How to get ArrayList value from a Object ArrayList in java?

Question

I have an ArrayList of Objects. I have added some objects and an ArrayList of Strings to this ArrayList. I can easily get the objects value from it. Now my question is how can I get the whole ArrayList of Strings from it?

Code snippet :

Person.java

public class Person {

    private String name;
    private int number;

    public Learn(String name, int number) {
        this.name = name;
        this.number= number;
    }

    public String getName() {
        return name;
    }

    public int getNumber() {
        return number;
    }
}

Now I have defined object List

List<Object> itemsList = new ArrayList<>();

Now it's time to add some Person in itemsList

public void addPerson(){
    itemsList.add(new Person("Alex", 0000062846));
    itemsList.add(new Person("Jack", 0000131332));
    itemsList.add(new Person("Anjela", 0000053715));
    itemsList.add(new Person("Brian", 0000085015));
}

Now, I will add a String List at the index of 2

public void addList(){
    List<String> strList = new ArrayList<>();
    strList.add("Hello");
    strList.add("How");
    strList.add("are");
    strList.add("you?");

    itemsList.add(2, strList);
}

Alright, it's time to get the values from itemsList

Person person = (Person) itemsList.get(0);
System.out.println(person.getName);  // Alex

Now my question is : How can I get strList from itemsList?

I Have a heterogeneous recyclerView in android. I want to pass a whole arrayList to this recyclerView which can be used to show a viewPager as a item — Ravi
– Ravi, Commented Mar 12, 2019 at 13:15
List<String> strList = (List<String>) items.getList(...) but really, why? — Federico klez Culloca
– Federico klez Culloca, Commented Mar 12, 2019 at 13:15
As you are creating such different list, while retrieving you have to write if conditions to check the instance of the item at index. This is not good design though. instanceof is a costly operation too. — Uma Sankar
– Uma Sankar, Commented Mar 12, 2019 at 13:18
This approach is such a mess. Why not make an ArrayList of type Interface which holds your classes. Then make each class implement that interface. Add a function to the interface which returns an identifier — Zun
– Zun, Commented Mar 12, 2019 at 13:21

Ryuzaki L · Accepted Answer · 2019-03-12 13:39:06Z

3

Use instanceof for that, by using instanceof find the Object type in List<Object>

for(Object obj : itemsList) {

    if(obj instanceof List) {    //if it is List type then type cast it

          List<String> str = (List<String>) obj;
          for(String s : str) {

             System.out.println(s);

                  }
          }
    }

But according to the discussion in comments, suppose if you have List<String> and List<Integers> in List<Object> itemsList then while at the time of type casting exception will be thrown, because in runtime List<String> and List<Integers> both are treated as `List

List<String> str = (List<String>) obj;    //exception thrown at this line

take a look at Type Erasure , so use generic lists for each type instead of `List to add different types

edited Mar 12, 2019 at 13:39

answered Mar 12, 2019 at 13:17

Ryuzaki L

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5 Comments

Glains Over a year ago

What happens if i decide to add another List<Integer> to items for later use, just to think one step further, to me this whole design approach looks very error prone.

Ryuzaki L Over a year ago

It will throw an exception , to avoid that jdk people introduced generic concept @Glains

Glains Over a year ago

The point i want to make is, while this does indeed solve the problem, it is error prone for the future if other properties are added to the list. This note could be added to the answer.

Federico klez Culloca Over a year ago

@Glains the question is error prone to begin with. That should be added to the answer.

Glains Over a year ago

@FedericoklezCulloca That is true, but you can still give a good answer, even to a bad question.

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