Java Generics in Collections

1. Introduction

Java Generics enable types (classes and interfaces) to be parameters when defining classes, interfaces, and methods. This feature is essential for the Collections Framework, allowing for strong type checks and reducing runtime errors.

2. Key Concepts

• Type Parameter: A placeholder for a type specified when creating an instance of a generic class.
• Wildcards: Special symbols (e.g., , , ) that allow flexibility in the type used.
• Type Erasure: The process of removing generic type information after compilation to ensure compatibility with legacy code.

3. Using Generics with Collections

Generics can be applied to various Collection classes, such as ArrayList, HashMap, and HashSet.

3.1 Example: Using Generics with ArrayList

import java.util.ArrayList;

public class GenericsExample {
    public static void main(String[] args) {
        ArrayList list = new ArrayList<>();
        list.add("Hello");
        list.add("World");

        for (String s : list) {
            System.out.println(s);
        }
    }
}
                

3.2 Wildcards Example

import java.util.ArrayList;

public class WildcardExample {
    public static void printList(ArrayList list) {
        for (Object elem : list) {
            System.out.println(elem);
        }
    }

    public static void main(String[] args) {
        ArrayList integers = new ArrayList<>();
        integers.add(1);
        integers.add(2);
        printList(integers);
    }
}
                

4. Best Practices

• Always use generics to avoid casting and ensure type safety.
• Prefer using List<T> instead of raw types.
• Use wildcard generics when you need more flexibility.
• Be cautious with List<? extends T> and List<? super T> as they have different implications.

5. FAQ