object 轉換期間線程“main”錯誤中的異常 [java.lang.ClassCastException] - 如何解決轉換問題？

Question

我正在編寫一個由多個類組成的程序（每個類有 2 個超類和幾個子類）。 在我的驅動程序文件中，我創建了兩個 arrays：第一個數組包含來自兩個完全獨立的類的對象（與每個類無關，沒有父/子類關系），第二個數組包含來自我的一個超類及其子類的對象。

在編寫和測試從每個 arrays 中查找和打印最便宜和最昂貴的對象的方法時，我遇到了一個與我的轉換相關的錯誤，我必須這樣做才能使該方法起作用。 我在下面包含了我的各種課程的較短版本：

//First superclass
package Plane;
public class Plane {
    
    
     private String brand;
     private double price; 
     private int horsepower;
    
     
    public Plane() {
        brand = "test";
        price = 50000.99; 
        horsepower = 500;
    }
    
    public Plane(String planeBrand, double planePrice, int planePower) {
        this.brand = planeBrand;
        this.price = planePrice;
        this.horsepower = planePower;
    }
    
    public String getBrand() {
        return this.brand;
    }
    public void setBrand(String planeBrand) {
        this.brand = planeBrand;
    }
    
    public double getPrice() {
        return this.price;
    }
    
    public void setPrice(double planePrice) {
        this.price = planePrice;
    }
    
    public int getPower() {
        return this.horsepower;
    }
    
    public void setPower(int planePower) {
        this.horsepower = planePower;
    }
    
    public Airplane(Plane plane) {
        this.brand = plane.getBrand();
        this.price = plane.getPrice();
        this.horsepower = plane.getPower();
    }
    
    public String toString() {
        return "The airplane is manufactured by " + this.brand + " and costs $" + this.price + ". It has " + this.horsepower + " horsepower.";
    }

    public boolean equals(Plane plane) {
        if (!(plane instanceof Plane) || plane == null) {
                return false;
            } else if (this.brand != plane.getBrand() || this.price != plane.getPrice() || this.horsepower != plane.getPower()) {
                    return false;
                } else {
                    return true;
                }
            }
        }

//Second superclass
package Boat;
public class Boat {

    private double weight;
    private double price;
    
    public UAV() {
        weight = 3949.5;
        price = 64000;
    }
    
    public UAV(double boatWeight, double boatPrice) {
        weight = boatWeight;
        price = boatPrice;
    }
    
    public double getWeight() {
        return this.weight;
    }
    
    public void setWeight(double boatWeight) {
        this.weight = boatWeight;
    }
    
    public double getPrice() {
        return this.price;
    }

    public void setPrice(double boatPrice) {
        this.price = boatPrice;
    }
    
    public Boat(Boat boat) {
        this.weight = boat.getWeight();
        this.price = boat.getPrice();
    }
    
    public String toString() {
        return "This boat weighs " + this.getWeight() + "kg and costs $" + this.getPrice() + ".";
    }
    
    public boolean equals(Boat boat) {
        if (!(boat instanceof Boat) || boat == null) {
            return false;
        } else if (this.price != boat.price || this.weight != boat.weight) {
            return false;
        } else {
            return true;
        }
    }
    
}

//Driver file that produces the error
public class Main {

    public static void main(String[] args) {
        
       Object[] mixedObjects = new Object[8];
        
        mixedObjects[0] = new Plane();
        mixedObjects[1] = new Plane();
        mixedObjects[2] = new Helicopter();
        mixedObjects[3] = new Helicopter();
        mixedObjects[4] = new Drone();
        mixedObjects[5] = new Drone();
        mixedObjects[6] = new Boat();
        mixedObjects[7] = new Boat();
        
        
        Object[] planeObjects = new Object[6];
        
        airplaneObjects[0] = new Plane();
        airplaneObjects[1] = new Plane();
        airplaneObjects[2] = new Helicopter();
        airplaneObjects[3] = new Helicopter();
        airplaneObjects[4] = new Drone();
        airplaneObjects[5] = new Drone();
        
        findLeastAndMostExpensiveBoat(mixedObjects);
        findLeastAndMostExpensiveBoat(planeObjects);
        //The top line is line 91 (SEE ERROR MESSAGE)

public static void findLeastAndMostExpensiveBoat(Object[] mixedObjects) {
        if(mixedObjects.length == 1 && mixedObjects[0] instanceof Boat) {
            System.out.println(mixedObjects[0] + " is the most and least expensive.");
        }
        else if(mixedObjects.length == 0) {
            System.out.println("Empty array");
        }
        else if (mixedObjects.length == 1 && !(mixedObjects[0] instanceof Boat)){
            System.out.println("No UAV object detected.");
        }
        else {
            int max = 0;
            int min = 0;
            //Maximum
            for(int i = 0 ; i< mixedObjects.length; i++) {
                if(mixedObjects[i] instanceof Boat) {
                    System.out.println(mixedObjects[i].getClass());
            //The following line is line 157 (SEE ERROR MESSAGE)
                    if(((Boat)mixedObjects[i]).getPrice() >        ((Boat)mixedObjects[max]).getPrice()) {
                        max = i;
                    }
                } else {
                    System.out.println("No Boat object detected.");
                }
            }
            //Mininmum
            for(int i = 0 ; i< mixedObjects.length; i++) {
                if(mixedObjects[i] instanceof Boat) {
                    if(((Boat)mixedObjects[i]).getPrice() < ((Boat)mixedObjects[min]).getPrice()) {
                        min = i;
                    }
                } else {
                    System.out.println("No Boat object detected.");
                }
            }
        } 
    }
} 

findLeastAndMostExpensiveBoat方法基本上檢查任何數組中最昂貴和最便宜的船。 如果沒有船只存在，那么它會打印一條消息說明這一點。 我運行代碼時得到的錯誤代碼是：

Exception in thread "main" java.lang.ClassCastException: class Plane.Plane cannot be cast to class Boat.Boat (Plane.Plane and Boat.Boat are in unnamed module of loader 'app')
    at Main.findLeastAndMostExpensiveBoat(Main.java:157)
    at Main.main(Main.java:91)

根據錯誤消息，第 91 行是： findLeastAndMostExpensiveBoat(planeObjects);

根據錯誤消息，第 157 行是： if(((Boat)mixedObjects[i]).getPrice() > ((Boat)mixedObjects[max]).getPrice())

我的代碼 go 到底哪里錯了？ 我的轉換語法是錯誤的，還是我需要解決更深層次的問題？

Answer 1

您正在通過if(mixedObjects[i] instanceof Boat) Boat object 是否是 Boat 的實例，但您當然不知道mixedObjects[max]是Boat的類型。 最小情況也是如此。 您也可以添加一個額外的條件，例如mixedObjects[max] instanceof Boat 。