Java Set copyOf()

Introduction

The Set.copyOf() method was introduced in Java 10 to allow you to create an unmodifiable copy of an existing set. This method ensures that the resulting set cannot be modified, meaning no elements can be added, removed, or altered. Any attempt to modify the set will throw an UnsupportedOperationException.

The Set.copyOf() method ensures immutability, making it useful when you need to provide a read-only view of a set. The method does not accept null values, and it will throw a NullPointerException if the source set contains null.

Program Steps

1. Create a Set: Create a mutable set of elements.
2. Use Set.copyOf(): Create an unmodifiable copy of the original set.
3. Attempt Modifications: Try to modify the unmodifiable set, which will result in an exception.
4. Display the Set: Print the contents of the copied set.

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Java Program

Example 1: Copying a Set of Strings

import java.util.Set;
import java.util.HashSet;

public class SetCopyOfExample {
    public static void main(String[] args) {
        // Step 1: Create a mutable set of strings
        Set<String> originalSet = new HashSet<>();
        originalSet.add("Apple");
        originalSet.add("Banana");
        originalSet.add("Orange");

        // Step 2: Create an unmodifiable copy of the set
        Set<String> unmodifiableSet = Set.copyOf(originalSet);

        // Step 3: Display the unmodifiable set
        System.out.println("Unmodifiable Set: " + unmodifiableSet);

        // Step 4: Attempt to modify the unmodifiable set (This will throw an exception)
        try {
            unmodifiableSet.add("Grapes");
        } catch (UnsupportedOperationException e) {
            System.out.println("Error: Cannot modify an unmodifiable set");
        }
    }
}

Output

Unmodifiable Set: [Banana, Orange, Apple]
Error: Cannot modify an unmodifiable set

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Explanation

Step 1: Create a Mutable Set

A mutable set originalSet is created using HashSet:

Set<String> originalSet = new HashSet<>();
originalSet.add("Apple");
originalSet.add("Banana");
originalSet.add("Orange");

The set contains three strings: "Apple", "Banana", and "Orange". This set is mutable, meaning elements can be added or removed.

Step 2: Use Set.copyOf() to Create an Unmodifiable Set

We use the Set.copyOf() method to create an unmodifiable copy of originalSet:

Set<String> unmodifiableSet = Set.copyOf(originalSet);

The unmodifiableSet is now an immutable version of originalSet. Any attempt to modify it will result in an UnsupportedOperationException.

Step 3: Display the Unmodifiable Set

The program prints the unmodifiable set:

System.out.println("Unmodifiable Set: " + unmodifiableSet);

Output:

Unmodifiable Set: [Banana, Orange, Apple]

The elements are printed in no specific order (because a Set does not guarantee element order).

Step 4: Attempt to Modify the Unmodifiable Set

Next, the program tries to add an element ("Grapes") to the unmodifiable set, which causes an UnsupportedOperationException:

try {
    unmodifiableSet.add("Grapes");
} catch (UnsupportedOperationException e) {
    System.out.println("Error: Cannot modify an unmodifiable set");
}

Output:

Error: Cannot modify an unmodifiable set

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Example 2: Copying a Set of Integers

import java.util.Set;
import java.util.HashSet;

public class SetCopyOfIntegerExample {
    public static void main(String[] args) {
        // Step 1: Create a mutable set of integers
        Set<Integer> originalSet = new HashSet<>();
        originalSet.add(1);
        originalSet.add(2);
        originalSet.add(3);

        // Step 2: Create an unmodifiable copy of the set
        Set<Integer> unmodifiableSet = Set.copyOf(originalSet);

        // Step 3: Display the unmodifiable set
        System.out.println("Unmodifiable Integer Set: " + unmodifiableSet);

        // Step 4: Attempt to modify the unmodifiable set (This will throw an exception)
        try {
            unmodifiableSet.add(4);
        } catch (UnsupportedOperationException e) {
            System.out.println("Error: Cannot modify an unmodifiable integer set");
        }
    }
}

Output

Unmodifiable Integer Set: [1, 2, 3]
Error: Cannot modify an unmodifiable integer set

Explanation

• Original Set: A mutable set of integers 1, 2, and 3.
• Unmodifiable Set: Set.copyOf() creates an immutable copy of originalSet.
• Modification Attempt: An attempt to modify the unmodifiable set throws an UnsupportedOperationException.

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Example 3: Copying a Set of Custom Objects

import java.util.Set;
import java.util.HashSet;

public class SetCopyOfCustomObjectExample {
    public static void main(String[] args) {
        // Step 1: Create a mutable set of Employee objects
        Set<Employee> originalSet = new HashSet<>();
        originalSet.add(new Employee("Ravi", 30));
        originalSet.add(new Employee("Amit", 28));
        originalSet.add(new Employee("Pooja", 35));

        // Step 2: Create an unmodifiable copy of the set
        Set<Employee> unmodifiableSet = Set.copyOf(originalSet);

        // Step 3: Display the unmodifiable set
        System.out.println("Unmodifiable Employee Set: " + unmodifiableSet);

        // Step 4: Attempt to modify the unmodifiable set (This will throw an exception)
        try {
            unmodifiableSet.add(new Employee("Neha", 27));
        } catch (UnsupportedOperationException e) {
            System.out.println("Error: Cannot modify an unmodifiable employee set");
        }
    }
}

class Employee {
    private String name;
    private int age;

    public Employee(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return name + " (" + age + ")";
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Employee employee = (Employee) o;
        return age == employee.age && name.equals(employee.name);
    }

    @Override
    public int hashCode() {
        return name.hashCode() + age;
    }
}

Output

Unmodifiable Employee Set: [Ravi (30), Amit (28), Pooja (35)]
Error: Cannot modify an unmodifiable employee set

Explanation

• Original Set: A mutable set of Employee objects.
• Unmodifiable Set: Set.copyOf() creates an immutable copy of the set.
• Modification Attempt: Attempting to add a new Employee object results in an UnsupportedOperationException.

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Conclusion

The Set.copyOf() method in Java 10 provides a simple way to create unmodifiable sets from existing sets. It ensures that the copied set cannot be modified, making it useful for creating immutable collections. The method can be used with any type of object (e.g., strings, integers, custom objects). Any attempts to modify the unmodifiable set will throw an UnsupportedOperationException, ensuring that the set remains immutable throughout its lifecycle.