程序不打印索引 0 處的元素

Question

我正在處理案例 3，我需要打印有關飛機 class 和 BatMobile class 的 3 個實例的一些詳細信息。 該循環僅打印出索引 1 和 2 處的元素以及 BatMobile class 的其他 3 個實例。 為什么會發生這種情況，如何打印兩個類的所有 3 個實例？

import java.util.Scanner;

public class vehicleInstancesAndMenu {
static int count;
Vehicle[] vehicles = new Vehicle[15];
public vehicleInstancesAndMenu() {
    menu();
}//constructor

public void vehicles_Instances() {
    
    vehicles[0] = new Airplane("Airbus A220-100", "Delta Airlines", "2019", 2, 120, 300, 221, 250);
    vehicles[1] = new Airplane("Airbus A319-100", "Delta Airlines", "2018", 2, 200, 450, 319, 300);
    vehicles[2] = new Airplane("Airbus A320-200", "Delta Airlines", "2018", 2, 220, 500, 320, 320);
    
    vehicles[3] = new Automobile("Corolla LE", "Toyota", "2020", 300, 6, 150);
    vehicles[4] = new Automobile("Civic SE", "Honda", "2018", 310, 6, 160);
    vehicles[5] = new Automobile("Sentra", "Nissan", "2019", 360, 6, 200);
    
    vehicles[6] = new Ship("APM - Maersk", "Maersk", "1904", 400000, "Carnival",2695, 35);
    vehicles[7] = new Ship("MSC", "Mediterranean Shipping Company", "1970", 30000 ,"Mediterranean Shipping Company",2565, 32);
    vehicles[8] = new Ship("COSCO", "China Ocean Shipping Company", "1990", 650000, "China Ocean Shipping Company",36201, 40);
    
    vehicles[9] =  new Tesla("Tesla Model 3", "Tesla", "2020", 1, 250, 200, 6, 300);
    vehicles[10] = new Tesla("Tesla Model S", "Tesla", "2020", 1, 300, 220, 6, 390);
    vehicles[11] = new Tesla("Tesla Model Y", "Tesla", "2020", 1, 400, 360, 6, 420);
    
    vehicles[12] = new BatMobile("Model A","Wayne Industries", "2020", 4 , 300, 600 , 40000, "Wayne Industries", 500, 320);
    vehicles[13] = new BatMobile("Model C", "Wayne Industries", "2019", 3 , 290, 400, 32000, "Wayne Industries", 450, 310);
    vehicles[14] = new BatMobile("Model T", "Wayne Industries", "2018",3 , 250, 360, 280000, "Wayne Industries", 370, 310);

    count = vehicles.length;
}

public void menu() {
    Scanner scan = new Scanner(System.in);
    String response;
    
    System.out.println("Welcome! \n1 - To see how many vehicles are in the system\n2 - To see the name and the class of each vehicle\n3 - To see which vehicles can fly.\n4 - To see which vehicles can float.\n5 - To see which vehicles can fly and float.\n6 - To see a description of each vehicle.\nh - to see brief help information about the system.\nq - To terminate the program.");
        response = scan.nextLine();
    
    switch(response) {
        case "1":
            if(response.equals("1")) {
                vehicles_Instances();
                System.out.println("There are "+vehicleInstancesAndMenu.count+" vehicles in the system.");  
            }
            
        case "2":
            if(response.equals("2")) {
                for(int i = 0; i< vehicles.length;i++) {
                    vehicles_Instances();
                    System.out.println( "Name: "+vehicles[i].getName()+" Class: "+vehicles[i].getClass().getSimpleName()+"\n" );
                }   
            }
            
        case "3":
            if(response.equals("3")) {
                
                Vehicle airplane = new Airplane(response, response, response, 0, 0, 0, 0, 0);
                Vehicle batMobile = new BatMobile(response, response, response, 0, 0, 0, 0, response, 0, 0);
                
                for(int i = 0; i < vehicles.length; i++) {
                    if( (vehicles[i] instanceof Airplane) || (vehicles[i] instanceof BatMobile) ){
                        System.out.println( "Name: "+vehicles[i].getName() + "\tType: "+vehicles[i].getClass().getSimpleName() );
                        System.out.println();
                    }else {
                    }
                    vehicles_Instances();
                }
            }

Answer 1

第三種情況：

   case "3":
        if(response.equals("3")) {
            vehicles_Instances(); // you probably want to populate the vehicles first
            Vehicle airplane = new Airplane(response, response, response, 0, 0, 0, 0, 0);
            Vehicle batMobile = new BatMobile(response, response, response, 0, 0, 0, 0, response, 0, 0);
            // the two above are not being used currently
            /*
            you can potentially do:
            vehicles[0] = airplane;
            vehicles[1] = batMobile;
            */
        
        for(int i = 0; i < vehicles.length; i++) {
            if( (vehicles[i] instanceof Airplane) || (vehicles[i] instanceof BatMobile) ){
                System.out.println( "Name: "+vehicles[i].getName() + "\tType: "+vehicles[i].getClass().getSimpleName() );
                System.out.println();
            }else {
            }

        }
    }

同樣在第二種情況下，您不想在每次循環迭代時初始化車輛：

 case "2":
            if(response.equals("2")) {
                vehicles_Instances(); // should be here, not inside the loop
                for(int i = 0; i< vehicles.length;i++) {
                    
                    System.out.println( "Name: "+vehicles[i].getName()+" Class: "+vehicles[i].getClass().getSimpleName()+"\n" );
                }   
            }

Answer 2

由於數組在第一次運行時使用 null 值初始化

`if( (vehicles[i] instanceof Airplane) || (vehicles[i] instanceof BatMobile) )`

它返回 false 並運行vehicles_Instances()填充數組，但隨后 for 循環遞增，因此它將打印索引 1。

所以只需運行vehicles_Instances(); 在 for 循環之前

Answer 3

這是因為您在 for 循環的第一次迭代之后調用了 vehicle_Instances()！ 您的循環不會遍歷第一個元素，因為尚未創建列表！ 在 for 循環執行之前實例化 vehicle_Instances() ！