Java Collectors toMap

Last updated: January 8, 2024

Written by: Rodrigo Graciano

Reviewed by: Michal Aibin

Java Streams

Java Collectors

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1. Overview

In this quick tutorial, we’re going to talk about the toMap() method of the Collectors class. We’ll use it to collect Streams into a Map instance.

For all the examples covered here, we’ll use a list of books as a starting point and transform it into different Map implementations.

Guide to Java Collectors

The article discusses Java 8 Collectors, showing examples of built-in collectors, as well as showing how to build custom collector.

Collect a Java Stream to an Immutable Collection

Learn how to collect Java Streams to immutable Collections.

2. List to Map

We’ll start with the simplest case, by transforming a List into a Map.

Here is how we define our Book class:

class Book {
    private String name;
    private int releaseYear;
    private String isbn;
    
    // getters and setters
}

And we’ll create a list of books to validate our code:

List<Book> bookList = new ArrayList<>();
bookList.add(new Book("The Fellowship of the Ring", 1954, "0395489318"));
bookList.add(new Book("The Two Towers", 1954, "0345339711"));
bookList.add(new Book("The Return of the King", 1955, "0618129111"));

For this scenario we’ll use the following overload of the toMap() method:

Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
  Function<? super T, ? extends U> valueMapper)

With toMap, we can indicate strategies for how to get the key and value for the map:

public Map<String, String> listToMap(List<Book> books) {
    return books.stream().collect(Collectors.toMap(Book::getIsbn, Book::getName));
}

And we can easily validate that it works:

@Test
public void whenConvertFromListToMap() {
    assertTrue(convertToMap.listToMap(bookList).size() == 3);
}

3. Solving Key Conflicts

The example above worked well, but what would happen with a duplicate key?

Let’s imagine that we keyed our Map by each Book‘s release year:

public Map<Integer, Book> listToMapWithDupKeyError(List<Book> books) {
    return books.stream().collect(
      Collectors.toMap(Book::getReleaseYear, Function.identity()));
}

Given our earlier list of books, we’d see an IllegalStateException:

@Test(expected = IllegalStateException.class)
public void whenMapHasDuplicateKey_without_merge_function_then_runtime_exception() {
    convertToMap.listToMapWithDupKeyError(bookList);
}

To resolve it, we need to use a different method with an additional parameter, the mergeFunction:

Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,
  Function<? super T, ? extends U> valueMapper,
  BinaryOperator<U> mergeFunction)

Let’s introduce a merge function that indicates that, in the case of a collision, we keep the existing entry:

public Map<Integer, Book> listToMapWithDupKey(List<Book> books) {
    return books.stream().collect(Collectors.toMap(Book::getReleaseYear, Function.identity(),
      (existing, replacement) -> existing));
}

Or in other words, we get first-win behavior:

@Test
public void whenMapHasDuplicateKeyThenMergeFunctionHandlesCollision() {
    Map<Integer, Book> booksByYear = convertToMap.listToMapWithDupKey(bookList);
    assertEquals(2, booksByYear.size());
    assertEquals("0395489318", booksByYear.get(1954).getIsbn());
}

4. Other Map Types

By default, a toMap() method will return a HashMap.

But we can return different Map implementations:

Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,
  Function<? super T, ? extends U> valueMapper,
  BinaryOperator<U> mergeFunction,
  Supplier<M> mapSupplier)

where the mapSupplier is a function that returns a new, empty Map with the results.

4.1. List to ConcurrentMap

Let’s take the same example and add a mapSupplier function to return a ConcurrentHashMap:

public Map<Integer, Book> listToConcurrentMap(List<Book> books) {
    return books.stream().collect(Collectors.toMap(Book::getReleaseYear, Function.identity(),
      (o1, o2) -> o1, ConcurrentHashMap::new));
}

We’ll go on and test our code:

@Test
public void whenCreateConcurrentHashMap() {
    assertTrue(convertToMap.listToConcurrentMap(bookList) instanceof ConcurrentHashMap);
}

4.2. Sorted Map

Lastly, let’s see how to return a sorted map. For that, we’ll use a TreeMap as a mapSupplier parameter.

Because a TreeMap is sorted according to the natural ordering of its keys by default, we don’t have to explicitly sort the books ourselves:

public TreeMap<String, Book> listToSortedMap(List<Book> books) {
    return books.stream() 
      .collect(
        Collectors.toMap(Book::getName, Function.identity(), (o1, o2) -> o1, TreeMap::new));
}

So in our case, the returned TreeMap will be sorted in alphabetical order by the book name:

@Test
public void whenMapisSorted() {
    assertTrue(convertToMap.listToSortedMap(bookList).firstKey().equals(
      "The Fellowship of the Ring"));
}

5. Conclusion

In this article, we looked into the toMap() method of the Collectors class. It allows us to create a new Map from a Stream.

We also learned how to resolve key conflicts and create different map implementations.

The code backing this article is available on GitHub. Once you're logged in as a Baeldung Pro Member, start learning and coding on the project.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.