我以为有人会提到这一点(第二点)，但是…私有构造函数有三种用法:

to prevent instantiation outside of the object, in the following cases: singleton factory method static-methods-only (utility) class constants-only class . to prevent sublcassing (extending). If you make only a private constructor, no class can extend your class, because it can't call the super() constructor. This is some kind of a synonym for final overloaded constructors - as a result of overloading methods and constructors, some may be private and some public. Especially in case when there is a non-public class that you use in your constructors, you may create a public constructor that creates an instance of that class and then passes it to a private constructor.

Yes.

私有构造函数用于防止实例初始化，例如您在java中使用的Math final类。单例也使用私有构造函数

私有构造函数背后的基本思想是限制JVM从外部实例化类，但是如果一个类有参数构造函数，那么它就会推断该类是有意实例化的。

是的，类可以有一个私有构造函数。需要禁止从其他类访问构造函数，并在定义的类中保持它的可访问性。

为什么要只在内部创建类的对象?出于任何原因都可以这样做，但一个可能的原因是您想实现单例对象。单例是一种设计模式，它只允许创建类的一个实例，这可以通过使用私有构造函数来实现。

受到Robert C. Martin的“Clean Code”的启发，我整理了一个例子:

/**
When overloading constructors, it is best practise to only allow the use of
different constructors than the standart one by explicitly enforcing the 
useage of a static function to highlight the use of the overloaded constructor

in example:
Animal a = Animal.CreateInsectOrArachnia(2, "black", 8); //hatch a black widow
*/


class Animal
{
   private int size;
   private String color;
   private int legs;

   public Animal(int size, String color)
   {
       this.size = size;
       this.color = color;
       this.legs = 4;
   }

   //will prevent the instanciation of Animal with this constructor
   private Animal(int size, String color, int legs)
   {
       this.size = size;
       this.color = color;
       this.legs = legs;
   }


   public static Animal CreateInsectOrArachnia(int size, String color, int legs)
   {
    return new Animal (size, color, legs);
   }

}

Martins明确指出，应该阻止用户访问“标准构造函数”以外的构造函数，并应该强制使用静态初始化函数，以强调“你所做的可能没有错，但它与该类的预期使用不同”。

[他没有使用这个确切的措辞，我试图把它挤进这个定义-对不起，罗伯特:^)]

作为旁注，完全可以将类中唯一的构造函数(即标准构造函数)完全声明为private，并让静态函数返回类实例-请参阅单例模式。但是，非常不鼓励实现单例模式，除非在一些通信只在一个方向上流动的用例中，例如在编写记录器类时

我以为有人会提到这一点(第二点)，但是…私有构造函数有三种用法:

to prevent instantiation outside of the object, in the following cases: singleton factory method static-methods-only (utility) class constants-only class . to prevent sublcassing (extending). If you make only a private constructor, no class can extend your class, because it can't call the super() constructor. This is some kind of a synonym for final overloaded constructors - as a result of overloading methods and constructors, some may be private and some public. Especially in case when there is a non-public class that you use in your constructors, you may create a public constructor that creates an instance of that class and then passes it to a private constructor.