Java generics multiple generic parameters

Question

I was wondering if it s possible to write a function that accepts multiple generic types as follows   public int void myfunction Set lt T gt  a  Set lt T gt  b        return 5     Set lt Integer gt  setA   new HashSet lt Integer gt     Set lt String gt  setB   new HashSet lt String gt     int result   myfunction setA  setB     Will that work  Does the  generic in each parameter mean that each parameter must have the same type T that s generic

User · Answer

a and b must both be sets of the same type  But nothing prevents you from writing  myfunction Set lt X gt  a  Set lt Y gt  b

User · Answer

Even more  you can inherit generics      SuppressWarnings  unchecked   public  lt T extends Something lt E gt   E extends Enum lt E gt   amp  SomethingAware gt  T getSomething Class lt T gt  clazz            return  T  somethingHolderMap get clazz

User · Answer

In your function definition you re constraining sets a and b to the same type  You can also write  public  lt X Y gt  void myFunction Set lt X gt  s1  Set lt Y gt  s2

User · Answer

Yes - it s possible  though not with your method signature  and yes  with your signature the types must be the same    With the signature you have given  T must be associated to a single type  e g  String or Integer  at the call-site  You can  however  declare method signatures which take multiple type parameters   public  lt S  T gt  void func Set lt S gt  s  Set lt T gt  t    Note in the above signature that I have declared the types S and T in the signature itself  These are therefore different to and independent of any generic types associated with the class or interface which contains the function    public class MyClass lt S  T gt       public        void foo Set lt S gt  s  Set lt T gt  t     same type params as on class    public  lt U  V gt  void bar Set lt U gt  s  Set lt V gt  t     type params independent of class     You might like to take a look at some of the method signatures of the collection classes in the java util package  Generics is really rather a complicated subject  especially when wildcards    extends and   super  are considered  For example  it s often the case that a method which might take a Set lt Number gt  as a parameter should also accept a Set lt Integer gt   In which case you d see a signature like this   public void baz Set lt   extends T gt  s     There are plenty of questions already on SO for you to look at on the subject    Java Generics  List  List lt Object gt   List lt   gt  Java Generics  Wildcards  What are the differences between Generics in C  and Java    and Templates in C      Not sure what the point of returning an int from the function is  although you could do that if you want

User · Answer

You can declare multiple type variables on a type or method  For example  using type parameters on the method    lt P  Q gt  int f Set lt P gt   Set lt Q gt       return 0

User · Answer

You can follow one of the below approaches   1  Basic  single type      One type public static  lt T gt  void fill List  lt T gt  list  T val         for int i 0  i lt list size    i             list set i  val              2  Multiple Types       multiple types as parameters public static  lt T1  T2 gt  String multipleTypeArgument T1 val1  T2 val2         return val1     val2       3  Below will raise compiler error as  T3 is not in the listing of generic types that are used in function declaration part     Raised compilation error public static  lt T1  T2 gt  T3 returnTypeGeneric T1 val1  T2 val2        return 0      Correct   Compiles fine  public static  lt T1  T2  T3 gt  T3 returnTypeGeneric T1 val1  T2 val2        return 0      Sample Class Code    package generics basics   import java util ArrayList  import java util List   public class GenericMethods        Declare the generic type parameter T in this method     After the qualifiers public and static  you put  lt T gt  and   then followed it by return type  method name  and its parameters    Observe   type of val is  T  and not   lt T gt            One type public static  lt T gt  void fill List  lt T gt  list  T val         for int i 0  i lt list size    i             list set i  val                multiple types as parameters public static  lt T1  T2 gt  String multipleTypeArgument T1 val1  T2 val2         return val1     val2           Q  To audience - gt  will this compile           public static  lt T1  T2 gt  T3 returnTypeGeneric T1 val1  T2 val2         return 0         public static  lt T1  T2  T3 gt  T3 returnTypeGeneric T1 val1  T2 val2         return null      public static void main String   args        List lt Integer gt  list   new ArrayList lt  gt         list add 10       list add 20       System out println list toString         fill list  100       System out println list toString          List lt String gt  Strlist   new ArrayList lt  gt         Strlist add  Chirag        Strlist add  Nayak        System out println Strlist toString         fill Strlist   GOOD BOY        System out println Strlist toString           System out println multipleTypeArgument  Chirag   100        System out println multipleTypeArgument 100  Nayak                class definition ends  Sample Output    10  20   100  100   Chirag  Nayak   GOOD BOY  GOOD BOY  Chirag 100 100 Nayak

[java] Java generics: multiple generic parameters?

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