Java EE 6 -为什么需要默认构造函数以及如何定义可选参数

我正在努力学习Java，我已经设置好了一切，我正在运行。我为我的第一个模型创建了一个构造函数，并设置了一个接受参数的构造函数。我添加了这个功能:

public Guest(String name) {
    this.name = name;
    this.signingDate = new Date(System.currentTimeMillis());
}

Netbeans抛出一个错误，说我需要一个默认构造函数。当我将其更改为:

时，错误消失了。

public Guest() {
}
public Guest(String name) {
    this.name = name;
    this.signingDate = new Date(System.currentTimeMillis());
}

我无法想象这就是在Java中创建可选参数的方式。这就引出了两个问题:

如何创建可选参数?

实体类必须有一个无参数的公共或受保护构造函数。

另一种需要无参数构造函数的情况是当您有子类时。

例如,


class Car {
 Engine engine;
 Car(Engine e)
   {
     // do something
   }
 }

你有一个子类，


class Benz extends Car
 {
   Benz()
   { 
   }
 }

Now you can't instantiate a Benz object. The reason is, when a new Benz() instantiation is attempeted, the parent constructor is first invoked.

i.e,


class Benz extends Car
 {
   Benz()
   {
     super(); //implicitly added 
   }
 }

By default, it tries to invoke a no-arg constructor of the super-class. So you'd need one, unless you have an explicit super(arg) call like this:


class Benz extends Car
 {
   Benz()
   {
     super(new Engine()); 
   }
 }

Note: A simple Java rule to remember A no-arg constructor is provided by default as long as you don't have another constructor with argument.

i.e

 class Car{}

等于

class Car{ Car() }

但是当你写

 class Car{ Car(Engine e){} }

没有默认的无参数构造函数。您必须显式地编写

 class Car{ Car(){} Car(Engine e)}

if you needed it.

Yes. Java supports method / constructor overloading.

You don't "need" the default constructor. However, NetBeans is being helpful to you, and informing you that you need one because it assumes you are creating a bean - which requires the option to be created without a constructor requiring parameters - because the calling code wouldn't know what to populate into these.

For "optional" parameters, you can define as many constructors as you would need - with each one omitting one or more of the "optional" parameters.

Why do I need a default constructor?

I think you are trying to call new Guest() and have only one constructor in Guest class that is Guest(String arg). And that's why it is showing error that you need to create a constructor with no arguments that is default constructor of a class.

How do I create optional parameters?

If you want to make a constructor with 0 more arguments you need to use ... in argument. Ex:

public class Testing {
    public Testing(String... str) {
    }
    public static void main(String[] args) {
        Testing testing = new Testing();
        Testing testing1 = new Testing("1");
        Testing testing2 = new Testing("2","1");
    }
}

代码

如果您正在使用POJO(普通旧Java对象)，那么只有在通过调用无参数构造函数(如Guest g = new Guest())创建该对象的实例时才需要默认构造函数。
这很明显——你想使用无参数构造函数——你需要有一个。

如果你没有指定任何构造函数，编译器会为你生成一个(默认为无参数)。如果你自己指定了至少一个构造函数，编译器将不会生成任何构造函数，因为它假定你知道自己在做什么。

然而，如果你正在创建一个JPA实体(所以你有@Entity在你的类的顶部)比你需要一个默认构造函数，因为它是JPA需求。
这是因为当您从数据库中获取类的实例时，该类的实例是由JPA提供程序创建的。JPA提供者必须有一种方法来实例化您的实体。如果您只有参数化的构造函数—JPA将如何知道它应该传递哪些参数?

在Java中没有可选的参数，就像在PHP世界中一样，你可以定义myMethod($var1, $var2 = null, $var3 = 2);。在Java世界中，我们使用重载方法和变量。

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