What is SuppressWarnings unchecked in Java

Question

Sometime when looking through code  I see many methods specify an annotation    SuppressWarnings  unchecked     What does this mean

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The SuppressWarning annotation is used to suppress compiler warnings for the annotated element  Specifically  the unchecked category allows suppression of compiler warnings generated as a result of unchecked type casts

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SuppressWarnings annotation is one of the three built-in annotations available in JDK and added alongside  Override and  Deprecated in Java 1 5    SuppressWarnings instruct the compiler to ignore or suppress  specified compiler warning in annotated element and all program elements inside that element  For example  if a class is annotated to suppress a particular warning  then a warning generated in a method inside that class will also be separated   You might have seen  SuppressWarnings  unchecked   and  SuppressWarnings  serial    two of most popular examples of  SuppressWarnings annotation  Former is used to suppress warning generated due to unchecked casting while the later warning is used to remind about adding SerialVersionUID in a Serializable class   Read more  https   javarevisited blogspot com 2015 09 what-is-suppresswarnings-annotation-in-java-unchecked-raw-serial html ixzz5rqQaOLUa

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In Java  generics are implemented by means of type erasure  For instance  the following code   List lt String gt  hello   List of  a    b    String example   hello get 0     Is compiled to the following   List hello   List of  a    b    String example    String  hello get 0     And List of is defined as   static  lt E gt  List lt E gt  of E e1  E e2     Which after type erasure becomes   static List of Object e1  Object e2     The compiler has no idea what are generic types at runtime  so if you write something like this   Object list   List of  a    b    List lt Integer gt  actualList    List lt Integer gt   list    Java Virtual Machine has no idea what generic types are while running a program  so this compiles and runs  as for Java Virtual Machine  this is a cast to List type  this is the only thing it can verify  so it verifies only that    But now add this line   Integer hello   actualList get 0     And JVM will throw an unexpected ClassCastException  as Java compiler inserted an implicit cast   java lang ClassCastException  java base java lang String cannot be cast to java base java lang Integer   An unchecked warning tells a programmer that a cast may cause a program to throw an exception somewhere else  Suppressing the warning with  SuppressWarnings  unchecked   tells the compiler that the programmer believes the code to be safe and won t cause unexpected exceptions   Why would you want to do that  Java type system isn t good enough to represent all possible type usage patterns  Sometimes you may know that a cast is safe  but Java doesn t provide a way to say so - to hide warnings like this   SupressWarnings  unchecked   can be used  so that a programmer can focus on real warnings  For instance  Optional empty   returns a singleton to avoid allocation of empty optionals that don t store a value   private static final Optional lt   gt  EMPTY   new Optional lt  gt     public static lt T gt  Optional lt T gt  empty          SuppressWarnings  unchecked       Optional lt T gt  t    Optional lt T gt   EMPTY      return t      This cast is safe  as the value stored in an empty optional cannot be retrieved so there is no risk of unexpected class cast exceptions

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It is an annotation to suppress compile warnings about unchecked generic operations  not exceptions   such as casts  It essentially implies that the programmer did not wish to be notified about these which he is already aware of when compiling a particular bit of code   You can read more on this specific annotation here      SuppressWarnings   Additionally  Oracle provides some tutorial documentation on the usage of annotations here      Annotations   As they put it        The  unchecked  warning can occur when interfacing with legacy code written before the advent of generics  discussed in the lesson titled Generics

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You can suppress the compiler warnings and tell the generics that the code which you had written is legal according to it   Example    SuppressWarnings  unchecked   public List lt ReservationMealPlan gt  retreiveMealPlan          List lt ReservationMealPlan gt  list new ArrayList lt ReservationMealPlan gt         TestMenuService testMenuService new TestMenuService em  this selectedInstance       list   testMenuService getMeal reservationMealPlan       return list

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As far I know  for now it has to do with suppressing warnings about generics  generics are a new programming construct not supported in JDK versions earlier than JDK 5  so any mixes of the old constructs with the new ones might pose some unexpected results    The compiler warns the programmer about it  but if the programmer already knows  they can turn those dreaded warnings off using SuppressWarnings

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Sometimes Java generics just doesn t let you do what you want to  and you need to effectively tell the compiler that what you re doing really will be legal at execution time   I usually find this a pain when I m mocking a generic interface  but there are other examples too  It s usually worth trying to work out a way of avoiding the warning rather than suppressing it  the Java Generics FAQ helps here  but sometimes even if it is possible  it bends the code out of shape so much that suppressing the warning is neater  Always add an explanatory comment in that case   The same generics FAQ has several sections on this topic  starting with  What is an  unchecked  warning   - it s well worth a read

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It could also mean that the current Java type system version isn t good enough for your case  There were several JSR propositions   hacks to fix this  Type tokens  Super Type Tokens  Class cast      If you really need this supression  narrow it down as much as possible  e g  don t put it onto the class itself or onto a long method   An example   public List lt String gt  getALegacyListReversed         SuppressWarnings  unchecked   List lt String gt  list           List lt String gt  legacyLibrary getStringList        Collections reverse list      return list

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Simply  It s a warning by which the compiler indicates that it cannot ensure type safety   JPA service method for example    SuppressWarnings  unchecked   public List lt User gt  findAllUsers        Query query   entitymanager createQuery  SELECT u FROM User u        return  List lt User gt  query getResultList        If I didn n anotate the  SuppressWarnings  unchecked   here  it would have a problem with line  where I want to return my ResultList   In shortcut type-safety means  A program is considered type-safe if it compiles without errors and warnings and does not raise any unexpected ClassCastException s at runtime   I build on http   www angelikalanger com GenericsFAQ FAQSections Fundamentals html

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A warning by which the compiler indicates that it cannot ensure type safety  The term  unchecked  warning is misleading   It does not mean that the warning is unchecked in any way   The term  unchecked  refers to the fact that the compiler and the runtime system do not have enough type information to perform all type checks that would be necessary to ensure type safety  In this sense  certain operations are  unchecked     The most common source of  unchecked  warnings is the use of raw types    unchecked  warnings are issued when an object is accessed through a raw type variable  because the raw type does not provide enough type information to perform all necessary type checks    Example  of unchecked warning in conjunction with raw types     TreeSet set   new TreeSet     set add  abc              unchecked warning  set remove  abc        warning   unchecked  unchecked call to add E  as a member of the raw type java util TreeSet                 set add  abc                                When the add method is invoked the compiler does not know whether it is safe to add a String object to the collection   If the TreeSet is a collection that contains String s  or a supertype thereof   then it would be safe   But from the type information provided by the raw type TreeSet the compiler cannot tell   Hence the call is potentially unsafe and an  unchecked  warning is issued      unchecked  warnings are also reported when the compiler finds a cast whose target type is either a parameterized type or a type parameter    Example  of an unchecked warning in conjunction with a cast to a parameterized type or type variable       class Wrapper lt T gt       private T wrapped      public Wrapper  T arg   wrapped   arg             public Wrapper  lt T gt  clone          Wrapper lt T gt  clon   null        try            clon    Wrapper lt T gt   super clone       unchecked warning         catch  CloneNotSupportedException e             throw new InternalError                   try            Class lt   gt  clzz   this wrapped getClass            Method   meth   clzz getMethod  clone   new Class 0            Object   dupl   meth invoke this wrapped  new Object 0            clon wrapped    T  dupl     unchecked warning         catch  Exception e           return clon               warning   unchecked  unchecked cast  found     java lang Object  required  Wrapper  lt T gt                     clon     Wrapper  lt T gt  super clone                                                         warning   unchecked  unchecked cast  found     java lang Object  required  T                    clon  wrapped    T dupl    A cast whose target type is either a  concrete or bounded wildcard  parameterized type or a type parameter is unsafe  if a dynamic type check at runtime is involved   At runtime  only the type erasure is available  not the exact static type that is visible in the source code   As a result  the runtime part of the cast is performed based on the type erasure  not on the exact static type    In the example  the cast to Wrapper would check whether the object returned from super clone is a Wrapper   not whether it is a wrapper with a particular type of members   Similarly  the casts to the type parameter T are cast to type Object at runtime  and probably optimized away altogether   Due to type erasure  the runtime system is unable to perform more useful type checks at runtime    In a way  the source code is misleading  because it suggests that a cast to the respective target type is performed  while in fact the dynamic part of the cast only checks against the type erasure of the target type   The  unchecked  warning is issued to draw the programmer s attention to this mismatch between the static and dynamic aspect of the cast   Please refer  What is an  unchecked  warning

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One trick is to create an interface that extends a generic base interface     public interface LoadFutures extends Map lt UUID  Future lt LoadResult gt  gt       Then you can check it with instanceof before the cast     Object obj   context getAttribute FUTURES   if    obj instanceof LoadFutures         String format    Servlet context attribute    s   is not of type                  LoadFutures  Its type is  s        String msg   String format format  FUTURES  obj getClass         throw new RuntimeException msg     return  LoadFutures  obj

[java] What is SuppressWarnings ("unchecked") in Java?

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