Regarding behaviour of arraylist.

[arien beck]

Hello all;
I am trying to run below code which produces an output as shown following the code sample

public class Test implements Constants {
	String firstname;
	String lastname;
	String email;

	public static void main(String[] args) throws UnknownHostException,
			MongoException, ParseException, IllegalAccessException,
			InvocationTargetException, NoSuchMethodException {

		ArrayList<Test> lst = new ArrayList<Test>();

		Test test = new Test();
		test.firstname = "ajay";
		test.lastname = "padvi";

		for (int i = 0; i < 2; i++) {
			if (i == 0) {
				test.email = "[e-mail]";
			} else {
				test.email = "[e-mail]";
			}
			System.out.println("+++" + test.email);
			lst.add(test);
		}
		System.out.println(lst.get(0).email);
		System.out.println(lst.get(1).email);
	}
}

Output
======

[e-mail]
[e-mail]
[e-mail]
[e-mail]

Please note that before adding "test" object to arraylist if I try to print an email field it produces correct output as shown by first two lines in output above.But I dont understands why fields are same if I try to retrieve those individually from arraylist after for loop.
And if I initialize test object within for loop like below

ArrayList<Test> lst = new ArrayList<Test>();
		
		for (int i = 0; i < 2; i++) {
			Test test = new Test();
			test.firstname = "ajay";
			test.lastname = "padvi";
			if (i == 0) {
				test.email = "[e-mail]";
			} else {
				test.email = "[e-mail]";
			}
			System.out.println("+++" + test.email);
			lst.add(test);
		}
		System.out.println(lst.get(0).email);
		System.out.println(lst.get(1).email);

It gives me desired output showing different emails in test each object in arraylist like below

[e-mail]
[e-mail]
[e-mail]
[e-mail]


Please explain this behaviour of arraylist and provide some thoughts regarding this.

[Winston Gutkowski]

arien beck wrote:Please note that before adding "test" object to arraylist if I try to print an email field it produces correct output as shown by first two lines in output above.But I dont understands why fields are same if I try to retrieve those individually from arraylist after for loop.

Because the two entries in your list point to the same Test object; so what you see will be whatever you last updated it to.

Winston

[Joanne Neal]

You don't add objects to Lists in java, you add references to objects. So in the first code you create a Test object and add a reference to it to the list. You now have two references to that one Test object - the one you created on line 12 and the one you added to the list. You then modify this object and add a second reference to it to the list - you now have three references to one object and this object will contain the data you added the second time thru the loop.

In the second piece of code, you craete a new Test object inside the loop, therefore you will create two Test object (each containing different data) and the list will contain two references - one to each of the two objects.

Does that help ?