How to have Java method return generic list of any type

Question

I would like to write a method that would return a java util List of any type without the need to typecast anything  List lt User gt  users   magicalListGetter User class    List lt Vehicle gt  vehicles   magicalListGetter Vehicle class    List lt String gt  strings   magicalListGetter String class    What would the method signature look like  Something like this  perhaps     public List lt  lt   gt    gt  magicalListGetter Class lt   gt  clazz        List lt   gt  list   doMagicalVooDooHere         return list

User · Answer

I m pretty sure you can completely delete the    lt stuff gt      which will generate a warning and you can use an    suppress warnings  If you really want it to be generic  but to use any of its elements you will have to do type casting  For instance  I made a simple bubble sort function and it uses a generic type when sorting the list  which is actually an array of Comparable in this case  If you wish to use an item  do something like  System out println  Double arrayOfDoubles 0     Double arrayOfDoubles 1    because I stuffed Double s  into Comparable s  which is polymorphism since all Double s  inherit from Comparable to allow easy sorting through Collections sort              INDENT TO DISPLAY CODE ON STACK-OVERFLOW  SuppressWarnings  unchecked   public static void simpleBubbleSort ascending  SuppressWarnings  rawtypes   Comparable   arrayOfDoubles      VARS       looping     int end             arrayOfDoubles length - 1   the last index in our loops     int iterationsMax   arrayOfDoubles length - 1         swapping      SuppressWarnings  rawtypes       Comparable tempSwap   0 0   a temporary double used in the swap process     int elementP1   1   element   1    an index for comparing and swapping     CODE       do up to  iterationsMax  many iterations     for  int iteration   0  iteration  lt  iterationsMax  iteration                    go through each element and compare it to the next element         for  int element   0  element  lt  end  element                          elementP1   element   1                 if the elements need to be swapped  swap them             if  arrayOfDoubles element  compareTo arrayOfDoubles elementP1    1                                  swap                 tempSwap   arrayOfDoubles element                   arrayOfDoubles element    arrayOfDoubles elementP1                   arrayOfDoubles elementP1    tempSwap                                   END public static void simpleBubbleSort ascending double   arrayOfDoubles

User · Answer

Something like this  publi   lt T gt  List lt T gt  magicalListGetter Class lt T gt  clazz        List list   doMagicalVooDooHere        return list

User · Answer

Another option is doing the following   public class UserList extends List lt User gt       public  lt T gt  T magicalListGetter Class lt T gt  clazz        List lt   gt  list   doMagicalVooDooHere        return  T list     List lt User gt  users   magicalListGetter UserList class

User · Answer

You can use the old way   public List magicalListGetter         List list   doMagicalVooDooHere         return list      or you can use Object and the parent class of everything   public List lt Object gt  magicalListGetter         List lt Object gt  list   doMagicalVooDooHere         return list      Note Perhaps there is a better parent class for all the objects you will put in the list  For example  Number would allow you to put Double and Integer in there

User · Answer

You can simply cast to List and then check if every element can be casted to T   public  lt T gt  List lt T gt  asList final Class lt T gt  clazz        List lt T gt  values    List lt T gt   this value      values forEach clazz  cast       return values

User · Answer

private Object actuallyT   public  lt T gt  List lt T gt  magicalListGetter Class lt T gt  klazz        List lt T gt  list   new ArrayList lt  gt         list add klazz cast actuallyT        try           list add klazz getConstructor   newInstance        If default constructor               return list      One can give a generic type parameter to a method too  You have correctly deduced that one needs the correct class instance  to create things  klazz getConstructor   newInstance

User · Answer

No need to even pass the class   public  lt T gt  List lt T gt  magicalListGetter         return new ArrayList lt T gt

User · Answer

Let us have List lt Object gt  objectList which we want to cast to List lt T gt    public  lt T gt  List lt T gt  list Class lt T gt  c  List lt Object gt  objectList               List lt T gt  list   new ArrayList lt  gt                for  Object o   objectList           T t   c cast o           list add t             return list

[java] How to have Java method return generic list of any type?

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