为什么我们需要一个空的构造函数来传递/保存来自 Firebase 的数据? [英] Why we need an empty Constructor to passing/save a Data From Firebase?

问题描述

也许这个问题有点简单，但我不明白为什么我们需要一个空的构造函数来从 Firebase 传递数据.下面是代码示例:

maybe this question little bit easy, but I don't get any idea why we need an empty constructor to passing a data from Firebase. Here is an example for the code:

public class Hero{
String Name, Weapon, Description, Price, Discount, Id;

public Hero() {
}

public Hero(String name, String weapon, String description, String price, String discount, String id) {
    Name = name;
    Weapon= weapon;
    Description = description;
    Price= price;
    Discount = discount;
    Id= id;
}}

然后我们需要为每个菜单设置一个 getter 和 setter..

then we need a getter and setter for each Menu..

但令人头晕的是，为什么我们需要一个空的构造函数?真的有必要吗?

But whats make dizzy is, Why we need an empty Constructor? Is that really necessary?

我们可以只创建一个类而不输入空的构造函数吗?这会是同样的结果吗?

Can we just make a class without typing an empty constructor? is this will be the same result?

推荐答案

类的字段将使用反射填充.但是你不能在没有构造函数的情况下创建一个默认"对象(意思是:没有预填充的字段).Firebase 无法自行确定构造函数的作用，因此您需要一个空构造函数:允许 Firebase 创建对象的新实例，然后使用反射继续填充.

The fields of the class will be filled in using reflection. But you cannot create a "default" object (meaning: with no fields pre-filled) without a constructor. Firebase cannot figure out on its own what your constructor does, so that's why you need an empty constructor: to allow Firebase to create a new instance of the object, which it then proceeds to fill in using reflection.

这不是 Firebase 特有的:您会在框架或库为您填充对象的任何地方找到空构造函数，例如 JPA/Hibernate.

This is not specific to Firebase: you'll find the empty constructor everywhere where a framework or library fills in an object for you, such as JPA/Hibernate.

为了完整起见，正如@Lutzi 提到的，一旦您定义了自己的构造函数，Java 定义的默认空构造函数就不再可供您使用，这就是您需要明确定义它的原因.

for completeness' sake, as @Lutzi mentioned, once you define your own constructor, the default empty constructor that Java defines is no longer available to you, which is why you need to define it explicitly.

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