我的 isEmpty 方法不起作用，我不确定为什么

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 方法可以为我做到这一点，但我的代码不会在这里编译。它一直说找不到方法。 我没有在我的代码中定义该方法，但我认为我不必这样做，因为它是一个java定义的方法，我已经导入了java.util.ArrayList。 如果不再使用此方法，是否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();
}
}

这是我在召唤它的猫

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：这是我的目录类的构造函数

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();
}
}

类的用法是这样的

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，则列表为空。