日期对象比较 - Java

Question

我遇到一个程序的问题，该程序需要我比较对Date对象的引用。场景是一个车辆展厅，车辆存储在数组列表中。

我有车辆类别

    import java.text.DecimalFormat;
    import java.util.*;
    import java.util.Date;
    public class Vehicle {
private String manufacturer;
private String model;
//private String customerNameSold = null;
private String vehicleID_VIN;
private String dateManufacture;
private String dateSold;
private boolean beenSold;
private char taxBand_A_M;
private double vehicleCost;
private String emissions;

private Customer customerNameSold;
private Date dom;
//private String manuDate;
private Date saleDate;
     //Constructor 7
     public Vehicle (String manu, String mod, String vin, String dateManu, char tax, double cost){
    manufacturer = manu;
    model = mod;
    vehicleID_VIN = vin;
    dateManufacture = dateManu;
    //Add String to Date
    dom = new Date(dateManu);
    taxBand_A_M = tax;
    vehicleCost = cost;
    dateSold = null;
    customerNameSold = null;
    beenSold = false;
}


public String toString() {
    DecimalFormat df = new DecimalFormat("#.00");
    //method calls - do they need this. then method call?
    String s = "nManufacturer: " + getManu()
            + " nModel: " + getModel()
            + " nVIN: " + getVin()
            + " nDate of Manufacture: " + getManuDate()
            + " nDate of Manufacture (String to Date): " + getManuDate2()
            + " nAge of Vehicle: " + getAgeOfVehicle() + " (in WEEKS)"
            + " nTax Band: " + getTax()
            + " nEmissions: " + cO2()
            + " nCost: £" + df.format(getCost())
            + " nHas Vehicle Been Sold: " + getBeenSold()
            + " nCustomer: " + getCust()
            + " nDate Sold: " + getDateSold()
            + " nDate Sold (String to Date): " + getDateSold2();

    return s;
}
public String getManu() {
    return manufacturer;
}
public String getModel() {
    return model;
}
    ////    public String getCust() {
    ////    
    ////        return customerNameSold;
    ////    }
public Customer getCust() {
    return customerNameSold;
}
public String getVin() {
    return vehicleID_VIN;
}
    public String getManuDate(){
    return dateManufacture;
}
public Date getManuDate2() {
    return dom;
}
public String getDateSold(){
return dateSold;
}
public Date getDateSold2() {
    return saleDate;
}

//CONVENTION SUGGESTS SHOULD BE 'isSold()'
public boolean getBeenSold() {
    return beenSold;
}
public char getTax() {

    return taxBand_A_M;
}
public double getCost() {
    //how to format to 2 decimal places?
    return vehicleCost;
}


public void buyVehicle(String sale, Customer cust) {


    customerNameSold = cust;
    //Add String to Date
    saleDate = new Date(sale);
   dateSold = sale;
    beenSold = true;
}
public String cO2() {

    switch (taxBand_A_M) {
        case 'A':
            emissions = "0-100";
            break;
        case 'B':
            emissions = "101-110";
            break;
        case 'C':
            emissions = "111-120";
            break;
        case 'D':
            emissions = "121-130";
            break;
        case 'E':
            emissions = "131-140";
            break;
        case 'F':
            emissions = "141-150";
            break;
        case 'G':
            emissions = "151-160";
            break;
        default:
            emissions = null;
            break;
    }
    return emissions;
}

public int getAgeOfVehicle() {
    Date now = new Date();
    long diff = now.getTime() - dom.getTime();
    long age = (diff / (1000L * 60 * 60 * 24 * 7));
    return (int) age;
}

}

展厅类

    import java.util.*;
    public class Showroom {
private String showroomName;
private ArrayList<Vehicle> theVehicles;
private Vehicle currVeh = null;
private ArrayList<Vehicle> recentlySold;
private Date dateSold;
private long diff;
private long age;

      //Constructor Method - Takes the name of the Showroom Object & Creates the array list of vehicles
      public Showroom(String name) {
      showroomName = name;
      theVehicles = new ArrayList<Vehicle>();
      }
public String getName() {
    return showroomName;
}
public void setName(String name) {
    //this.showroomName if passed in parameter was named showroomName also
    showroomName = name;
}
    public boolean addVehicle(Vehicle newVehicle) {

    theVehicles.add(newVehicle);
    currVeh = newVehicle;
    return true;

}
public boolean addVehicleAfterCurrent(Vehicle newVehicle) {

    theVehicles.add(theVehicles.indexOf(currVeh) + 1, newVehicle);
    currVeh = newVehicle;
    return true;

}

public Vehicle findVehicle(String vehicleVIN) {
    for (Vehicle v : theVehicles) {
        if (v.getVin().equalsIgnoreCase(vehicleVIN)) {
            System.out.println("Vehicle Found:n"
                    + v.getManu() + "n"
                    + v.getModel() + "n"
                    + v.getVin() + "n"
                    + theVehicles.indexOf(v) + "n");
            return v;
            //OR CALL THE toString() METHOD - v.toString()
        }
    }
    System.out.println("Sorry - The Vehicle was not found in the Showroom!n"
            + theVehicles.indexOf(vehicleVIN) + "n");
    return null;
}
public Vehicle setCurrentVehicle(Vehicle cv) {
    currVeh = cv;
    return currVeh;
}
public Vehicle getCurrentVehicle() {
    System.out.println("nCurrentVehicle: " + currVeh);
    return currVeh;
}
public Vehicle nextVehicle() {

    int index = theVehicles.indexOf(currVeh);
    if (index < 0 || index + 1 == theVehicles.size()) {
        System.out.println("nEnd of the list");
        return null;
    }
    Vehicle v = theVehicles.get(index + 1);
    setCurrentVehicle(v);
    System.out.println("nThe Former Next Vehicle & Now Current Vehicle: " + v);

    return currVeh;
}
public Vehicle previousVehicle() {
    int index = theVehicles.indexOf(currVeh);
    if (index <= 0) {
        System.out.println("nNegative Index -1 Before Start of List");
        return null;
    }
    Vehicle v = theVehicles.get(index - 1);
    setCurrentVehicle(v);
    System.out.println("nThe Former Previous Vehicle & Now Current Vehicle: " + v);

    return currVeh;
}
public void outputArray() {
    for (Vehicle nextVehicle : theVehicles) {
        System.out.println(nextVehicle.getModel() + "n" + theVehicles.indexOf(nextVehicle));
    }
}

public void outputShowroomDetails() {
    System.out.println("nSHOWROOM NAME: " + showroomName);
    //output each vehicle in turn
    System.out.println("THE VEHICLES IN THE SHOWROOM:");
    if (theVehicles.isEmpty()) {
        System.out.println("n*** There are no Vehicles in the Showroom! ***");
    } else {
        for (Vehicle nextVehicle : theVehicles) {
            System.out.println(nextVehicle.toString() + "n" + theVehicles.indexOf(nextVehicle));
        }
    }
}
public Vehicle setCurrentVehicleByVIN(String vin) {

    System.out.println("n*** ATTEMPTING TO SET CURRENT VEHICLE BY VIN:n"
            + vin);
    Vehicle v = findVehicle(vin);
    if (v != null) {
        System.out.println("nTHE CURRENT VEHICLE: " + "nARRAY LIST INDEX: " + theVehicles.indexOf(v)
                + v.toString());
    }
    currVeh = v;
    return currVeh;
}

public boolean deleteVehicle(String vin) {
    System.out.println("nATTEMPTING TO DELETE VEHICLE:n"
            + "VEHICLE VIN to DELETE: " + vin);
    Vehicle v = findVehicle(vin);
    if (v != null) {
        theVehicles.remove(v);
        System.out.println("VEHICLE *** " + vin + " *** REMOVED!");
        return true;
    }
    return false;
}
//Method not working - null pointer 'long diff' line
public ArrayList<Vehicle> getVehiclesSoldRecently() {
    recentlySold = new ArrayList<Vehicle>();

    //Each vehicle has a sale date
    //Determine the difference between sale date & todays date
    //If the difference is greater than 14 days (2 weeks)  it won't be added to the array
    //If the difference is less than or equal to 14 days (2 weeks), then they will be added to the array

    if (theVehicles.isEmpty()) {
        System.out.println("n*** The Showroom is Empty!***");
    } else 
    {
        for (Vehicle v : theVehicles) {
            Date now = new Date();


            dateSold = v.getDateSold2();

            long diff = now.getTime() - v.getDateSold2().getTime();
            long age = (diff / (1000L * 60 * 60 * 24 * 7));

            if (age <= 2) {
                recentlySold.add(v);
                System.out.println("nVEHICLES RECENTLY SOLD: " + v.toString());
            }
        }
    return recentlySold;
    }

}

&客户类

    public class Customer {
private String custName = null;
private String custPhone = null;
private String custEmail = null;
public Customer() {
}
public Customer(String name) {
    custName = name;
    custPhone = "n/a";
    custEmail = "n/a";
}
public Customer(String name, String phone) {
    custName = name;
    custPhone = phone;
    custEmail = "n/a";
}
//***CAN'T HAVE A CONSTRUCTOR DETAILING NAME & EMAIL ONLY
//AS A (String, String) CONSTRUCTOR ALREADY DEFINED
public Customer(String name, String phone, String email) {
    custName = name;
    custPhone = phone;
    custEmail = email;
}
//***AUTO GENERATED GETTERS & SETTERS - this.*
public String getCustName() {
    return custName;
}
public void setCustName(String custName) {
    this.custName = custName;
}
public String getCustPhone() {
    return custPhone;
}
public void setCustPhone(String custPhone) {
    this.custPhone = custPhone;
}
public String getCustEmail() {
    return custEmail;
}
public void setCustEmail(String custEmail) {
    this.custEmail = custEmail;
}
//***AUTO GENERATED TOSTRING METHOD (THOUGH EDITED FOR FORMAT)
@Override
public String toString() {
    String cust = "n*** CUSTOMER ***"
            + "nName: " + getCustName()
            + "nPhone: " + getCustPhone()
            + "nEmail: " + getCustEmail();
    //System.out.println(cust);
    return cust;
}

}

展厅司机是

    public class ShowroomDriver {
public static void main(String args[]) {
    Showroom showDrive = new Showroom("ShowroomDriver Showroom");
    System.out.println("n*** OUTPUT SHOWROOM DETAILS ***");
    showDrive.outputShowroomDetails();
    System.out.println("n*** CREATE / ADD 4 VEHICLES ***");
    Vehicle sdv1 = new Vehicle("Audi", "R8 Spider", "FAVE 101", "MAR-04-2011", 'E', 45000);
    //Vehicle sdv1 = new Vehicle("Audi", "R8 Spider", "FAVE 101", 03 / 04 / 2011, 'E', 45000);
    System.out.println("nTesting toString: " + sdv1.toString());
    Vehicle sdv2 = new Vehicle("Tesla", "Model S", "ELEC TRIC", "JAN-01-2013", 'A', 55000);
    //Vehicle sdv2 = new Vehicle("Tesla", "Model S", "ELEC TRIC", 01 / 01 / 2013, 'A', 55000);
    System.out.println("nTesting to String: " + sdv2.toString());
    Vehicle sdv3 = new Vehicle("Ford", "Cortina", "1212 NUM", "JUN-06-2006", 'D', 55000);
    //Vehicle sdv3 = new Vehicle("Ford", "Cortina", "1212 NUM", 06 / 06 / 2006, 'D', 55000);
    System.out.println("nTesting to String: " + sdv3.toString());
    Vehicle sdv4 = new Vehicle("VW", "Golf MK 1", "DUB DUB", "NOV-11-1971", 'E', 25000);
    //Vehicle sdv4 = new Vehicle("VW", "Golf MK 1", "DUB DUB", 11/11/1971, 'E', 25000);
    System.out.println("nTesting to String: " + sdv4.toString());
    showDrive.addVehicle(sdv1);
    showDrive.addVehicle(sdv2);
    showDrive.addVehicle(sdv3);
    showDrive.addVehicle(sdv4);

    System.out.println("n*** OUTPUT SHOWROOM DETAILS ***");
    showDrive.outputShowroomDetails();
    System.out.println("n*** BUY 2 VEHICLES ***");
    Customer cust1 = new Customer("Andrew Antivan", "01785 111 111");
    Customer cust2 = new Customer("Belinda Belle", "01782 222 222", "[e-mail]");
    sdv1.buyVehicle("JUL-07-2013", cust1);
    sdv2.buyVehicle("MAR-01-2013", cust2);
    showDrive.outputShowroomDetails();
    System.out.println("n***CREATE 4 VEHICLES ***");
    Vehicle purV1 = new Vehicle("Ford", "Fiesta", "NAT NAT", "MAR-09-2006", 'E', 5000);
    Vehicle purV2 = new Vehicle("Vauxhall", "Corsa", "LEE 123", "JUL-07-2011", 'D', 5500);
    Vehicle purV3 = new Vehicle("Toyota", "Aygo", "JOHN 32A", "FEB-02-2010", 'E', 2000);
    Vehicle purV4 = new Vehicle("Marvel", "Bat Mobile", "KA BOOM", "MAR-11-2008", 'C', 3000);
    System.out.println("n*** SELL 2 of 4 VEHICLES ***");


    System.out.println("n*** ADD THE 4 NEW VEHICLES TO SHOWROOM ***");
    showDrive.addVehicle(purV1);
    showDrive.addVehicle(purV2);
    showDrive.addVehicle(purV3);
    showDrive.addVehicle(purV4);

     purV1.buyVehicle("DEC-12-2012", cust1);
    purV2.buyVehicle("DEC-12-2012", cust1);
    purV3.buyVehicle("OCT-18-2013", cust2);
    purV4.buyVehicle("OCT-19-2013", cust2);
            showDrive.outputShowroomDetails();
            showDrive.getVehiclesSoldRecently();


}

}

程序以增量方式修改。最初，制造和销售日期作为字符串对象传入（硬编码），但现在必须将它们添加为日期对象。

只有当我尝试调用getVehiclesSoldNear（）方法时，我遇到的问题才发生。我收到空指针异常警告，表明由于该方法中的"diff..."行而出现问题。

收到的错误：

线程"main"中的异常 java.lang.NullPointerException

at Showroom.getVehiclesSoldLate（Showroom.java：197）

    diff=now.getTime()-v.getDateSold2().getTime();

at ShowroomDriver.main（ShowroomDriver.java：93）

    showDrive.getVehiclesSoldRecently();

任何帮助指出我出错的地方将不胜感激。

谢谢。

*编辑*

除了getDates之外的所有方法的方法签名必须保持不变，这意味着必须传入String参数，然后将其转换为Date对象。

Answer 1

这意味着：

v.getDateSold2()

返回null：

diff=now.getTime()-v.getDateSold2().getTime();

因此，您实际要做的是在空对象上调用getTime()。在尝试访问它之前，您需要检查v.getDateSold2()是否为 null。

如果它为 null，您可以忽略它或打印其他内容。