我的 isEmpty 方法不起作用，我不知道为什么

Question

I am just trying to check whether an ArrayList is empty or not so I looked into java libraries and found that there is an isEmpty method in java that does this for me but my code won't compile here. It keeps saying it can't find the method. I haven't defined the method in my code but I didn't think I had to since it is a java defined method and I already imported java.util.ArrayList. If this method isn't used anymore is there any other way I can check if an ArrayList is empty?

else if((userChoice.equals("S")) || (userChoice.equals("s"))){
    boolean empty = this.cat.isEmpty();
    if (empty == true){
        System.out.println("there is nothing i this catalog");
    }
    else{
        this.cat.printList();
    }
}

Here is the cat that im calling it on

this.name = name;
this.cat = new Catalog();
//CREATE ARRAYLIST HERE
cat.addItem(new Music(1111,"White and Nerdy",2.50,"\"Weird Al\" Yankovic"));
cat.addItem(new Music(2222,"Amish Paradise",2.22,"\"Weird Al\" Yankovic"));
cat.addItem(new Music(3333,"The Saga Begins",2.00,"\"Weird Al\" Yankovic"));
cat.addItem(new Movie(4444,"UHF",9.99,"\"Weird Al\" Yankovic"));
cat.addItem(new Movie(5555,"The Dark Crystal",8.99,"\"Jim Henson"));
cat.addItem(new Movie(6666,"Die Hard",13.99,"Bruce Willis"));
cat.addItem(new Book(7777,"When I Grow Up",7.98,"\"Weird Al\" Yankovic"));
cat.addItem(new Book(8888,"The Chronicles of Pern: First Fall",5.99,"\"Anne McCaffrey"));
cat.addItem(new Book(9999,"Get gud you scrub",2.50,"Steve \"Troll\" Rathier"));

EDIT: Here is the constructor for my catalog class

public Catalog()
    {
        //makes empty arraylist
        // initialise instance variables
        items = new ArrayList<>(MAX);
        size = 0;

    }

Answer 1

There is an easy way for this:

public class MyClass {
    List<Object> myObjects;
    Object[] myArrayObjects;

    void myMethod() {
        if (myArrayObjects == null || myArrayObjects.length == 0) {
            //Array is empty
        } else {
            //Array has some data
        }
        if (myObjects == null || myObjects.size() == 0) {
            //List is empty
        } else {
            //List has some data
        }
    }
}

another solution which is better I think, is something like this

    public class Catalog {
        List<Object> items;

        public Catalog() {
            items = new ArrayList<>();
        }

        public void addItem(Object object) {
            items.add(object);
        }

        public boolean isEmpty() {
            return items.isEmpty();
        }

        public int size() {
            return items.size();
        }
    }

and usage of the class is like this

public class MyClass {

    Catalog catalog;

    public MyClass() {
        this.catalog = new Catalog();
    }

    void printCatalog() {
        if (catalog.isEmpty()) {
            //catalog is empty
        } else {
            //catalog has some data you can print them
        }
    }

    void addItems(Object object) {
        catalog.addItem(object);
    }
}

Answer 2

java.util.ArrayList also has size() method. You can use that to check for the list size, if it's 0, your list is empty.