Java Academy · Lesson

toMap, joining, and summarizing

Convert streams to maps, concatenate strings with joining, and compute stats with summarizingInt.

toMap, joining, and summarizing is a free Java Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Java Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Collectors.toMap Basics

Collectors.toMap(keyMapper, valueMapper) converts a stream to a Map. Each element provides one key-value pair.

import java.util.*;
import java.util.stream.*;

record Country(String code, String name) {}
List<Country> countries = List.of(
    new Country("US","United States"),
    new Country("DE","Germany"),
    new Country("JP","Japan")
);

Map<String, String> codeToName =
    countries.stream().collect(
        Collectors.toMap(Country::code, Country::name)
    );
System.out.println(codeToName.get("DE")); // Germany

Handling Duplicate Keys

toMap throws IllegalStateException on duplicate keys. Supply a merge function as the third argument to resolve conflicts:

List<String> words = List.of("apple","ant","banana","bat","cherry");

Map<Character, String> firstByLetter =
    words.stream().collect(
        Collectors.toMap(
            w -> w.charAt(0),
            w -> w,
            (existing, replacement) -> existing // keep first
        )
    );
System.out.println(firstByLetter); // {a=apple, b=banana, c=cherry}

toMap with value transformation

The value mapper can be any function — transform the element before storing:

record User(String login, String email) {}
List<User> users = List.of(
    new User("alice","alice@example.com"),
    new User("bob","bob@example.com")
);

Map<String, String> loginToEmail =
    users.stream().collect(
        Collectors.toMap(User::login, u -> u.email().toUpperCase())
    );
System.out.println(loginToEmail.get("alice")); // ALICE@EXAMPLE.COM

toUnmodifiableMap

Use Collectors.toUnmodifiableMap() to produce an immutable map — equivalent to wrapping with Collections.unmodifiableMap() but cleaner:

Map<String, Integer> wordLengths =
    Stream.of("Java","Stream","API")
          .collect(Collectors.toUnmodifiableMap(w -> w, String::length));
// wordLengths.put("test",4); // throws UnsupportedOperationException

Collectors.joining

Collectors.joining() concatenates string elements. It accepts optional delimiter, prefix, and suffix:

List<String> names = List.of("Alice","Bob","Carol");

System.out.println(names.stream().collect(Collectors.joining()));
// AliceBobCarol

System.out.println(names.stream().collect(Collectors.joining(", ")));
// Alice, Bob, Carol

System.out.println(names.stream().collect(Collectors.joining(", ", "[", "]")));
// [Alice, Bob, Carol]

joining for CSV Output

Build CSV rows or SQL IN clauses with joining:

List<Integer> ids = List.of(1,2,3,4,5);

String inClause = ids.stream()
    .map(Object::toString)
    .collect(Collectors.joining(",", "(", ")"));
System.out.println("WHERE id IN " + inClause);
// WHERE id IN (1,2,3,4,5)

summarizingInt / Long / Double

Collectors.summarizingInt() computes all statistics in one pass and returns an IntSummaryStatistics:

record Product(String name, int price) {}
List<Product> products = List.of(
    new Product("A",100), new Product("B",200), new Product("C",150)
);

IntSummaryStatistics stats =
    products.stream().collect(Collectors.summarizingInt(Product::price));

System.out.println("Count: " + stats.getCount());   // 3
System.out.println("Sum: "   + stats.getSum());     // 450
System.out.println("Min: "   + stats.getMin());     // 100
System.out.println("Max: "   + stats.getMax());     // 200
System.out.println("Avg: "   + stats.getAverage()); // 150.0

summingInt and averagingInt

For simpler cases where you only need one statistic:

int total = products.stream().collect(Collectors.summingInt(Product::price));
System.out.println("Total: " + total); // 450

double avg = products.stream().collect(Collectors.averagingInt(Product::price));
System.out.println("Average: " + avg); // 150.0

groupingBy with summarizingInt

Combine summarization per group:

record Sale(String region, int amount) {}
List<Sale> sales = List.of(
    new Sale("North",100), new Sale("South",200),
    new Sale("North",150), new Sale("South",300)
);

Map<String, IntSummaryStatistics> statsByRegion =
    sales.stream().collect(
        Collectors.groupingBy(Sale::region,
            Collectors.summarizingInt(Sale::amount))
    );
statsByRegion.forEach((r,s) ->
    System.out.printf("%s: sum=%d avg=%.1f%n", r, s.getSum(), s.getAverage()));

Collecting to LinkedHashMap

By default toMap produces a HashMap (unordered). Use the 4-arg overload to specify a LinkedHashMap for insertion-order preservation:

Map<String, Integer> ordered =
    Stream.of("banana","apple","cherry")
          .collect(Collectors.toMap(
              w -> w,
              String::length,
              (a,b) -> a,
              LinkedHashMap::new
          ));
System.out.println(ordered.keySet()); // [banana, apple, cherry]

Practical: Config Parser

Parse key=value lines into a map with toMap:

List<String> lines = List.of("host=localhost","port=5432","db=myapp");

Map<String, String> config =
    lines.stream()
         .map(l -> l.split("=", 2))
         .collect(Collectors.toMap(a -> a[0], a -> a[1]));

System.out.println(config.get("port")); // 5432

Quick Check

You use Collectors.toMap(k, v) on a stream that contains two elements with the same key. What happens?

Recap: toMap, joining, summarizing

Key takeaways:

toMap(keyMapper, valueMapper) — throws on duplicate keys without merge fn
joining(delimiter, prefix, suffix) — concatenates strings elegantly
summarizingInt gives count+sum+min+max+avg in one pass
summingInt/averagingInt for single-statistic needs
Use 4-arg toMap with LinkedHashMap::new for ordered results

Frequently asked questions

Is the “toMap, joining, and summarizing” lesson free?

Yes — the full text of “toMap, joining, and summarizing” is free to read here on the web, and the Java Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Java Academy course, upgrade to CoddyKit PRO.

What will I learn in “toMap, joining, and summarizing”?

Convert streams to maps, concatenate strings with joining, and compute stats with summarizingInt. You practise Java Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Java Academy?

No prior experience is required. Java Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

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