How to Dynamically Set the Generic Type of an ArrayList in Java?

How can I set the generic type of an ArrayList dynamically during runtime in Java?

In Java, generic types for collections like ArrayList must be specified at compile-time due to Java's type-erasure mechanism. However, you can work around this limitation using the concept of generics and wildcards, allowing you to create an ArrayList that could accept different types at runtime.

import java.util.ArrayList;

public class DynamicArrayList<T> {
    private ArrayList<T> list = new ArrayList<>();
    
    public void addElement(T element) {
        list.add(element);
    }
    
    public T getElement(int index) {
        return list.get(index);
    }

    public static void main(String[] args) {
        DynamicArrayList<String> stringList = new DynamicArrayList<>();
        stringList.addElement("Hello");
        System.out.println(stringList.getElement(0));

        DynamicArrayList<Object> genericList = new DynamicArrayList<>();
        genericList.addElement(123);
        genericList.addElement("World");
        System.out.println(genericList.getElement(0));
        System.out.println(genericList.getElement(1));
    }
}

Causes

• Java uses type erasure for generics, which means generic type information is not available at runtime.
• To dynamically set the type, you can use raw types or wildcards.

Solutions

• Use an ArrayList<Object> to accept any type, then cast the objects when needed.
• Utilize the concept of bounded wildcards to constrain the type of objects during runtime.

Mistake: Failing to specify the generic type, leading to raw type warnings.

Solution: Always try to specify the generic type for type safety, using Object as a last resort.

Mistake: Using incompatible types in the same ArrayList.

Solution: Ensure that all objects added to a generic ArrayList are compatible with the specified type.