How to Create an ArrayList in Java that Accommodates Multiple Object Types?

What is the method to create an ArrayList in Java that can store both integer and string types?

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

In Java, the ArrayList class allows you to create dynamically sized lists. However, it is type-specific, meaning you typically define the type of objects it will hold at the time of declaration. To store different types of objects—such as integers and strings—you can use a wrapper class like `Object` to allow the collection to accept various data types.

import java.util.ArrayList;
import java.util.List;

public class MixedArrayList {
    public static void main(String[] args) {
        List<Object> mixedList = new ArrayList<>();
        mixedList.add(1);
        mixedList.add("Hello");
        
        // Retrieving elements with casting
        int num = (Integer) mixedList.get(0);
        String word = (String) mixedList.get(1);
        
        System.out.println("Integer: " + num);
        System.out.println("String: " + word);
    }
}

Causes

• ArrayLists are type-safe, which means they restrict elements to a defined type during declaration.
• To handle multiple types, you need to utilize polymorphism provided by Java's type hierarchy.

Solutions

• Declare your ArrayList with the type `Object`: `List<Object> mixedList = new ArrayList<>();`
• You can add both Integer and String objects without issue: `mixedList.add(1); mixedList.add("Hello");`
• When retrieving elements, explicit type casting is necessary to convert them back to their original types.

Mistake: Forgetting to cast retrieved elements back to their original types.

Solution: Always cast the objects to their original type when retrieving them from the list.

Mistake: Using generics for mixed types like `List<Integer>` and `List<String>`.

Solution: Use `List<Object>` for a mixed type ArrayList to hold various data types.