Remove dates from an array list using iterator

[Josiah smith]

I am trying to modify the ArrayListofDates so that after generating the random dates it loops thru the Dates and checks to see if it is already in the arraylist. If there is a duplicate it then should prompt the user if they want to remove that date from the array. I am trying to use the contains and remove method of array list. Here is the code and it compiles fine but does not work as intended.

import java.util.*;  
 import javax.swing.*;  

public class ArrayListofDates {  
     public static void main(String[] args) {  
    String input;  
     int size;  
     int userAnswer;  
    
     ArrayList myDatesList = new ArrayList();  
     input = JOptionPane.showInputDialog( "Enter number of random dates" );  
     size = Integer.parseInt( input );  
    
     //**array list of random Dates  
     for (int i=1; i<=size; i++) {  
         myDatesList.add(generateRandomDate());  //**pseudo data...  
     }  
    
     //iterate over them  
     Iterator it=myDatesList.iterator();  
     System.out.println();  
     System.out.println("Here are your dates: ");  
    
     Date thisDate;  
     while (it.hasNext()) {  
         //reference to the Date in the list.  
         //***next() returns Object, must cast to Date:  
         thisDate = (Date)(it.next());  
         //thisDate = (it.next());     //**failure to cast would be compile error  
         System.out.print(thisDate);  
         //***access a method of the Date object:  
         System.out.println("     Month is: " + thisDate.getMonth());  
     }  
     System.out.println();  
     //Used to Identify and Display duplicate Dates  
     //Used to prompt user if they want to delete the duplicate Dates  
    // I believe this section of Code is where I am getting my logic error
         while(it.hasNext()) {  
             
            Object element = (Date)(it.next());  
            Set uniqueEntries = new HashSet();  
                 if (!uniqueEntries.add(element)) { // if current element is a duplicate,  
    
                     userAnswer = JOptionPane.showConfirmDialog(null,  
                            element + " Is a duplicate Date in your list. Do you want to delete this duplicate?",  
                            "Delete the Duplicate?",  
                            JOptionPane.YES_NO_OPTION);  
             if (userAnswer == JOptionPane.YES_OPTION) {  
                 it.remove();    // remove it  
                 }  
         }  
      }  
  }  

    //generate a random Date.  Note this is not a method of the Date class.  
     //it creates and returns a Date object.  
     public static Date generateRandomDate() {  
     int year, month, day=0;  
    
     year = (int)(Math.random()*20+1995);  // +/- 10 years from 2005  
         month = (int)(Math.random()*12+1);  
     switch (month) {  
     case 1:  
     case 3:  
     case 5:  
     case 7:  
     case 8:  
     case 10:  
     case 12:  
         day = (int)(Math.random()*31+1);  
         break;  
     case 4:  
     case 6:  
     case 9:  
     case 11:  
         day = (int)(Math.random()*30+1);  
         break;  
     case 2:  
         if (year%4==0 && year%100!=0 || year%400==0)  //leap year test  
             day = (int)(Math.random()*29+1);  
         else 
             day = (int)(Math.random()*28+1);  
         break;  
     }  
     //instantiate a Date object using the 3-arg constructor,  
     //then return the object to caller.  
         return new Date(month,day,year);  
  }  
}

Any suggestions would be greatly appreciated.

[Praveen Kumar M K]

Hi Josiah,

In the code that you have posted I couldn't see where you were using "contains" but I see a set.add(element) function. Either ways, since you are referring to a list of Dates, so you would have to override and write your own contains definition.

Regards,
Praveen.

[Praveen Kumar M K]

My bad! This is the actual documentation for contains method

boolean contains(Object o) -
"Returns true if this list contains the specified element. More formally, returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e))."

As you can see, the contains will return true if and only if the "equals" function returns true(assuming you have atleast 1 element in the list). But by-default the equals method here refers to the equals definition of Object class.

So please ignore the suggestion in the previous post, you would have to override the equals method instead(to compare 2 Dates).

[Josiah smith]

Do you have any examples of code that uses iterators and the contains method. The code I created works for integers within an array, but when I make the array contain dates it doesn't produce the correct output.

Thanks for your help

[Praveen Kumar M K]

You are right, basic datatypes like Integers and Strings have their own version of equals method, so you can use a contains function on these types of lists directly.

I'll give you an example, see if you can write the actual code.

class CustomDate {
	int day;
	int month;
	int year;
	
	public boolean equals(CustomDate date) {
		//Add compare logic here
		//if 2 dates are equal return true
		//else return false
	}
}

After you write the equals logic according to your requirements, subsequent calls to contains method of a CustomDate list would further call this equals method.

Note - If you override equals method for a class, you need to override another default method called hashCode. Do read up more on this before you do the override.

[Campbell Ritchie]

Praveen Kumar M K wrote:. . . basic datatypes like Integers and Strings . . .

Note - If you override equals method for a class, you need to override another default method called hashCode. Do read up more on this before you do the override.

Why are you calling Integer and String basic? There is nothing basic about them, as if they were primitives.

Please don’t double-space code in the code tags, nor write line numbers, which are added automatically.
Beware of dates like 8 2 2012. If the month is 2, that counts as March in the Calendar class. I would have preferred to get a daysInMonth value from the switch block and use the random call once only.

And welcome to the Ranch

[Josiah smith]

Hello,
Sorry If i double spaced i have been working on this for days. This is what I am coming up with to detect and prompt removal for the duplicate dates. I am having a rough time though. I think my error lies in the condional statement. I have been working on this for days and nobody has really been able to help me. Thank you very much for your time.
V/r,
Josiah

       
       //Used to prompt user if they want to delete the duplicate Dates
        for (int i=1; i<=size; i++) {
            Object element = (Date)(it.next());
            if(myDatesList.get(i).equals(myDatesList.get(it.hasNext()))) {
                   userAnswer = JOptionPane.showConfirmDialog(null,
						   element + " Is a duplicate date in your list. Do you want to delete this duplicate?",
						   "Delete the Duplicate?",
						   JOptionPane.YES_NO_OPTION);
	        if (userAnswer == JOptionPane.YES_OPTION) {
	            it.remove();	// remove it
                }
            }
        }

[D. Ogranos]

Josiah smith wrote:Hello,
Sorry If i double spaced i have been working on this for days. This is what I am coming up with to detect and prompt removal for the duplicate dates. I am having a rough time though. I think my error lies in the condional statement. I have been working on this for days and nobody has really been able to help me. Thank you very much for your time.
V/r,
Josiah

I found your original way of using a Set quite ok.

I'd suggest using a slight trick with the dates however: you can get the date "value" as a long with the date.getTime() method. Now just define your set to contain Long (note the capical 'L') elements. Then you can simple write something like

Set<Long> uniqueEntries = new HashSet<Long>();
...
Date current = (Date) it.next();
if (uniqueEntries.add(current.getTime())) {   // Note: autoboxing of the date.getTime() to Long!
  ...
}

EDIT: an addition to your original post. You are aware that you are running through the iterator already once, before you do your actual "test and remove" run?

EDIT2: and make sure to define the set outside of the while loop!