Sets, Queues & Iterators

Sets store unique elements (no duplicates). Queues follow FIFO order (First-In, First-Out). Iterators provide a safe way to traverse and modify collections. Together, these tools handle diverse data organization needs. This is a foundational concept in enterprise application development that professional developers rely on daily. The explanations below are written to be beginner-friendly while covering the depth and nuance that comes from real-world Java experience. Take your time with each section and practice the examples

40 min•By Priygop Team•Last updated: Feb 2026

Sets, Queues & Iterators

Example

import java.util.*;

public class CollectionsDemo {
    public static void main(String[] args) {
        // === HASHSET — unique elements, no order ===
        System.out.println("=== HashSet ===");
        HashSet<String> languages = new HashSet<>();
        languages.add("Java");
        languages.add("Python");
        languages.add("JavaScript");
        languages.add("Java");      // Duplicate — ignored!
        languages.add("Python");    // Duplicate — ignored!
        System.out.println("Languages: " + languages);
        System.out.println("Size: " + languages.size());  // 3 (no dups)
        
        // Set operations
        HashSet<String> webLangs = new HashSet<>();
        webLangs.add("JavaScript");
        webLangs.add("HTML");
        webLangs.add("CSS");
        
        // Intersection
        HashSet<String> common = new HashSet<>(languages);
        common.retainAll(webLangs);
        System.out.println("Common: " + common);
        
        // Union
        HashSet<String> allLangs = new HashSet<>(languages);
        allLangs.addAll(webLangs);
        System.out.println("All: " + allLangs);
        
        // === QUEUE — First-In, First-Out ===
        System.out.println("\n=== Queue ===");
        Queue<String> queue = new LinkedList<>();
        queue.add("Customer 1");
        queue.add("Customer 2");
        queue.add("Customer 3");
        
        System.out.println("Queue: " + queue);
        System.out.println("Next: " + queue.peek());     // Look, don't remove
        System.out.println("Served: " + queue.poll());    // Remove first
        System.out.println("After: " + queue);
        
        // === ITERATOR — safe traversal ===
        System.out.println("\n=== Iterator ===");
        ArrayList<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8));
        Iterator<Integer> it = numbers.iterator();
        while (it.hasNext()) {
            int num = it.next();
            if (num % 2 == 0) {
                it.remove();  // Safely remove even numbers
            }
        }
        System.out.println("Odd numbers: " + numbers);
    }
}

Try It Yourself: Unique Word Counter

Try It Yourself: Unique Word CounterJava

Java Editor

✓ ValidTab = 2 spaces

import java.util.*;

public class UniqueWords {
    public static void main(String[] args) {
        String paragraph = "Java is a programming language and Java is " +
            "widely used in enterprise applications because Java is " +
            "platform independent and Java supports object oriented programming";
        
        String[] words = paragraph.toLowerCase().split(" ");
        
        // Count unique words using Set
        HashSet<String> uniqueWords = new HashSet<>(Arrays.asList(words));
        System.out.println("Total words: " + words.length);
        System.out.println("Unique words: " + uniqueWords.size());
        
        // Find duplicate words
        HashSet<String> seen = new HashSet<>();
        HashSet<String> duplicates = new HashSet<>();
        for (String word : words) {
            if (!seen.add(word)) {
                duplicates.add(word);
            }
        }
        System.out.println("\nDuplicate words: " + duplicates);
        
        // Word frequency using HashMap
        HashMap<String, Integer> frequency = new HashMap<>();
        for (String word : words) {
            frequency.put(word, frequency.getOrDefault(word, 0) + 1);
        }
        
        System.out.println("\n=== Word Frequencies ===");
        frequency.entrySet().stream()
            .sorted(Map.Entry.<String, Integer>comparingByValue().reversed())
            .forEach(e -> System.out.println("  " + e.getKey() + ": " + e.getValue()));
    }
}

import java.util.*;

public class UniqueWords {
    public static void main(String[] args) {
        String paragraph = "Java is a programming language and Java is " +
            "widely used in enterprise applications because Java is " +
            "platform independent and Java supports object oriented programming";
        
        String[] words = paragraph.toLowerCase().split(" ");
        
        // Count unique words using Set
        HashSet<String> uniqueWords = new HashSet<>(Arrays.asList(words));
        System.out.println("Total words: " + words.length);
        System.out.println("Unique words: " + uniqueWords.size());
        
        // Find duplicate words
        HashSet<String> seen = new HashSet<>();
        HashSet<String> duplicates = new HashSet<>();
        for (String word : words) {
            if (!seen.add(word)) {
                duplicates.add(word);
            }
        }
        System.out.println("\nDuplicate words: " + duplicates);
        
        // Word frequency using HashMap
        HashMap<String, Integer> frequency = new HashMap<>();
        for (String word : words) {
            frequency.put(word, frequency.getOrDefault(word, 0) + 1);
        }
        
        System.out.println("\n=== Word Frequencies ===");
        frequency.entrySet().stream()
            .sorted(Map.Entry.<String, Integer>comparingByValue().reversed())
            .forEach(e -> System.out.println("  " + e.getKey() + ": " + e.getValue()));
    }
}

Output

Click ▶ Run to see the result

Edit the code on the left, then click Run

Java|37 lines|1488 chars|✓ Valid syntax

UTF-8

Quick Quiz: Sets & Queues

Next Module →

Sets, Queues & Iterators

40 min•By Priygop Team•Last updated: Feb 2026

Sets, Queues & Iterators

Example

import java.util.*;

public class CollectionsDemo {
    public static void main(String[] args) {
        // === HASHSET — unique elements, no order ===
        System.out.println("=== HashSet ===");
        HashSet<String> languages = new HashSet<>();
        languages.add("Java");
        languages.add("Python");
        languages.add("JavaScript");
        languages.add("Java");      // Duplicate — ignored!
        languages.add("Python");    // Duplicate — ignored!
        System.out.println("Languages: " + languages);
        System.out.println("Size: " + languages.size());  // 3 (no dups)
        
        // Set operations
        HashSet<String> webLangs = new HashSet<>();
        webLangs.add("JavaScript");
        webLangs.add("HTML");
        webLangs.add("CSS");
        
        // Intersection
        HashSet<String> common = new HashSet<>(languages);
        common.retainAll(webLangs);
        System.out.println("Common: " + common);
        
        // Union
        HashSet<String> allLangs = new HashSet<>(languages);
        allLangs.addAll(webLangs);
        System.out.println("All: " + allLangs);
        
        // === QUEUE — First-In, First-Out ===
        System.out.println("\n=== Queue ===");
        Queue<String> queue = new LinkedList<>();
        queue.add("Customer 1");
        queue.add("Customer 2");
        queue.add("Customer 3");
        
        System.out.println("Queue: " + queue);
        System.out.println("Next: " + queue.peek());     // Look, don't remove
        System.out.println("Served: " + queue.poll());    // Remove first
        System.out.println("After: " + queue);
        
        // === ITERATOR — safe traversal ===
        System.out.println("\n=== Iterator ===");
        ArrayList<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8));
        Iterator<Integer> it = numbers.iterator();
        while (it.hasNext()) {
            int num = it.next();
            if (num % 2 == 0) {
                it.remove();  // Safely remove even numbers
            }
        }
        System.out.println("Odd numbers: " + numbers);
    }
}

Try It Yourself: Unique Word Counter

Try It Yourself: Unique Word CounterJava

Java Editor

✓ ValidTab = 2 spaces

import java.util.*;

public class UniqueWords {
    public static void main(String[] args) {
        String paragraph = "Java is a programming language and Java is " +
            "widely used in enterprise applications because Java is " +
            "platform independent and Java supports object oriented programming";
        
        String[] words = paragraph.toLowerCase().split(" ");
        
        // Count unique words using Set
        HashSet<String> uniqueWords = new HashSet<>(Arrays.asList(words));
        System.out.println("Total words: " + words.length);
        System.out.println("Unique words: " + uniqueWords.size());
        
        // Find duplicate words
        HashSet<String> seen = new HashSet<>();
        HashSet<String> duplicates = new HashSet<>();
        for (String word : words) {
            if (!seen.add(word)) {
                duplicates.add(word);
            }
        }
        System.out.println("\nDuplicate words: " + duplicates);
        
        // Word frequency using HashMap
        HashMap<String, Integer> frequency = new HashMap<>();
        for (String word : words) {
            frequency.put(word, frequency.getOrDefault(word, 0) + 1);
        }
        
        System.out.println("\n=== Word Frequencies ===");
        frequency.entrySet().stream()
            .sorted(Map.Entry.<String, Integer>comparingByValue().reversed())
            .forEach(e -> System.out.println("  " + e.getKey() + ": " + e.getValue()));
    }
}

import java.util.*;

Output

Click ▶ Run to see the result

Edit the code on the left, then click Run

Java|37 lines|1488 chars|✓ Valid syntax

UTF-8

Sets, Queues & Iterators