如何访问在Bus类构造方法中创建的Trip对象？

我在Bus类的构造方法里创建了Trip对象，但没法在Main类里访问它。一开始以为这个对象存在Trip类里，控制台提示找不到符号；后来又以为在Bus类里，还是访问不了。

Main类代码 #

public class Interface
{
    public static void main(String args[])
    {       
        Bus bus1 = new Bus(1);
        Trip.trip1.toString();
    }
}

Bus类代码 #

public class Bus
{
    private int tripNumber;
    private String model;
    private String type;
    private int age;
    private int capacity;
    private int remainingCapacity;
    private boolean[][] seats;
    
    public Bus(int tripNumber)
    {
        this.tripNumber = tripNumber;
        
        if (tripNumber==1)
        {
            this.model = "Setra";
            this.type = "2+2";
            this.age = 8;
            this.capacity = 40;
            this.remainingCapacity = 23;
            
            Trip trip1 = new Trip(this, tripNumber);
        }
    }
    public String toString()
    {
        return ("\n\tBus Information:\n\t\tBus: " + this.model + "\n\t\tType: " + this.type + "\n\t\tAge: " + this.age + "\n\t\tCapacity" + this.capacity + "\n\t\tRemainingCapacity" + this.remainingCapacity);
    }
}

Trip类代码 #

public class Trip
{
    private int tripNumber;
    private String date;
    private String origin;
    private String destination;
    private String departureTime;
    private String arrivalTime;
    private Bus assignedBus;
    
    public Trip(Bus bus, int tripNumber)
    {
        if (tripNumber==1)
        {
            this.tripNumber = 1;
            this.assignedBus = bus;
            this.date = "27/11/2022";
            this.origin = "Ankara";
            this.destination = "Istanbul";
            this.departureTime = "00:15";
            this.arrivalTime = "06:30";
        }
    }
    
    public String toString()
    {
        return tripNumber + ") Trip Information: \n\tDate: " + this.date + "\n\tFrom: " + this.origin + " to " + this.destination + "\n\tTrip time: " + this.departureTime + " to " + this.arrivalTime + this.assignedBus.toString();
    }
}

问题根源 #

• 你在Bus构造方法里创建的Trip trip1是局部变量，仅在构造方法的if代码块范围内有效，Bus类外部甚至类内其他方法都无法访问。
• 你尝试用Trip.trip1访问，这是错误地将trip1当成了Trip类的静态成员变量，但Trip类中并未定义这个静态变量，因此编译器报“找不到符号”错误。

修复步骤 #

1. 给Bus类添加Trip类型的成员变量
   在Bus类的私有成员声明区添加一行：

   private Trip assignedTrip;
   

2. 修改Bus构造方法的对象赋值逻辑
   将构造方法内的局部变量创建代码，改为给成员变量赋值：

   // 替换原来的Trip trip1 = new Trip(this, tripNumber);
   this.assignedTrip = new Trip(this, tripNumber);
   

3. 给Bus类添加getter方法
   提供外部访问Bus类中Trip对象的入口：

   public Trip getAssignedTrip() {
       return assignedTrip;
   }
   

4. 修改Main类的调用逻辑
   通过Bus实例获取Trip对象，再调用toString方法：

   public class Interface {
       public static void main(String args[]) {       
           Bus bus1 = new Bus(1);
           // 如果需要打印信息，加上System.out.println
           System.out.println(bus1.getAssignedTrip().toString());
       }
   }
   

额外提示 #

• 若tripNumber不等于1，assignedTrip会是null，调用toString前建议添加非空判断，避免空指针异常。
• 从业务逻辑看，Bus与Trip是关联关系，将Trip实例存储在Bus类中是更合理的设计。

内容的提问来源于stack exchange，提问作者Arson1st