How to Convert an ArrayList<Object> to ArrayList<String> in Java?

How can I convert ArrayList<Object> to ArrayList<String>?

ArrayList<Object> list = new ArrayList<Object>();
list.add(1);
list.add("Java");
list.add(3.14);
System.out.println(list.toString());

In Java, converting an ArrayList<Object> to an ArrayList<String> requires handling type casting carefully, as you cannot directly cast a list of one type to another. Instead, you need to iterate over the original list, checking each element's type and performing the conversion accordingly.

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<Object> list = new ArrayList<>();
        list.add(1);
        list.add("Java");
        list.add(3.14);
        
        ArrayList<String> list2 = new ArrayList<>();
        for (Object obj : list) {
            if (obj instanceof String) {
                list2.add((String) obj);
            }
        }
        System.out.println(list2.toString()); // Outputs: [Java]
    }
}

Causes

• Attempting to directly cast the list using (String)list leads to a compile-time error because Java uses generics and does not allow such casting.
• The original list may contain elements that are not instances of String, leading to ClassCastException at runtime if not handled.

Solutions

• Create a new ArrayList<String> instance.
• Iterate over each element in the original ArrayList<Object> and check if the element is an instance of String using the instanceof operator.
• Cast each valid element to String and add it to the new ArrayList.

Mistake: Not checking instance before casting, which results in a runtime error.

Solution: Always use instanceof to check if the object is of the expected type before casting.

Mistake: Trying to cast the entire ArrayList directly, causing a compile-time error.

Solution: Create a new ArrayList and populate it using a loop.