Java中的[a bug]按引用传递

Question

这里是主要的java类，非常简单，只需询问用户房间的宽度和长度以及每平方米的价格。然后它应该显示房间的面积和总成本

package exercise;

import javax.swing.JOptionPane;

public class carpetshopping {

public static void main(String[] args) {
    // TODO Auto-generated method stub

    String input;
    double width;
    double length;
    double price;

    input = JOptionPane.showInputDialog("Please enter the width of the room");
    width = Double.parseDouble(input);

    input = JOptionPane.showInputDialog("Please enter the length of the room");
    length = Double.parseDouble(input);

    input = JOptionPane.showInputDialog("What about the price per unit area?");
    price = Double.parseDouble(input);

    RoomDimension dim = new RoomDimension(length, width);

    System.out.println(dim);

    RoomCarpet car = new RoomCarpet(dim, price);

    System.out.println(car);
}

}

这是房间的尺寸。在java中，它有两个字段：长度和宽度（都是双字段），这两个字段将获得房间的尺寸并计算房间面积

package exercise;

public class RoomDimension {

public double length;
public double width;

public RoomDimension(double len, double w) {
    // TODO Auto-generated constructor stub
    length = len;
    width = w;
}

public RoomDimension(RoomDimension size) {
    // TODO Auto-generated constructor stub
    length = size.length;
    width = size.width;

}

public double getArea() {

    return length * width;
}

public String toString() {

    return "The area of this room is " + this.getArea();
}

}

这是房间的地毯。java中，它有两个字段，一个是价格，另一个是RoomDimension中的对象。java，它将计算房间的总成本

package exercise;

public class RoomCarpet {

public RoomDimension room;
public double carpetCost;

public RoomCarpet(RoomDimension room1, double carpetCost) {
    // TODO Auto-generated constructor stub
    room = new RoomDimension(room1);
    carpetCost = carpetCost;
}

public double getTotalCost() {

    return room.getArea() * carpetCost;
}

public String toString() {

    return "The total cost is " + this.getTotalCost();
}

}

我的问题是：无论用户输入什么价格，总成本总是0.0 有人帮我吗？Java的新宝贝，万分感谢

Answer 1

问题出在RoomCarpet类中。在构造器中，使用相同的变量carpetCost = carpetCost会混淆JVM，这会导致实例变量隐藏。编译器会感到困惑，无法为成本变量指定正确的值

你的改良地毯课：

class RoomCarpet {

public RoomDimension room;
public double carpetCost;

public RoomCarpet(RoomDimension room1, double carpetCost) {
    room = new RoomDimension(room1);
    this.carpetCost = carpetCost;
}

public double getTotalCost() {
    return room.getArea() * carpetCost;
}

public String toString() {
    return "The total cost is " + this.getTotalCost();
}
}

It is illegal in Java to declare two local variables with the same name inside the same or enclosing scopes. Interestingly, you can have local variables, including formal parameters to methods, which overlap with the names of the class’ instance variables. However, when a local variable has the same name as an instance variable, the local variable hides the instance variable.

Answer 2

你的问题在你的RoomHeart类中，在要指定的构造函数中
你在写

public RoomCarpet(RoomDimension room1, double carpetCost) {
    // TODO Auto-generated constructor stub
    room = new RoomDimension(room1);
    carpetCost = carpetCost;
}

这里的问题是，编译器两次都使用参数变量来表示“地毯成本”，所以基本上是将变量分配给它自己，甚至不触及要更改的实例变量
要解决此问题，请使用“this”引用实例变量：

public RoomCarpet(RoomDimension room1, double carpetCost) {
    // TODO Auto-generated constructor stub
    room = new RoomDimension(room1);
    this.carpetCost = carpetCost;
}

为了避免将来出现这种情况，只需习惯通过“this”引用实例变量即可因为编译器知道你说的是哪个变量

顺便说一下。。。如果您还没有使用eclipse（我认为这是因为您的问题），我强烈建议您使用它。我花了大约一秒钟的时间才在代码中找到问题，因为eclipse已经为我标记了有问题的部分，并告诉我它可能有问题；）

你好，瑞文