Java HashMap & HashSet

Key-value maps and unique sets.

✍️ 2 exercises in this lesson

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HashMap

Stores key → value pairs: Map<String, Integer> ages = new HashMap<>();

put(), get(), getOrDefault(), containsKey(), remove(), keySet(), entrySet()

HashSet

Stores unique values: duplicates are ignored. add(), contains(), remove().

HashMap and HashSet do not keep order. Use LinkedHashMap / TreeMap when order matters.

Key Takeaways: Java HashMap & HashSet

Mastering Java HashMap & HashSet helps you write clean and reliable Java code. Follow standard conventions, test edge cases, and keep logic simple. Try running the code example below and complete the exercises to build confidence.

Java HashMap & HashSet Code Example

Java

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Map<String, Integer> stock = new HashMap<>();
        stock.put("Laptop", 12);
        stock.put("Mouse", 150);
        stock.put("Laptop", 10);   // replaces the old value

        System.out.println(stock.get("Laptop"));
        System.out.println(stock.getOrDefault("Monitor", 0));
        for (Map.Entry<String, Integer> e : new TreeMap<>(stock).entrySet()) {
            System.out.println(e.getKey() + ": " + e.getValue());
        }

        Set<String> tags = new HashSet<>(List.of("java", "spring", "java"));
        System.out.println(tags.size());
    }
}

Output

10
0
Laptop: 10
Mouse: 150
2

Run this example on your own computer. The "Get Started" lesson shows how to set it up.

Java HashMap & HashSet Practice Exercises

Check what you learned in “Java HashMap & HashSet”.

/ 2 solved

Fill in the blank

Read a value by key with a default.

int qty = stock.("Monitor", 0);

✓ Correct! Not quite. Try again.

Multiple choice

What does a HashSet do with duplicates?

Ignores them Stores them twice Throws an error Sorts them

✓ Correct! Not quite. Try again.

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