can someone tell me what is...?

Its called generics, though you don't need to know that to solve this task. They will touch upon it in later lessons. I will give a quick explanation though: In the class X an Y are just variables that can be any class (hence generic). You don't have to use X and Y in your code, you could create a generic class using any name for the generics, only governed by Java's naming rules. This code be exactly identical to the code in the task:

public static class Pair<Raalph, Charles> {
   public Raalph x;
   public Charles y;

   public Pair(Raalph x, Charles y) {
      this.x = x;
      this.y = y;
   }
}

The generics are declared in angle brackets then referenced in code When creating an object you declare what class that you want those generics to be and the X and Y for that object will be replaced with those classes. You have used them already: the List, Map, Set classes are all generic based: new ArrayList<Integer>() - this will create a List for holding Integers new HashSet<String>() - this will create a Set for holding Strings new Pair<Integer, Integer>(0, 0) - This will create a pair for Integer Integer, with them being set to 0 and 0. If I were to create a new Pair object using String Integer instead: new Pair<String, Integer>("0", 0) the class code for that particular object would be compiled to:

public static class Pair<String, Integer> {
   public String x;
   public Integer y;

   public Pair(String x, Integer y) {
      this.x = x;
      this.y = y;
   }
}

Where you can see that the X has been replaced in ALL cases by the class String, and the Y has been replaced in all cases with Integer. This is how you use generics.