HashSet

import java.util.HashSet;

public class UniqueTags {
    public static void main(String[] args) {
        // Create a set of tags for a blog post
        HashSet<String> tags = new HashSet<>();
        
        System.out.println("=== Adding Blog Tags ===");
        
        // Add tags
        boolean added;
        
        added = tags.add("java");
        System.out.println("Add 'java': " + added + " → " + tags);
        
        added = tags.add("programming");
        System.out.println("Add 'programming': " + added + " → " + tags);
        
        added = tags.add("tutorial");
        System.out.println("Add 'tutorial': " + added + " → " + tags);
        
        // Try to add duplicate
        added = tags.add("java");
        System.out.println("Add 'java' again: " + added + " → " + tags);
        System.out.println("Duplicate ignored!");
        
        // Add more tags
        tags.add("beginner");
        tags.add("coding");
        tags.add("java");
        System.out.println("\nWith more tags: " + tags);
        
        System.out.println("\n=== Blog Post Tags ===");
        System.out.println("Total unique tags: " + tags.size());
        System.out.println("Tags: " + tags);
        
        // Display as hashtags
        System.out.println("\n=== Formatted ===");
        for (String tag : tags) {
            System.out.print("#" + tag + " ");
        }
        System.out.println();
    }
}

import java.util.HashSet;
import java.util.Arrays;

public class CheckMembership {
    public static void main(String[] args) {
        // Create an allowlist of premium users
        HashSet<String> premiumUsers = new HashSet<>(Arrays.asList(
            "alice", "bob", "carol", "david"
        ));
        
        System.out.println("=== Premium Membership ===");
        System.out.println("Premium users: " + premiumUsers);
        
        // Check if user has access
        String currentUser = "eve""bob";
        
        System.out.println("\n=== Access Check ===");
        System.out.println("User: " + currentUser);
        
        if (premiumUsers.contains(currentUser)) {
            System.out.println("✓ Premium access granted!");
            System.out.println("Welcome to exclusive content.");
        } else {
            System.out.println("✗ Not a premium user.");
            System.out.println("Upgrade to access premium features!");
        }
        
        // Blocked users check
        HashSet<String> blockedUsers = new HashSet<>(Arrays.asList(
            "spammer", "troll", "eve"
        ));
        
        System.out.println("\n=== Security Check ===");
        if (blockedUsers.contains(currentUser)) {
            System.out.println("⛔ User is blocked!");
        } else {
            System.out.println("✓ User is not blocked.");
        }
        
        // Check multiple users
        System.out.println("\n=== Batch Check ===");
        String[] usersToCheck = {"alice", "eve", "bob", "frank"};
        
        for (String user : usersToCheck) {
            String status = premiumUsers.contains(user) ? "Premium" : "Regular";
            System.out.println(user + ": " + status);
        }
    }
}

import java.util.HashSet;
import java.util.Arrays;

public class CheckMembership {
    public static void main(String[] args) {
        // Create an allowlist of premium users
        HashSet<String> premiumUsers = new HashSet<>(Arrays.asList(
            "alice", "bob", "carol", "david"
        ));
        
        System.out.println("=== Premium Membership ===");
        System.out.println("Premium users: " + premiumUsers);
        
        // Check if user has access
        String currentUser = "eve""bob";
        
        System.out.println("\n=== Access Check ===");
        System.out.println("User: " + currentUser);
        
        if (premiumUsers.contains(currentUser)) {
            System.out.println("✓ Premium access granted!");
            System.out.println("Welcome to exclusive content.");
        } else {
            System.out.println("✗ Not a premium user.");
            System.out.println("Upgrade to access premium features!");
        }
        
        // Blocked users check
        HashSet<String> blockedUsers = new HashSet<>(Arrays.asList(
            "spammer", "troll", "eve"
        ));
        
        System.out.println("\n=== Security Check ===");
        if (blockedUsers.contains(currentUser)) {
            System.out.println("⛔ User is blocked!");
        } else {
            System.out.println("✓ User is not blocked.");
        }
        
        // Check multiple users
        System.out.println("\n=== Batch Check ===");
        String[] usersToCheck = {"alice", "eve", "bob", "frank"};
        
        for (String user : usersToCheck) {
            String status = premiumUsers.contains(user) ? "Premium" : "Regular";
            System.out.println(user + ": " + status);
        }
    }
}

import java.util.HashSet;
import java.util.Arrays;

public class RemoveItem {
    public static void main(String[] args) {
        HashSet<String> cart = new HashSet<>(Arrays.asList(
            "Laptop", "Mouse", "Keyboard", "Monitor", "Headphones"
        ));
        
        System.out.println("=== Shopping Cart ===");
        System.out.println("Items: " + cart);
        System.out.println("Count: " + cart.size());
        
        // Remove an item
        String toRemove = "Mouse";
        boolean removed = cart.remove(toRemove);
        
        System.out.println("\n=== Removing Item ===");
        System.out.println("Remove '" + toRemove + "': " + removed);
        System.out.println("Cart: " + cart);
        
        // Try to remove non-existent item
        String notInCart = "Tablet";
        removed = cart.remove(notInCart);
        System.out.println("\nRemove '" + notInCart + "': " + removed);
        System.out.println("(Item wasn't in cart, so nothing changed)");
        
        // Remove more items
        cart.remove("Keyboard");
        cart.remove("Monitor");
        System.out.println("\nAfter more removals: " + cart);
        
        // Clear all
        System.out.println("\n=== Clearing Cart ===");
        System.out.println("Before clear: " + cart.size() + " items");
        cart.clear();
        System.out.println("After clear: " + cart.size() + " items");
        System.out.println("Cart is empty: " + cart.isEmpty());
        
        System.out.println("\n=== Final Cart ===");
        System.out.println("Items: " + cart);
        System.out.println("Count: " + cart.size());
    }
}

import java.util.HashSet;
import java.util.Arrays;

public class SetUnion {
    public static void main(String[] args) {
        // Two teams' skill sets
        HashSet<String> teamA = new HashSet<>(Arrays.asList(
            "Java", "Python", "SQL", "Git"
        ));
        
        HashSet<String> teamB = new HashSet<>(Arrays.asList(
            "JavaScript", "Python", "CSS", "Git"
        ));
        
        System.out.println("=== Team Skills ===");
        System.out.println("Team A: " + teamA);
        System.out.println("Team B: " + teamB);
        
        // Union: All skills combined
        HashSet<String> allSkills = new HashSet<>(teamA);  // Copy first
        allSkills.addAll(teamB);  // Add all from second
        
        System.out.println("\n=== Union (All Skills) ===");
        System.out.println("Combined: " + allSkills);
        System.out.println("(Duplicates automatically removed)");
        
        // Count unique skills
        System.out.println("\n=== Statistics ===");
        System.out.println("Team A skills: " + teamA.size());
        System.out.println("Team B skills: " + teamB.size());
        System.out.println("Unique combined: " + allSkills.size());
        System.out.println("Overlap count: " + 
            (teamA.size() + teamB.size() - allSkills.size()));
        
        // Practical example: Merge user permissions
        System.out.println("\n=== Permission Merge ===");
        
        HashSet<String> basicPerms = new HashSet<>(Arrays.asList(
            "read", "comment"
        ));
        
        HashSet<String> editorPerms = new HashSet<>(Arrays.asList(
            "read", "write", "edit", "comment"
        ));
        
        HashSet<String> adminPerms = new HashSet<>(Arrays.asList(
            "read", "write", "edit", "delete", "manage"
        ));
        
        // Build editor permissions (basic + editor)
        HashSet<String> fullEditorPerms = new HashSet<>(basicPerms);
        fullEditorPerms.addAll(editorPerms);
        System.out.println("Editor has: " + fullEditorPerms);
        
        // Build admin permissions (all combined)
        HashSet<String> fullAdminPerms = new HashSet<>(basicPerms);
        fullAdminPerms.addAll(editorPerms);
        fullAdminPerms.addAll(adminPerms);
        System.out.println("Admin has: " + fullAdminPerms);
    }
}

import java.util.HashSet;
import java.util.ArrayList;
import java.util.Arrays;

public class RemoveDuplicates {
    public static void main(String[] args) {
        // List with duplicate entries
        ArrayList<String> emails = new ArrayList<>(Arrays.asList(
            "alice@example.com",
            "bob@example.com",
            "alice@example.com",  // duplicate
            "carol@example.com",
            "bob@example.com",    // duplicate
            "david@example.com",
            "alice@example.com"   // duplicate
        ));
        
        System.out.println("=== Email List (with duplicates) ===");
        System.out.println("Emails: " + emails);
        System.out.println("Count: " + emails.size());
        
        // Remove duplicates using HashSet
        HashSet<String> uniqueEmails = new HashSet<>(emails);
        
        System.out.println("\n=== After Removing Duplicates ===");
        System.out.println("Unique: " + uniqueEmails);
        System.out.println("Count: " + uniqueEmails.size());
        System.out.println("Removed " + (emails.size() - uniqueEmails.size()) + " duplicates");
        
        // Convert back to ArrayList if needed
        ArrayList<String> cleanedList = new ArrayList<>(uniqueEmails);
        System.out.println("\nAs ArrayList: " + cleanedList);
        
        // Preserve original order using LinkedHashSet
        System.out.println("\n=== Preserving Order ===");
        java.util.LinkedHashSet<String> orderedUnique = 
            new java.util.LinkedHashSet<>(emails);
        System.out.println("LinkedHashSet: " + orderedUnique);
        System.out.println("(Maintains insertion order!)");
        
        // Count duplicates
        System.out.println("\n=== Duplicate Analysis ===");
        java.util.HashMap<String, Integer> counts = new java.util.HashMap<>();
        for (String email : emails) {
            counts.put(email, counts.getOrDefault(email, 0) + 1);
        }
        
        for (String email : counts.keySet()) {
            int count = counts.get(email);
            if (count > 1) {
                System.out.println(email + " appeared " + count + " times");
            }
        }
    }
}