java - Java 如何处理多个构造函数？

Question

这个问题已经有答案了:

Method overload resolution in java (7 个回答)

已关闭 7 年前。

如果一个类包含两个接受不同类型参数的构造函数，如下所示:

public class Planet {
   public double xxPos; //its current x position
   public double yyPos; //its current y position
   public double xxVel; //its current veolicity in the x direction
   public double yyVel; //its current veolicity in the y direction
   public double mass; //its mass
   public String imgFileName; //The name of an image in the images directory that depicts the planet

   // constructor is like __init__ from python, this sets up the object when called like: Planet(arguments)
   public Planet(double xP, double yP, double xV, double yV, double m, String img) {
      xxPos = xP;
      yyPos = yP;
      xxVel = xV;
      yyVel = yV;
      mass = m;
      imgFileName = img;

   }

   // second constructor
   // how come testplanetconstructor knows to use this second one?
   // does it know based on the argument type its being passed?
   public Planet(Planet p) {
      xxPos = p.xxPos;
      yyPos = p.yyPos;
      xxVel = p.xxVel;
      yyVel = p.yyVel;
      mass = p.mass;
      imgFileName = p.imgFileName;
   }
}

我的主要问题是:1) 另一个具有调用该类的 main 的类如何确定使用哪个构造函数？

如果是这种情况，如果您有两个构造函数:2)相同类型和数量的参数？3)相同类型但不同数量的参数？

我意识到后续问题是你可能永远不应该做的事情(又名困惑的事情)。我只是好奇。

Answer 1

1) How does another class with a main that calls this class determine which constructor to use?

编译器通过检查唯一方法签名来遵循与静态绑定(bind)重载方法相同的过程。了解方法签名see this

public static void main(String args[]) {
    double d1 = 0;
    double d2 = 0;
    double d3 = 0;
    double d4 = 0;
    double d5 = 0;
    String img = "";
    Planet p = new Planet(d1, d2, d3, d4, d5, img);// constructor with valid argument set
}

2) the same type and number of arguments?

实际上不可能在单个类中编写具有相同签名的两个方法/构造函数。例如以下代码永远不会编译

Planet(int i) {// compilation error
    return 0;
}
Planet(int j) {// compilation error
    return 0;
}

3) the same type but different number of arguments?

这是可能的，就像使用不同签名创建/调用方法一样。例如

Planet p1 = new Planet(d1, d2, d3, d4, d5, img);
Planet p2 = new Planet(p1);