我的 isEmpty 方法不起作用，我不知道为什么

Question

我只是想检查 ArrayList 是否为空，所以我查看了 java 库，发现 java 中有一个 isEmpty 方法，但我的代码不会在这里编译。 一直说找不到方法。 我没有在我的代码中定义该方法，但我认为我不必这样做，因为它是 java 定义的方法，并且我已经导入了 java.util.ArrayList。 如果不再使用此方法，是否还有其他方法可以检查 ArrayList 是否为空？

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();
    }
}

这是我打电话给它的猫

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"));

编辑：这是我的目录 class 的构造函数

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

    }

Answer 1

有一个简单的方法：

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
        }
    }
}

我认为另一个更好的解决方案是这样的

    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();
        }
    }

class 的用法是这样的

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 也有size()方法。 您可以使用它来检查列表大小，如果为 0，则您的列表为空。