java - 为什么我的数据没有保存在数组中

Question

当我添加动物时，你能告诉我为什么它不起作用吗？我以前工作过，当我将它添加到我在菜单中创建的 ArrayList 时。但我想将 BLL 和 UI 分开。

期待收到你们的来信:)

public class Animal
{

    private String name; //Name of animal
    private String kindOfAnimal; //Kind of animal
    private int currentAge; //Current age of animal
    private int expectedAge; //Expected lifetime

    private List<Animal> animals = new ArrayList<>();

    public Animal(String name, String kindOfAnimal, int currentAge, int expectedAge)
    {
        this.name = name;
        this.kindOfAnimal = kindOfAnimal;
        this.currentAge = currentAge;
        this.expectedAge = expectedAge;
        this.animals = new ArrayList<>();
    }

    public int getExpectedAge()
    {
        return expectedAge;
    }

    public void setExpectedAge(int expectedAge)
    {
        this.expectedAge = expectedAge;
    }

    public String getName()
    {
        return name;
    }

    public void setName(String name)
    {
        this.name = name;
    }

    public String getKindOfAnimal()
    {
        return kindOfAnimal;
    }

    public void setKindOfAnimal(String kindOfAnimal)
    {
        this.kindOfAnimal = kindOfAnimal;
    }

    public int getCurrentAge()
    {
        return currentAge;
    }

    public void setCurrentAge(int currentAge)
    {
        this.currentAge = currentAge;
    }

    public String toString()
    {
        return "Type: " + getKindOfAnimal() + '\n'
                + "Name: " + getName() + '\n'
                + "Age: " + getCurrentAge() + '\n'
                + "Lifetime: " + getExpectedAge();
    }

    public void setAnimalList(Animal animal)
    {
        animals.add(animal);
    }

    public List<Animal> getAnimals()
    {
        return animals;
    }
}

菜单类

   public Menu()
    {
        menu();
    }

    public void menu()
    {
        Scanner sc = new Scanner(System.in);

        System.out.println("Please type in a number and press enter");
        System.out.println("1. Add animal");
        System.out.println("2. Show animals");

        if (sc.hasNextInt())
        {
            int input = sc.nextInt();

            if (input >= 0 && input < 10)
            {
                if (input == 1)
                {
                    addAnimal();
                }
                else if (input == 2)
                {
                    showAnimals();
                }

            }
            else
            {
                System.out.println("Please enter a number between the range");
                menu();
            }
        }
        else
        {
            System.out.println("Wrong input, please input a number!");
            menu();
            System.out.println();
        }

    }

    private void addAnimal()
    {
        System.out.println("ADD ANIMAL");
        System.out.println();

        Scanner sc = new Scanner(System.in);

        try
        {
        System.out.println("Name: ");
        String name = sc.nextLine();

        System.out.println("Kind of animal: ");
        String kindOfAnimal = sc.nextLine();


        System.out.println("Current age: ");
        int currentAge = sc.nextInt();

        System.out.println("Expected age: ");
        int expectedAge = sc.nextInt();

        Animal animal = new Animal(name, kindOfAnimal, currentAge, expectedAge);

//        animals.add(animal.toString());

        animal.setAnimalList(animal);
        }

        catch (Exception e)
        {
            System.out.println("Please enter the correct information");
        }
        System.out.println();
        menu();
    }

    public void showAnimals()

    {

            Animal animal = new Animal(null, null, 1, 1);

        if (animal.getAnimals().isEmpty())
        {
            System.out.println("There is no animals yet!");
            menu();
        }
        else
        {
        for (int i = 0; i < animal.getAnimals().size(); i++)
        {
            System.out.println(animal.getAnimals());

        }
        menu();
        }
    }

Answer 1

您的问题出在 showAnimals() 内的这一行:

Animal animal = new Animal(null, null, 1, 1);

问题是，每次您想要显示动物列表时，您都会使用该行代码创建一个新列表。所以，它总是空的。

尝试将动物列表从 Animal 类移至 Menu 类。这样，您将只有一份列表副本，并且可以在其中添加和显示动物。

public class Menu {

private List<Animal> animals = new ArrayList<>();

  public Menu()
  {
    menu();
  }


etc...


  private void addAnimal()
  {
    System.out.println("ADD ANIMAL");
    System.out.println();

    Scanner sc = new Scanner(System.in);

    try
    {
      System.out.println("Name: ");
      String name = sc.nextLine();

      System.out.println("Kind of animal: ");
      String kindOfAnimal = sc.nextLine();


      System.out.println("Current age: ");
      int currentAge = sc.nextInt();

      System.out.println("Expected age: ");
      int expectedAge = sc.nextInt();

      Animal animal = new Animal(name, kindOfAnimal, currentAge, expectedAge);

      animals.add(animal);
    }

    catch (Exception e)
    {
      System.out.println("Please enter the correct information");
    }

    System.out.println();
    menu();
}

  public void showAnimals()
  {
    if (animals.isEmpty())
    {
        System.out.println("There is no animals yet!");
        menu();
    }
    else
    {
      for (int i = 0; i < animals.size(); i++)
      {
        System.out.println(animals.get(i));
      }
      menu();
    }
}