Java 10 introduced the List.copyOf(), Set.copyOf(), and Map.copyOf() methods as convenient ways to create unmodifiable copies of existing collections. These methods are part of the java.util package and provide a simpler way to create immutable collections compared to using older methods like Collections.unmodifiableList().

Here’s how you can use them:

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1. List.copyOf()

The List.copyOf() method creates an unmodifiable copy of the provided Collection. The returned list:

• Is immutable (you cannot add, remove, or modify elements).
• Rejects null elements (throws a NullPointerException).

Example:

package org.kodejava.util;

import java.util.List;

public class ListCopyExample {
    public static void main(String[] args) {
        // Create a mutable list
        List<String> originalList = List.of("A", "B", "C");

        // Create an unmodifiable copy
        List<String> unmodifiableList = List.copyOf(originalList);

        // Print the copied list
        System.out.println(unmodifiableList);

        // Throws UnsupportedOperationException if modification is attempted
        // unmodifiableList.add("D");

        // Throws NullPointerException if original list has nulls
        // List<String> listWithNull = new ArrayList<>();
        // listWithNull.add(null);
        // List.copyOf(listWithNull);
    }
}

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2. Set.copyOf()

The Set.copyOf() method creates an unmodifiable copy of the provided Collection, ensuring that:

• The returned set contains no duplicate elements.
• Null elements are not allowed.
• The original collection can be a List, Set, or any Collection.

Example:

package org.kodejava.util;

import java.util.Set;

public class SetCopyExample {
   public static void main(String[] args) {
      // Create a mutable set
      Set<String> originalSet = Set.of("A", "B", "C");

      // Create an unmodifiable copy
      Set<String> unmodifiableSet = Set.copyOf(originalSet);

      // Print the copied set
      System.out.println(unmodifiableSet);

      // Throws UnsupportedOperationException
      // unmodifiableSet.add("D");
   }
}

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3. Map.copyOf()

The Map.copyOf() method creates an unmodifiable copy of the provided map. Similar to List.copyOf() and Set.copyOf():

• The returned map is immutable.
• Null keys or values are not allowed.
• Elements retain the original insertion order (if applicable, e.g., for LinkedHashMap).

Example:

package org.kodejava.util;

import java.util.Map;

public class MapCopyExample {
   public static void main(String[] args) {
      // Create a mutable map
      Map<Integer, String> originalMap = Map.of(1, "One", 2, "Two", 3, "Three");

      // Create an unmodifiable copy
      Map<Integer, String> unmodifiableMap = Map.copyOf(originalMap);

      // Print the copied map
      System.out.println(unmodifiableMap);

      // Throws UnsupportedOperationException
      // unmodifiableMap.put(4, "Four");
   }
}

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Notes:

1. Immutable Behavior:
   • Any attempt to modify the unmodifiable collections (e.g., using add() or put()) throws UnsupportedOperationException.
   • These methods return a new collection, but if the input collection is already immutable and meets the conditions, it may return the original collection (performance optimization).
2. Handling Nulls:
   • If any input collection contains null elements, these methods will throw a NullPointerException.
3. Differences from Existing Methods:
   • Unlike Collections.unmodifiableList()/Set()/Map(), these methods create a copy, ensuring that changes to the source collection won’t affect the new collection.
4. Static Imports:
   • These methods belong to static utility classes (List, Set, and Map) and are invoked directly as static methods.

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Summary:

• Use these methods to get immutable copies of collections.
• They reject null values by design.
• Collections become unmodifiable and can’t be changed after creation.

They are great for enhancing immutability and safety of the application!