Java-ArrayList <>和LinkedList <>-类作为标识符

Question

https://imgur.com/nb6sjuK

I don't get it what the identifier does in this example, I'm new with programming and what I realize is that ArrayList<String> creates the list of Strings and same with LinkedList<String> but what happens when you push a class in the identifier, like - LinkedList<Song> - and Song is a class . Thanks in advance.

public class Main {
    private static ArrayList<Album> albums = new ArrayList<Album>();

    LinkedList<Song> playlist = new LinkedList<Song>();

    public static void main(String[] args) {


    // write your code here

        Album album = new Album("StormBringer" , "Deep Purple");
        album.addSong("Stormbringer" , 4.6);
        album.addSong("Love don't mean a thing" , 4.22);

Answer 1

String is a class like Song is also a class. It means that you can only insert objects of this class into your List . Objects of other classes are not allowed to be added, because the type does not match. Trying to do so will result in a compile error. So your list is type save. This feature is called generics .

Answer 2

but what happens when you push a class in the identifier ?

First of all it's not an identifier it's a Type , and this is how Generics works, you pass a Type between <> , so the compiler knows the expected Type for this Generic class (here the Collection). And Collections List , Set ... are good examples for Generics implementations.

And like Song is a class String is also a predefined class in the java.lang package, so when you pass a class to your collection, the compiler will expect that all the elements in this collection are of this Type .

For example in your code:

LinkedList<Song> playlist = new LinkedList<Song>();

All playlist items are expected to be instances of Song class.