How to crate Multiple ArrayList from one ArrayList

[Jay Vaghela]

I have an ArrayList called

 List<FormBean> list = new ArrayList<FormBean>();

The Bean FormBean contains a Date field, the value of the date can be different.

public static class FormBean{
		private Date joiningDate;
// joiningDate's getter and setter.
}

Now, based on the joining date I want to create multiple ArryaList.
Like

SimpleDateFormat sdf = new SimpleDateFormat ("MM/dd/yyyy");

FormBean f1 = new FormBean();
f1.setJoiningDate(sdf.format("08/01/2020"));
list.add(f1);
FormBean f2 = new FormBean();
f2.setJoiningDate(sdf.format("08/01/2022"));
list.add(f2);
FormBean f3 = new FormBean();
f3.setJoiningDate(sdf.format("08/01/2024"));
list.add(f3);

FormBean f4 = new FormBean();
f4.setJoiningDate(sdf.format("08/01/2020"));
list.add(f4);
FormBean f5 = new FormBean();
f5.setJoiningDate(sdf.format("08/01/2022"));
list.add(f5);
FormBean f6 = new FormBean();
f6.setJoiningDate(sdf.format("08/01/2024"));
list.add(f6);

So if there are 3 different dates then I want to create three ArrayList.
Please advice me how to create that list.

[Paul Clapham]

But it's not 3, right? It's "multiple"? So how do you want to store these "multiple" array lists? You can't start programming until you decide what your code is supposed to do.

[Jay Vaghela]

The number of JoiningDate is fixed to 3, (I gave it just an example, but the number of multiple arraylist is fixed to 3)
So I will store them in new ArrayList

List<FormBean> firstDate = new ArrayList<FormBean>();
List<FormBean> secondDate = new ArrayList<FormBean>();
List<FormBean> thirdDate = new ArrayList<FormBean>();

[Jay Vaghela]

I want to know what condition I should put inside the loop

for(FormBean form: list){
// how to decide ?

}

Also, I have override the equal method based on the joiningDate

public boolean equals(Object obj) {
			    
 			if (obj == null)
 			return false;
 
 			if (obj == this)
 			return true;
 
 			if (!(obj instanceof FormBean ))
 			return false;
 
 			Date joiningDate= ((FormBean )obj).getJoiningDate();
 			if (joiningDate== null || getJoiningDate() == null)
 				return false;

			return  (joiningDate.getTime()==getJoiningDate().getTime());

	}

[Jay Vaghela]

Thanks Paul,
Ok, So, instead of joiningDate just consider it this way, if a person allowed to marry only 3 times in a life then his marriage date would be maximum 3.

[Paul Clapham]

for(FormBean form: list){
// how to decide ?
  if (form.getJoiningDate is first date) {
    firstDate.add(form);
  }
// and so on
}

[Jay Vaghela]

firstDate is an ArrayList and form.getJoiningDate is a type of Date.
I can use .contains() method but not sure the exact way.
because how to add the first element in firstDate ArrayList?

[Garrett Rowe]

How about something like:

public Map<Date, List<FormBean>> groupByDate(List<FormBean> beans) {
  //...
}

List<List<FormBean>> beans = new ArrayList<List<FormBean>>(groupByDate(myBeans).values());

It might be more flexible if requirements change in the future.

[Jay Vaghela]

Sorry, but not getting what you mean to say.

[Garrett Rowe]

The groupByDate() method takes a List<FormBean> and returns a Map<Date, List<FormBean>>, you can use this data structure as-is or optionally transform it into a List<List<FormBean>> where every element in each sub-list has the same Date.