如何最好地解释和使用 Java 中的空构造函数? [英] How to best explain and use empty Constructors in Java?

问题描述

我一直在自学Java.我了解定义类的范围，但仍然没有得到空构造函数用法的概念.
通常我们应该为构造函数传递参数来构建实例对象.但是，我经常看到构造函数的参数为​​空.例如:

I have been self-learning Java. I understand the scope of defining a class, but still didn't get the concept of an empty constructor usage.
Usually we should pass parameters for constructor to build instance object. But, I often see empty parameter for constructor. For example:

 class Person {
   String name;
       int age; 

 public Person();

 public Person(String name, int age){
     this.name = name;
     this.age = age;
     } 
 }

我研究并阅读了一个使用fish"类来解释的例子.所以，这就是我目前所理解的:在定义一个类时，我们首先为一个对象定义属性，然后我们创建一个构造函数，该构造函数将使用方法构建该对象.空构造函数构建 GENERIC 对象，带参数的构造函数构建具有更具体信息的对象.比如说上面的例子，如果我使用空构造函数创建一个实例对象:

I researched and read an example that using a class "fish" to explain. So, this is what I understood so far: when defining a class, we first define properties for an object, then we create a constructor that will build the object with methods. Empty constructor build GENERIC object, and constructor with parameters build objects with more specific information. Let’s say the example above, if I create an instance object using the empty constructor:

Person p1 = new Person();  

-- 它仍然会创建一个对象，但其中没有任何属性?那么，空构造函数到底是用来做什么的呢?我在很多示例代码中看到了它.它非常有用/常见吗?

-- it will still create an object but without any properties in it? So, what exactly the empty constructor is used for? I saw it in a lot of example codes. Is it very useful/common?

感谢您的关注和回答！

推荐答案

如果你写了一个带参数的构造函数，那么你需要声明默认的(如果你想使用它)

If you write a constructor with parameters, then you need to declare the default one (if you want to use it)

class Person {
    String name;
    int age; 

    public Person(String name, int age){
        this.name = name;
        this.age = age;
    } 
 } 

您现在不能这样做:

Person p = new Person();

为了使用默认"构造函数(不带参数)，您需要声明它:

In order to use the "default" constructor (with no parameters) you will need to declare it:

class Person {
    String name;
    int age; 

    public Person(){
        name = "Man With No Name";  //sometimes you will want to set the variables 
        age = 21;     //to some default values
    }

    public Person(String name, int age){
        this.name = name;
        this.age = age;
    } 
 } 

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