I found a function that basically looks like this:
<R> MyClass<R> functionName(){}
I have no Idea what the <R> right in front means and I didn't really find anything by googling it. I know what generics are, I just dont know what they do in that specific spot.
It will let you write methods that can act as general purpose helper methods for any type.
Here is an example of reversing the contents of a List implementation and return it in an ArrayList.
List<String> liststr = new ArrayList<>(List.of("A","B","C"));
List<String> reversed = reverse(liststr);
System.out.println(reversed);
List<Integer> listint = new ArrayList<>(List.of(1,2,3,4));
List<Integer> reverseint = reverse(listint);
System.out.println(reverseint);
And here is the method.
public static <T> List<T> reverse(ArrayList<T> list) {
List<T> rev = new ArrayList<>();
for (int i = list.size()-1; i >= 0; i--) {
rev.add(list.get(i));
}
return rev;
}
This could have been done a different way but it would require casting of the return value and might also generate a runtime cast exception.
As in all generic methods and classes, type conflicts will be resolved at compile time and not at runtime.
The first is a kind of declaring that functionName will use generics, generally we use R to represent "return", which means this function will return an instance of MyClass which manipulates R objects.
For better understanding, this is a way to call functionName if it was static:
public class AClass{
class MyClass<TYPE>{}
static <R> MyClass<R> functionName(){return null;}
public static void main(String[] args) {
AClass.<String>functionName();
}
}
This means that the R is a variable, which is assigned when the method is executed and can be assigned any class. This might help: What are Generics in Java?
