如何从java中的Object ArrayList获取ArrayList值？

Question

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

Answer 1

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