Collections checkedCollection() method in Java with Examples

The Collections.checkedCollection() method returns a runtime type-safe view of the specified collection. It ensures that only elements of the specified type can be added to the collection. If an element of an invalid type is inserted, a ClassCastException is thrown.

• Prevents insertion of elements of the wrong type.
• Throws ClassCastException if an invalid type is added.
• Useful when working with legacy or raw collections.

import java.util.*;

public class CheckedCollectionDemo {
    public static void main(String[] args) {

        // Creating a collection
        Collection<String> names = new ArrayList<>();

        // Creating a type-safe view
        Collection<String> checkedNames =
                Collections.checkedCollection(names, String.class);

        // Adding elements
        checkedNames.add("Alice");
        checkedNames.add("Bob");
        checkedNames.add("Charlie");

        // Displaying the collection
        System.out.println("Names: " + checkedNames);
    }
}

Names: [Alice, Bob, Charlie]

Explanation: This example creates a type-safe view of an ArrayList using Collections.checkedCollection(). The returned collection allows only String elements to be added, helping prevent accidental insertion of objects of the wrong type at runtime.

Syntax: 

public static <E> Collection<E> checkedCollection(
Collection<E> c, Class<E> type)

Parameters: This method takes two parameters 

• c –> The collection for which a runtime type-safe view is to be created.
• type –> The Class object representing the type of elements allowed in the collection.

Return Type: Returns a runtime type-safe view of the specified collection that allows only elements of the specified type.

Example: Using checkedCollection() with String

import java.util.*;

public class CheckedCollectionExample1 {
    public static void main(String[] args) {

        // Creating an ArrayList of String
        List<String> list = new ArrayList<>();

        // Adding elements
        list.add("Java");
        list.add("Python");
        list.add("C++");
        list.add("JavaScript");

        System.out.println("Original List: " + list);

        // Creating a type-safe view of the collection
        Collection<String> checkedList =
                Collections.checkedCollection(list, String.class);

        System.out.println("Type-safe Collection: " + checkedList);
    }
}

Original List: [Java, Python, C++, JavaScript]
Type-safe Collection: [Java, Python, C++, JavaScript]

Explanation: This example creates a type-safe view of a String collection using Collections.checkedCollection(). It ensures that only String objects can be added to the collection at runtime.

Example: Using checkedCollection() with Integer

import java.util.*;

public class CheckedCollectionExample2 {
    public static void main(String[] args) {

        // Creating an ArrayList of Integer
        List<Integer> list = new ArrayList<>();

        // Adding elements
        list.add(10);
        list.add(20);
        list.add(30);
        list.add(40);

        System.out.println("Original List: " + list);

        // Creating a type-safe view of the collection
        Collection<Integer> checkedList =
                Collections.checkedCollection(list, Integer.class);

        System.out.println("Type-safe Collection: " + checkedList);
    }
}

Original List: [10, 20, 30, 40]
Type-safe Collection: [10, 20, 30, 40]

Explanation: This example creates a type-safe view of an Integer collection using Collections.checkedCollection(). The returned collection allows only Integer elements, helping prevent runtime type errors.

Advantages

• Provides runtime type safety for collections.
• Prevents insertion of objects of the wrong type.
• Helps detect programming errors early.
• Useful when working with legacy code that uses raw collections.
• Improves the reliability and maintainability of collection-based code.