Can someone please explain why I need 2 < T > before and after ArrayList?

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So in line 16 I have the method declaration for newArrayList. For it to be accepted, it needs < T > before and after ArrayList. Please let me know if I have understood this correctly. The second < T > is the return type of the ArrayList. So this method will return an ArrayList that contains objects of type T. However, I don't know the purpose of the first < T > from the method declaration. What does that do?

Implement helper methods in the Solution class that create an appropriate collection and add the passed objects to it.
The newArrayList and newHashSet methods have the T type parameter.
The newHashMap method has K (key) and V (value) type parameters. The newHashMap method must accept lists that contain descendants of K and V.
The returned collections must be of the same type as the objects passed to the method.

Hint: In the newHashMap method, you need to check to make sure that the lists of keys and values are the same size. If they aren't, then throw an IllegalArgumentException.

Requirements:

The newArrayList method must have a T type parameter.
The newArrayList method must return an ArrayList that contains the objects passed to the method.
The newHashSet method must have a T type parameter.
The newHashSet method must return a HashSet that contains the objects passed to the method.
The newHashMap method must have K and V type parameters.
The newHashMap method must return a HashMap that contains the keys and values passed to the method.
The newHashMap method must throw an IllegalArgumentException if the lists of keys and values aren't the same size.

package com.codegym.task.task35.task3509; import java.util.*; /* Collections & Generics */ public class Solution { public static void main(String[] args) { } public static <T> ArrayList <T> newArrayList(T... elements) { //write your code here ArrayList<T> arrayList = new ArrayList<>(); for (Object obj : elements){ arrayList.add((T) obj); } return arrayList; } public static HashSet newHashSet(Object... elements) { //write your code here return null; } public static HashMap newHashMap(List keys, List values) { //write your code here return null; } }

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Thomas

Level 41 , Bayreuth, Germany

13 July 2021, 12:05

solution

it's better to write it that way

<T> ArrayList<T>

The first <T> indicates that the method uses generics, or better, to declare the method using generics (just like type declarations). The second one... here you're right with your assumption (parameterized retVal) btw. you can avoid casting if you use the generic type parameter in the for each loop

for (T obj : elements)
    arrayList.add(obj);

Guadalupe Gagnon

Level 37 , Tampa, United States

13 July 2021, 13:24

useful

Just a little additional info, not related to the help post: The rules for generic names you decide to use in Java are actually the same as the rules that govern variable names. Best practice is to use a single capital letter, typically T (type). K (key). V (value), E (elements) and a few others; however, if you care to be a little more stylish, there isn't anything stopping you from writing code like this:

public static <Guadalupe> ArrayList <Guadalupe>  newArrayList(Guadalupe... elements) {
   //write your code here
   ArrayList<Guadalupe> arrayList = new ArrayList<>();
   for (Object obj : elements){
      arrayList.add((Guadalupe) obj);
   }
   return arrayList;
}

Andrei

Level 41

14 July 2021, 09:35

Is that an optional < T > ? Or is it mandatory ?

Andrei

Level 41

14 July 2021, 09:36

That's great to know, thanks Guadalupe!

Thomas

Level 41 , Bayreuth, Germany

14 July 2021, 15:55

it's a declaration... so it's mandatory

Thomas

Level 41 , Bayreuth, Germany

25 July 2021, 11:54

just to extend my last answer with some conditions... - if the class has a type parameter and you use the same type in your methods, then you do not need to parameterize the method (then you do not need the <T> type declaration before the ret val declaration in the method definition). - if the types differ (like class uses T as type parameter and you need to use another one, eg. U for your method then you of course need to declare it [either for the class or just the method]) - if you use static methods, then these do not have access to the type parameter of the class and you need to declare the type for that methods. Here you have both cases, no type parameter for the class and static methods so you have to declare the generic type parameter for that method.

matemate123

Level 50 , Kraków, Poland

11 October 2023, 11:02

Thanks for additional info Thomas and Guadalupe! It's nice syntactic sugar we can use here:

public static <T> ArrayList<T> newArrayList(T... elements) {
       return new ArrayList<>(Arrays.asList(elements));
   }

PS: I must think about my helper generic class <Guadalupe> which help me solve a few programming problems :D