How to add objects in an ArrayList multiple times

Question

I add the details of a Person to an ArrayList twice, but when I want to print the two objects, it just repeats the final details twice (and it does not print the first).

import java.util.ArrayList;

public class Arraylist
{
    Person person = new Person();

    ArrayList<Person> array = new ArrayList<>();

    Person personTwo;
    Person personThree;
    void dataEntery(String name , int age , double marks, int itc, int pf)
    {
        person.addDetail(name,age,marks);
        person.marksDetail(itc,pf);

        array.add(person);
    }

    void print()
    {
        int index =0;
        while(index<array.size()) {
            personTwo = array.get(index);
            System.out.println(personTwo.name);
            System.out.println(personTwo.age);
            System.out.println(personTwo.marks);
            System.out.println(personTwo.itc);
            System.out.println(personTwo.pf);
            index++;
        }
    }
}

Can anyone explain why the first isn't printing and why the last prints twice?

You are adding the same object twice. Java is pass reference by value so you are mutating the same object. Also, don't call your class Arraylist; it's confusing. — Boris the Spider
– Boris the Spider, Commented Oct 21, 2014 at 19:44

Elliott Frisch · Accepted Answer · 2014-10-21 19:41:58Z

5

You must create new Person instances or you simply add (and update) the same Person instance (because you only have one reference).

Person person = new Person(); // <-- something like this.
person.addDetail(name,age,marks);
person.marksDetail(itc,pf);

array.add(person);

answered Oct 21, 2014 at 19:41

Elliott Frisch

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Comments

Eran · Accepted Answer · 2014-10-21 19:48:56Z

1

I don't see where you are creating the person instance, but you have to create a new instance before each time you add a Person to the list. Otherwise, the list would contain the same instance multiple times.

The details you are seeing printed reflect the last udpates you made to that single instance, which are the details of the second Person.

void dataEntery(String name , int age , double marks, int itc, int pf)
{
    Person person = new Person();
    person.addDetail(name,age,marks);
    person.marksDetail(itc,pf);

    array.add(person);

}

edited Oct 21, 2014 at 19:48

answered Oct 21, 2014 at 19:43

Eran

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2 Comments

Ahsan Rafiq Over a year ago

Its means every time the reference variable should be created

Eran Over a year ago

@ahsanawan It means that every time a new instance should be created. You could use the same variable (if, for example, you declare it as a member of the class - though there's no reason to do that), but you have to assign a new instance to it before adding it to the list.

Christian Waldmann · Accepted Answer · 2015-08-18 17:44:12Z

0

Ok, so I figured out another way to do it. I feel really stupid about this, but I just now realized I could simply do this:

Fish bass = new Fish("bass", 5);

fishlist.add(bass);

for (Fish fish: fishlist) {
    if (fish.caught) {
        fishcaughtlist.add(new Fish(fish.name, fish.weight);
    }
}

So this will add a new instance that just takes all the info from bass. Thank you for helping tho :)

answered Aug 18, 2015 at 17:44

Christian Waldmann

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