Mengen (Set) in Java 17: Ausführlicher Leitfaden

Sets in Java

What is a Set and why is it needed?

Imagine you have a box where you can only put unique items. If you try to put two identical apples in it, the second one simply won't fit — the box already has that apple. In programming, such a "box" is called a Set.

A Set is a collection that stores elements without duplicates. The order of elements in a regular set doesn't matter. The main rule is: each element can only appear once.

When is this useful?

• You need to collect only unique pizza ingredients
• Store a contact list without duplicates
• Check if a product is on the mandatory list
• Merge two shopping lists, removing duplicates

Basic Syntax

In Java 17, the most commonly used implementation is HashSet — the fastest and simplest set implementation. Here's how to work with it:

// Create an empty set for strings
Set<String> names = new HashSet<>();

// Add elements
names.add("Apple");
names.add("Banana");
names.add("Apple"); // Won't be added — already exists!

// Check the size
System.out.println(names.size()); // Output: 2

// Check if an element exists
if (names.contains("Banana")) {
    System.out.println("Banana is in the set");
}

// Remove an element
names.remove("Banana");

// Iterate over all elements
for (String fruit : names) {
    System.out.println(fruit);
}

Analysis of Typical Tasks

Task 1: Unique Pizza Ingredients

The user enters ingredients separated by commas. You need to output only the unique ones.

import java.util.*;

public class UniqueIngredients {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("Enter ingredients separated by commas:");
        String input = scanner.nextLine();
        
        // Split the string by commas
        String[] parts = input.split(",");
        
        // Create a set for unique ingredients
        Set<String> ingredients = new HashSet<>();
        
        // Add each ingredient (remove whitespace)
        for (String ingredient : parts) {
            ingredients.add(ingredient.trim());
        }
        
        System.out.println("Unique ingredients: " + ingredients);
        System.out.println("Count: " + ingredients.size());
    }
}

Example output:

Enter ingredients separated by commas:
cheese, tomato, cheese, basil, tomato, olives
Unique ingredients: [cheese, tomato, basil, olives]
Count: 4

Task 2: Merging Shopping Lists

You have your own list, guests add theirs. You need to get the final list without duplicates.

import java.util.*;

public class ShoppingList {
    public static void main(String[] args) {
        // Your preliminary list
        Set<String> myList = new HashSet<>();
        myList.add("Cola");
        myList.add("Chips");
        myList.add("Cookies");
        
        // Guests added their wishes
        Set<String> guestsAdded = new HashSet<>();
        guestsAdded.add("Juice");
        guestsAdded.add("Chips");  // Duplicate!
        guestsAdded.add("Nuts");
        
        // Merge — all unique elements from both sets
        myList.addAll(guestsAdded);
        
        System.out.println("Final shopping list:");
        for (String item : myList) {
            System.out.println("- " + item);
        }
    }
}

Example output:

Final shopping list:
- Cola
- Chips
- Cookies
- Juice
- Nuts

Task 3: Removing Extras from a List

You have a list of products. You decided to remove some of them. You need to delete them from the set.

import java.util.*;

public class RemovingExtras {
    public static void main(String[] args) {
        Set<String> purchases = new HashSet<>();
        purchases.add("Milk");
        purchases.add("Bread");
        purchases.add("Eggs");
        purchases.add("Cheese");
        purchases.add("Butter");
        
        System.out.println("Current list: " + purchases);
        System.out.println("What to remove? Enter separated by commas:");
        
        Scanner scanner = new Scanner(System.in);
        String input = scanner.nextLine();
        String[] toRemove = input.split(",");
        
        for (String item : toRemove) {
            purchases.remove(item.trim());
        }
        
        System.out.println("Remaining: " + purchases);
    }
}

Example output:

Current list: [Milk, Bread, Eggs, Cheese, Butter]
What to remove? Enter separated by commas:
Bread, Butter
Remaining: [Milk, Eggs, Cheese]

Task 4: Checking Product Availability

The store has a mandatory list of products. You need to check if all of them are in stock.

import java.util.*;

public class ProductCheck {
    public static void main(String[] args) {
        Set<String> mandatory = new HashSet<>();
        mandatory.add("Bread");
        mandatory.add("Milk");
        mandatory.add("Eggs");
        
        Scanner scanner = new Scanner(System.in);
        System.out.println("Enter products in stock separated by commas:");
        String input = scanner.nextLine();
        
        Set<String> stock = new HashSet<>();
        for (String product : input.split(",")) {
            stock.add(product.trim());
        }
        
        // Check if all mandatory products are in stock
        if (stock.containsAll(mandatory)) {
            System.out.println("All mandatory products are in stock!");
        } else {
            System.out.println("Some mandatory products are missing!");
        }
    }
}

Example output:

Enter products in stock separated by commas:
Bread, Milk, Eggs, Cheese
All mandatory products are in stock!