What does the entry mean: Collection collection = new ArrayList();?

Question

Java

Collection collection = new ArrayList(); //?WTF?

ArrayList x = new ArrayList<>(); // this entry is crisp and clear,

it's a mess in my head because of the 1st line, I don't understand what it means. Google says this is a type of etiquette rule. this answer is not clear to me. I would like a more precise answer or a link to an exact explanation.

Answer 1

Dive into OOP, get acquainted with the principles of SOLID, everything will become clear.