问题
I am currently designing a GUI for a bank database application. In my GUI I have a "List accounts opened before" button that I am trying to code to list all of the accounts in the database in a text area that have dates before a date that the user inputs into a text field. I am very confused about the implementation behind a Comparable interface and how to correctly compare two dates in a array of objects. In my mind my ShowBefore methods logic is correct, however I think that is not the case and I do not know why. My problem is with the BankDatabase's showBefore() method, Date's compareTo() method, and the GUI's ShowAllActions button. Any help would be greatly appreciated. I receive
"Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at BankDatabase.showBefore(BankDatabase.java:234) at TransactionManager.showAllAccountsActionPerformed(TransactionManager.java:474) at TransactionManager.access$1200(TransactionManager.java:17) at TransactionManager$13.actionPerformed(TransactionManager.java:202) "
When I input any date into the gui.
*I only posted the bare minimum methods required
public class BankDatabase
{
private static final int GROW_SIZE = 2;
private static final int NOT_FOUND = -1;
private static final int ARRAY_SIZE = 100; //i added this
private Account[] bank;
private int num;
/**
default constructor
*/
public BankDatabase()
{
num = 0;
bank = new Account[ARRAY_SIZE];
}
public String showBefore( Date prevDate)
{
String temp = new String();
for ( int i = 0; i < size(); i++)
{
if ( bank[i].getDate().compareTo(prevDate) == -1 )
{
temp += bank[i].toString(); //
temp += "\n";
}
}
return temp;
}
import java.util.Calendar;
import java.util.StringTokenizer;
public class Date implements Comparable {
private int invalidCheck = 0;
private int day;
private int month;
private int year;
/**
Parameterized Constructor.
@param d date
*/
public Date(String d)
{
StringTokenizer st = new StringTokenizer(d, "/");
month = Integer.parseInt(st.nextToken());
day = Integer.parseInt(st.nextToken());
year = Integer.parseInt(st.nextToken());
}
/**
Copy Constructor.
@param d date
*/
public Date(Date d)
{
month = d.month;
day = d.day;
year = d.year;
}
/**
Creates an instance with todays date
*/
public Date()
{
Calendar c = Calendar.getInstance();
this.day = c.get(Calendar.DAY_OF_MONTH);
this.month = c.get(Calendar.MONTH) + 1;
this.year = c.get(Calendar.YEAR);
}
/**
Compare “this” with (Date) o; if “this” is before o, return -1; if “this” is equal
to o return 0; if “this” is after o, return 1.
@param o
@return the value of the date
*/
public int compareTo(Object o)
{
Date d = new Date((Date) o);
if(d.year > year)
return -1;
if(d.year < year)
return 1;
if(d.month > month)
return -1;
if(d.month < month)
return 1;
if(d.day > day)
return -1;
if(d.day < day)
return 1;
return 0;
}
/**
checks to see if certain dates are valid. Also checks
if it is a leap year to verify the correct amount of days
in February.
@return true if the date is valid, false otherwise
*/
public boolean isValid()
{
if (( month == Month.JAN || month == Month.MAR
|| month == Month.MAY || month == Month.JUL
|| month == Month.OCT || month == Month.OCT
|| month == Month.DEC )
&& ( day <= Month.DAYS_ODD && day > invalidCheck ) )
return true;
if (( month == Month.APR || month == Month.JUN
|| month == Month.SEP
|| month == Month.NOV )
&& ( day <= Month.DAYS_EVEN && day > invalidCheck ) )
return true;
boolean leapYear = false;
if ( year % Month.QUADRENNIAL == invalidCheck
|| year % Month.CENTENNIAL == invalidCheck
|| year % Month.QUATERCENTENNIAL == invalidCheck )
{
leapYear = true;
}
if (leapYear)
{
if (month == Month.FEB && day <= Month.DAYS_FEB + 1)
return true;
}
if (month == Month.FEB && day <= Month.DAYS_FEB)
return true;
return false;
}
/**
Return the name and date of the TeamMember as a String.
@return name::price as a string.
*/
public String toString()
{
return month + "/" + day + "/" + year;
}
/**
day,month, and year are equal if they have the
same day, month, and year
@param obj
@return true if they are equal, false otherwise.
*/
public boolean equals(Object obj)
{
if (obj instanceof Date)
{
Date d = (Date) obj;
return d.day == day && d.month == month && d.year == year;
}
return false;
}
}
public abstract class Account
{
private static int numAccount = 1000; //incremented by 1 for each acc op.
protected final int PERCENTAGE = 100;
protected final int MONTH_PER_YEAR = 12;
protected Profile holder; //account holder's profile
protected int accNumber; //a sequence number from numAccount
protected double balance;
protected Date openOn; //the date the account is opened on
/**
parameterized constructor
@param name String that will be the name
@param phone String that will be the phone associated with the account
*/
public Account(String name, String phone)
{
holder = new Profile(name, phone);
accNumber = numAccount++;
balance = 0;// 0 if deposit has to happen after
openOn = new Date();
}
public Date getDate()
{
return openOn;
}
public abstract String toString(); //subclass must implement this method
public class TransactionManager extends javax.swing.JFrame
{
BankDatabase database = new BankDatabase();
TransactionSimulator sim = new TransactionSimulator();
ButtonGroup accType = new ButtonGroup();
private Vector client;
/**
Creates new form TransactionManager
*/
public TransactionManager()
{
initComponents();
database = new BankDatabase();
accType.add(checking);
accType.add(savings);
accType.add(moneyMarket);
cbSpecialSavings.setEnabled(false);
client = new Vector<String>();
}
private void showAllAccountsActionPerformed(java.awt.event.ActionEvent evt)
{
Date openOn = new Date(dateBefore.getText());
outputArea.append(database.showBefore(openOn));
}
回答1:
About your showBefore() at least make some additional checks
public String showBefore( Date prevDate)
{
StringBuilder builder = new StringBuilder("");
for ( int i = 0; i < size(); i++)
{
Account account = bank[i];
if (account != null) {
Date date = account.getDate();
if (date != null && date.compareTo(prevDate) == -1 ) {
builder.append(account.toString() + "\n")
}
}
}
return builder.toString();
}
and keep in mind what StringBuilder is not synchronized
In other case read more about Java 8 features such as Stream API, New Date/Time API, lambda
public String showBefore(LocalDate prevDate)
{
StringBuilder builder = new StringBuilder("");
Arrays.asList(bank).parallelStream().forEach(account -> {
if (account != null) {
LocalDate date = account.getDate();
if (date != null && date.compareTo(prevDate) == -1 ) {
builder.append(account.toString()).append("\n");
}
}
});
return builder.toString();
}
回答2:
if(d.month > month)
return -1;
if(d.month > month)
return 1;
You have equivalent if clauses in equals method.
来源：https://stackoverflow.com/questions/37044392/comparing-two-dates-using-comparable-interface