Map lt String String gt  map            this map   New immutableMap map             public Map lt String String gt  getMap             return map             Override public String toString             return map toString             public final class New       public static  lt K V gt  Map lt K V gt  immutableMap          Map lt   extends K    extends V gt  original                        optimise as you wish            return Collections unmodifiableMap              new HashMap lt String String gt  original                      static Map lt String  String gt  getItems HttpSession session        Items items    Items          session getAttribute  attributeKey        return items getMap         Except too many levels of indirection

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Here s one way I handle this when I override the equals   operation   public abstract class Section lt T extends Section gt  extends Element lt Section lt T gt  gt        Object attr1                 Compare one section object to another               param obj the object being compared with this section object        return true if this section and the other section are of the same       sub-class of section and their component fields are the same  false       otherwise                    Override     public boolean equals Object obj            if  obj    null                   this exists  but obj doesn t  so they can t be equal              return false                        prepare to cast            Section lt   gt  other           if  getClass      obj getClass                     looks like we re comparing apples to oranges             return false            else                  it must be safe to make that cast              other    Section lt   gt   obj                        and then I compare attributes between this and other         return this attr1 equals other attr1             This seems to work in Java 8  even compiled with -Xlint unchecked

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Just typecheck it before you cast it   Object someObject   session getAttribute  attributeKey    if someObject instanceof HashMap  HashMap lt String  String gt  theHash    HashMap lt String  String gt  someObject      And for anyone asking  it s quite common to receive objects where you aren t sure of the type   Plenty of legacy  SOA  implementations pass around various objects that you shouldn t always trust    The horrors    EDIT Changed the example code once to match the poster s updates  and following some comments I see that instanceof doesn t play nicely with generics   However changing the check to validate the outer object seems to play well with the commandline compiler   Revised example now posted

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Two ways  one which avoids the tag completely  the other using a naughty but nice utility method  The problem is pre-genericised Collections    I believe the rule of thumb is   cast objects one thing at a time  - what this means when trying to use raw classes in a genericised world is that because you don t know what is in this Map lt      gt   and indeed the JVM might even find that it isn t even a Map    it obvious when you think about it that you can t cast it   If you had a Map lt String    gt  map2 then HashSet lt String gt  keys    HashSet lt String gt  map2 keySet   does not give you a warning  despite this being an  act of faith  for the compiler  because it might turn out to be a TreeSet     but it is only a single act of faith   PS to the objection that iterating as in my first way  is boring  and  takes time   the answer is  no pain no gain   a genericised collection is guaranteed to contain Map Entry lt String  String gt s  and nothing else   You have to pay for this guarantee   When using generics systematically this payment  beautifully  takes the form of coding compliance  not machine time  One school of thought might say that you should set Eclipse s settings to make such unchecked casts errors  rather than warnings   In that case you would have to use my first way   package scratchpad   import java util HashMap  import java util Iterator  import java util Map  import java util Vector   public class YellowMouse           First way      Map lt String  String gt  getHashMapStudiouslyAvoidingSuppressTag HttpSession session          Map lt      gt  theHash    Map lt      gt  session getAttribute  attributeKey           Map lt String  String gt  yellowMouse   new HashMap lt String  String gt           for  Map Entry lt      gt  entry   theHash entrySet              yellowMouse put   String entry getKey     String entry getValue                     return yellowMouse                 Second way      Map lt String  String gt  getHashMapUsingNaughtyButNiceUtilityMethod HttpSession session          return uncheckedCast  session getAttribute  attributeKey                     NB this is a utility method which should be kept in your utility library  If you do that it will        be the  only  time in your entire life that you will have to use this particular tag         SuppressWarnings    unchecked         public static synchronized  lt T gt  T uncheckedCast Object obj            return  T  obj

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The obvious answer  of course  is not to do the unchecked cast   If it s absolutely necessary  then at least try to limit the scope of the  SuppressWarnings annotation  According to its Javadocs  it can go on local variables  this way  it doesn t even affect the entire method   Example    SuppressWarnings  unchecked   Map lt String  String gt  myMap    Map lt String  String gt   deserializeMap      There is no way to determine whether the Map really should have the generic parameters  lt String  String gt   You must know beforehand what the parameters should be  or you ll find out when you get a ClassCastException   This is why the code generates a warning  because the compiler can t possibly know whether is safe

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In Android Studio if you want to disable inspection you can use      noinspection unchecked Map lt String  String gt  myMap    Map lt String  String gt   deserializeMap

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A quick guess if you post your code can say for sure but you might have done something along the lines of  HashMap lt String  Object gt  test   new HashMap      which will produce the warning when you need to do  HashMap lt String  Object gt  test   new HashMap lt String  Object gt       it might be worth looking at   Generics in the Java Programming Language  if your unfamiliar with what needs to be done

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The Objects Unchecked utility function in the answer above by Esko Luontola is a great way to avoid program clutter   If you don t want the SuppressWarnings on an entire method  Java forces you to put it on a local  If you need a cast on a member it can lead to code like this    SuppressWarnings  unchecked   Vector lt String gt  watchedSymbolsClone    Vector lt String gt   watchedSymbols clone    this watchedSymbols   watchedSymbolsClone    Using the utility is much cleaner  and it s still obvious what you are doing   this watchedSymbols   Objects uncheckedCast watchedSymbols clone       NOTE  I feel its important to add that sometimes the warning really means you are doing something wrong like    ArrayList lt Integer gt  intList   new ArrayList lt Integer gt     intList add 1   Object intListObject   intList        this line gives an unchecked warning - but no runtime error ArrayList lt String gt  stringList     ArrayList lt String gt   intListObject  System out println stringList get 0       cast exception will be given here   What the compiler is telling you is that this cast will NOT be checked at runtime  so no runtime error will be raised until you try to access the data in the generic container

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The Objects Unchecked utility function in the answer above by Esko Luontola is a great way to avoid program clutter   If you don t want the SuppressWarnings on an entire method  Java forces you to put it on a local  If you need a cast on a member it can lead to code like this    SuppressWarnings  unchecked   Vector lt String gt  watchedSymbolsClone    Vector lt String gt   watchedSymbols clone    this watchedSymbols   watchedSymbolsClone    Using the utility is much cleaner  and it s still obvious what you are doing   this watchedSymbols   Objects uncheckedCast watchedSymbols clone       NOTE  I feel its important to add that sometimes the warning really means you are doing something wrong like    ArrayList lt Integer gt  intList   new ArrayList lt Integer gt     intList add 1   Object intListObject   intList        this line gives an unchecked warning - but no runtime error ArrayList lt String gt  stringList     ArrayList lt String gt   intListObject  System out println stringList get 0       cast exception will be given here   What the compiler is telling you is that this cast will NOT be checked at runtime  so no runtime error will be raised until you try to access the data in the generic container

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This makes the warnings go away      static Map lt String  String gt  getItems HttpSession session            HashMap lt      gt  theHash1    HashMap lt String String gt  session getAttribute  attributeKey            HashMap lt String String gt  theHash    HashMap lt String String gt  theHash1      return theHash

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Just typecheck it before you cast it   Object someObject   session getAttribute  attributeKey    if someObject instanceof HashMap  HashMap lt String  String gt  theHash    HashMap lt String  String gt  someObject      And for anyone asking  it s quite common to receive objects where you aren t sure of the type   Plenty of legacy  SOA  implementations pass around various objects that you shouldn t always trust    The horrors    EDIT Changed the example code once to match the poster s updates  and following some comments I see that instanceof doesn t play nicely with generics   However changing the check to validate the outer object seems to play well with the commandline compiler   Revised example now posted

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Take this one  it s much faster than creating a new HashMap  if it s already one  but still secure  as each element is checked against it s type      SuppressWarnings  unchecked   public static  lt K  V gt  HashMap lt K  V gt  toHashMap Object input  Class lt K gt  key  Class lt V gt  value           assert input instanceof Map   input          for  Map Entry lt      gt  e     HashMap lt      gt   input  entrySet                 assert key isAssignableFrom e getKey   getClass       Map contains invalid keys              assert value isAssignableFrom e getValue   getClass       Map contains invalid values                    if  input instanceof HashMap             return  HashMap lt K  V gt   input         return new HashMap lt K  V gt   Map lt K  V gt   input

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The obvious answer  of course  is not to do the unchecked cast   If it s absolutely necessary  then at least try to limit the scope of the  SuppressWarnings annotation  According to its Javadocs  it can go on local variables  this way  it doesn t even affect the entire method   Example    SuppressWarnings  unchecked   Map lt String  String gt  myMap    Map lt String  String gt   deserializeMap      There is no way to determine whether the Map really should have the generic parameters  lt String  String gt   You must know beforehand what the parameters should be  or you ll find out when you get a ClassCastException   This is why the code generates a warning  because the compiler can t possibly know whether is safe

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Unfortunately  there are no great options here  Remember  the goal of all of this is to preserve type safety    Java Generics  offers a solution for dealing with non-genericized legacy libraries  and there is one in particular called the  empty loop technique  in section 8 2  Basically  make the unsafe cast  and suppress the warning  Then loop through the map like this    SuppressWarnings  unchecked   Map lt String  Number gt  map   getMap    for  String s   map keySet     for  Number n   map values       If an unexpected type is encountered  you will get a runtime ClassCastException  but at least it will happen close to the source of the problem

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Warning suppression is not a solution  You should not be doing two level casting in one statement   HashMap lt String  String gt  getItems javax servlet http HttpSession session            first  cast the returned Object to generic HashMap lt     gt      HashMap lt      gt  theHash    HashMap lt      gt  session getAttribute  attributeKey            next  cast every entry of the HashMap to the required type  lt String  String gt      HashMap lt String  String gt  returingHash   new HashMap lt  gt         for  Entry lt      gt  entry   theHash entrySet              returingHash put  String  entry getKey     String  entry getValue               return returingHash

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I may have misunderstood the question an example and a couple of surrounding lines would be nice   but why don t you always use an appropriate interface  and Java5    I see no reason why you would ever want to cast to a HashMap instead of a Map lt KeyType ValueType gt   In fact  I can t imagine any reason to set the type of a variable to HashMap instead of Map   And why is the source an Object  Is it a parameter type of a legacy collection  If so  use generics and specify the type you want

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Here is a shortened example that avoids the  unchecked cast  warning by employing two strategies mentioned in other answers    Pass down the Class of the type of interest as a parameter at runtime  Class lt T gt  inputElementClazz   Then you can use  inputElementClazz cast anyObject   For type casting of a Collection  use the wildcard   instead of a generic type T to acknowledge that you indeed do not know what kind of objects to expect from the legacy code  Collection lt   gt  unknownTypeCollection   After all  this is what the  unchecked cast  warning wants to tell us  We cannot be sure that we get a Collection lt T gt   so the honest thing to do is to use a Collection lt   gt   If absolutely needed  a collection of a known type can still be built  Collection lt T gt  knownTypeCollection     The legacy code interfaced in the example below has an attribute  input  in the StructuredViewer  StructuredViewer is a tree or table widget   input  is the data model behind it   This  input  could be any kind of Java Collection   public void dragFinished StructuredViewer structuredViewer  Class lt T gt  inputElementClazz        IStructuredSelection selection    IStructuredSelection  structuredViewer getSelection           legacy code returns an Object from getFirstElement         the developer knows hopes it is of type inputElementClazz  but the compiler cannot know     T firstElement   inputElementClazz cast selection getFirstElement             legacy code returns an object from getInput  so we deal with it as a Collection lt   gt      Collection lt   gt  unknownTypeCollection    Collection lt   gt   structuredViewer getInput            for some operations we do not even need a collection with known types     unknownTypeCollection remove firstElement           nothing prevents us from building a Collection of a known type  should we really need one     Collection lt T gt  knownTypeCollection   new ArrayList lt T gt         for  Object object   unknownTypeCollection            T aT   inputElementClazz cast object           knownTypeCollection add aT           System out println aT getClass                structuredViewer refresh        Naturally  the code above can give runtime errors if we use the legacy code with the wrong data types  e g  if we set an array as the  input  of the StructuredViewer instead of a Java Collection    Example of calling the method   dragFinishedStrategy dragFinished viewer  Product class

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If you are sure that the type returned by session getAttribute   is HashMap then you can not typecast to that exact type  but rely on only checking the generic HashMap  HashMap lt     gt  getItems javax servlet http HttpSession session          HashMap lt     gt  theHash    HashMap lt     gt  session getAttribute  attributeKey        return theHash       Eclipse will then surprise warnings  but of course this can lead to runtime errors that can be hard to debug  I use this approach in not operation-critical contexts only

[java] How do I address unchecked cast warnings?

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