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Diamond " <> " [About List and ArrayList] what is that thing doing? :)

I'm so curious about this sign <> , I think JAVA call this for diamond.

When I code like this in Intellij it warns that I cant use by this code

List <String> hello = new ArrayList <String>();

it should be like this:

List <String> hello = new ArrayList<>();

Is it because of the new update JAVA8? :smile:

Next question:

List <> hello = new ArrayList<Integer>();

it warns me strange, I don't understand please explain me

Third Question:

// Question 3A)
List <Integer,Integer> number = new ArrayList<>();  

//Question 3B)
List <Integer,String> number = new ArrayList <Integer,String>(); 

//Question 3C)
List <> number = new ArrayList <Integer,String>();  

these three tests gave me warnings

please explain me every single questions STEP BY STEP ty :pray:

this post has been updated 3/01 :relaxed: Thank you MOD Seth Kroger :v:

1 Answer

Seth Kroger
Seth Kroger
56,412 Points

Sadly < and > are for HTML tags which makes it problematic to include them in posts. You can use &lt; &gt; to get around that. Or you can use three backticks, which is the key to the left of the 1 on many keyboards, for code and code blocks.


// ...code here...


  // ...code here...

Now on to your questions:

First: The warning is just a warning, not an actual error. Both ways are correct. The "diamond operator" <> was added in Java 7 to simplify things because once you've declared the type it doesn't make a lot of sense to keep repeating it. The new way is preferred but the old way is still valid and there's a lot of code out there written like that.

Second: The diamond operator only works after you've already declared the type once. Using List<> someList; doesn't work because you haven't declared what it should be a list of yet.

Third: Lists are defined as having only one generic type parameter. Passing it two causes an error because it's undefined. So, in Java you can only have a list of one type (and all it's subtypes) of object.

Sir :pray: thanks for the wonderful answer! Now I understand this is about the Generic type? I think I must study about that! The third one : so you mean that I should ONLY type ONE type? Which means <String,String> is not allowed only one right?

But I just wondering in JAVA there is no called Tupels right? I mean what if I want a String and an Integer??

.. I think I have mixed with Haskell language..eww. :angry: