set.size() / isEmpty() / iterator()

// Returns the number of elements in the set.
set.size();

// Checks whether the set is empty.
set.isEmpty();

// Retrieves an iterator for the set.
Iterator<Type> it = set.iterator();

// Iterates over all elements using an enhanced for loop.
for (Type variable : set) { ... }

// Iterates over all elements using a lambda expression (Java 8+).
set.forEach(element -> process);

// Converts the set to an array.
set.toArray();

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

Set<String> colors = new HashSet<>();
colors.add("red");
colors.add("green");
colors.add("blue");

// Check the size.
System.out.println(colors.size()); // Outputs "3".

// Check whether the set is empty.
System.out.println(colors.isEmpty()); // Outputs "false".

// Iterate over all elements using an iterator.
Iterator<String> it = colors.iterator();
while (it.hasNext()) {
    System.out.println(it.next()); // Each element is printed.
}

// Iterate over all elements using an enhanced for loop.
for (String color : colors) {
    System.out.println(color);
}

// Iterate using forEach and a lambda expression (Java 8+).
colors.forEach(color -> System.out.println(color));

// Convert to an array.
Object[] arr = colors.toArray();
System.out.println(arr.length); // Outputs "3".

size() returns the number of elements in the set. Unlike lists, sets do not support index-based access, so use an enhanced for loop, an iterator, or forEach() to iterate over elements.

Adding or removing elements while iterating with an iterator throws a ConcurrentModificationException. If you need to remove elements during iteration, use the iterator's remove() method.

For creating a set and using add, check, and remove operations, see 'new HashSet<>() / add() / contains() / remove()'.