CODEWITHSUNDEEP
javagenericsarraylistreflection
DarioBB
DarioBB

Reputation: 663

How to make this method generic for all passed Class types?

I would like to make this method generic for all class types: 

public static void printListSample(List<Class<?>> dateExtend) {
    System.out.println("dateExtend size: "+dateExtend.size());
    if(dateExtend.size() > 0){
        for(Class<?> mp : dateExtend){
            System.out.print("\n print entry - ");
            if(mp.getDate() != null){
                String dateDb = new SimpleDateFormat("dd.MM.yyyy.").format(mp.getDate());
                System.out.print("value: "+dateDb);
            }
        }
    }
    System.out.println();
}

Notice Class< ? > - now like this it doesen't work; but I think I am on the right track.

I would like to dinamically cast Class passed in the parameter and then dinamically cast it for for-loop (so I don't have to do if (instance of SomeClass.. )

How to do this? I would need example code, really don't know how to write this.

Edit: getDate is method from my custom object I passed as param. So Java would need to figure out dynamically what class type was object that was passed.

I would like to write generic method for printing List (or ArrayList) of any passed type: 

<MyClass> or <String> or <SomeOtherClass>,

so I wouldn't have to worry about types and casting would be done automatically.

Edit (2): I would like to determine class type INSIDE method (I presume I can grab it from:

<T> or <?>

with class info I am sending) ?

Upvotes: 0

Views: 88

Answers (3)

Rick
Rick

Reputation: 39

Have you tried using

   public static <T> void printListSample(List<T> dateExtend){
       ...
       if...
          for(T mp : dateExtend){
              ...
          }
   }

you will likely have the issue that the getDate() function is undefined for type T. There are a couple solutions to this. 1) you could cast mp (not usually considered a good meathod) 2) you could extend T in your input to only classes with a getDate function 3) you could use a try catch 

   //option 2
   public static <T extends MyClass> void printListSample(List<T> dateExtend){

   }

"myClass" will need to be a parent class so that no matter what type T is in your list, it has inhered the function getDate();

Upvotes: 0

Andy Turner
Andy Turner

Reputation: 140494

No Class instance has a getDate() method on it.

I suspect you need something like List<? extends SomeClassWithAGetDateMethod> as the parameter:

public static void printListSample(List<? extends SomeClassWithAGetDateMethod> dateExtend) {
    System.out.println("dateExtend size: "+dateExtend.size());
    if(dateExtend.size() > 0){
        for(SomeClassWithAGetDateMethod mp : dateExtend){
            if (mp.getDate() != null) {
                ...
            }
        }
    }
}

Incidentally, you don't need to check if a List is empty before iterating it: just iterate it, and if it has no elements, nothing in the loop is run.

if (list.size() > 0) { for (ElementType e : list) { ... } }

is equivalent to:

for (ElementType e : list) { ... }

(and also, !list.isEmpty() can be more efficient (and is no less efficient) than checking if the size is greater than zero).

Upvotes: 4

Guillaume Barr&#233;
Guillaume Barr&#233;

Reputation: 4218

Because you are trying to invoke mp.getDate() I imagine that all the classes you want to accept in your list have this method;

If it's the case you can have a look at Generic Methods and Bounded Type Parameters

In order to have something like this :

public interface WithDate {
    public Date getDate();
}

public static void printListSample(List<? extends WithDate> dateExtend) {
    System.out.println("dateExtend size: "+dateExtend.size());
    if(dateExtend.size() > 0){
        for(WithDate mp : dateExtend){
            System.out.print("\n print entry - ");
            if(mp.getDate() != null){
                String dateDb = new SimpleDateFormat("dd.MM.yyyy.").format(mp.getDate());
                System.out.print("value: "+dateDb);
            }
        }
    }
    System.out.println();
}

Upvotes: 1

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