Is there a Java equivalent of Python's defaultdict?

You can use MultiMap from Apache Commons.

in addition to apache collections, check also google collections:

A collection similar to a Map, but which may associate multiple values with a single key. If you call put(K, V) twice, with the same key but different values, the multimap contains mappings from the key to both values.

Using just the Java runtime library you could use a HashMap and add an ArrayList to hold your values when the key does not exist yet or add the value to the list when the key does exist.

The solution from @tendayi-mawushe did not work for me with Primitive types (e.g. InstantiationException Integer), here is one implementation that works with Integer, Double, Float. I often use Maps with these and added static constructors for conveninence

import java.util.HashMap;
import java.util.Map;

/** Simulate the behaviour of Python's defaultdict */
public class DefaultHashMap<K, V> extends HashMap<K, V> {
    private static final long serialVersionUID = 1L;

    private final Class<V> cls;
    private final Number defaultValue;

    @SuppressWarnings({ "rawtypes", "unchecked" })
    public DefaultHashMap(Class factory) {
        this.cls = factory;
        this.defaultValue = null;
    }

    public DefaultHashMap(Number defaultValue) {
        this.cls = null;
        this.defaultValue = defaultValue;
    }

    @SuppressWarnings("unchecked")
    @Override
    public V get(Object key) {
        V value = super.get(key);
        if (value == null) {
            if (defaultValue == null) {
                try {
                    value = cls.newInstance();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            } else {
                value = (V) defaultValue;
            }
            this.put((K) key, value);
        }
        return value;
    }

    public static <T> Map<T, Integer> intDefaultMap() {
        return new DefaultHashMap<T, Integer>(0);
    }

    public static <T> Map<T, Double> doubleDefaultMap() {
        return new DefaultHashMap<T, Double>(0d);
    }

    public static <T> Map<T, Float> floatDefaultMap() {
        return new DefaultHashMap<T, Float>(0f);
    }

    public static <T> Map<T, String> stringDefaultMap() {
        return new DefaultHashMap<T, String>(String.class);
    }
}

And a test, for good manners:

import static org.junit.Assert.assertEquals;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;

import org.junit.Test;

public class DefaultHashMapTest {

    @Test
    public void test() {
        Map<String, List<String>> dm = new DefaultHashMap<String, List<String>>(
                ArrayList.class);
        dm.get("nokey").add("one");
        dm.get("nokey").add("two");
        assertEquals(2, dm.get("nokey").size());
        assertEquals(0, dm.get("nokey2").size());
    }

    @Test
    public void testInt() {
        Map<String, Integer> dm = DefaultHashMap.intDefaultMap();
        assertEquals(new Integer(0), dm.get("nokey"));
        assertEquals(new Integer(0), dm.get("nokey2"));
        dm.put("nokey", 3);
        assertEquals(new Integer(0), dm.get("nokey2"));
        dm.put("nokey3", 3);
        assertEquals(new Integer(3), dm.get("nokey3"));
    }

    @Test
    public void testString() {
        Map<String, String> dm = DefaultHashMap.stringDefaultMap();
        assertEquals("", dm.get("nokey"));
        dm.put("nokey1", "mykey");
        assertEquals("mykey", dm.get("nokey1"));
    }
}

In most common cases where you want a defaultdict, you’ll be even happier with a properly designed Multimap or Multiset, which is what you’re really looking for. A Multimap is a key -> collection mapping (default is an empty collection) and a Multiset is a key -> int mapping (default is zero).

Guava provides very nice implementations of both Multimaps and Multisets which will cover almost all use cases.

But (and this is why I posted a new answer) with Java 8 you can now replicate the remaining use cases of defaultdict with any existing Map.

• getOrDefault(), as the name suggests, returns the value if present, or returns a default value. This does not store the default value in the map.
• computeIfAbsent() computes a value from the provided function (which could always return the same default value) and does store the computed value in the map before returning.

If you want to encapsulate these calls you can use Guava’s ForwardingMap:

public class DefaultMap<K, V> extends ForwardingMap<K, V> {
  private final Map<K, V> delegate;
  private final Supplier<V> defaultSupplier;

  /**
   * Creates a map which uses the given value as the default for <i>all</i>
   * keys. You should only use immutable values as a shared default key.
   * Prefer {@link #create(Supplier)} to construct a new instance for each key.
   */
  public static DefaultMap<K, V> create(V defaultValue) {
    return create(() -> defaultValue);
  }

  public static DefaultMap<K, V> create(Supplier<V> defaultSupplier) {
    return new DefaultMap<>(new HashMap<>(), defaultSupplier);
  }

  public DefaultMap<K, V>(Map<K, V> delegate, Supplier<V> defaultSupplier) {
    this.delegate = Objects.requireNonNull(delegate);
    this.defaultSupplier = Objects.requireNonNull(defaultSupplier);
  }

  @Override
  public V get(K key) {
    return delegate().computeIfAbsent(key, k -> defaultSupplier.get());
  }
}

Then construct your default map like so:

Map<String, List<String>> defaultMap = DefaultMap.create(ArrayList::new);

I wrote the library Guavaberry containing such data structure: DefaultHashMap.

It is highly tested and documented. You can find it and integrate it pretty easily via Maven Central.

The main advatage is that it uses lambda to define the factory method. So, you can add an arbitrarly defined instance of a class (instead of relying on the existence of the default constructor):

DefaultHashMap<Integer, List<String>> map = new DefaultHashMap(() -> new ArrayList<>());
map.get(11).add("first");

I hope that can be of help.

In Java 8+ you can use:

map.computeIfAbsent(1, k -> new ArrayList<Integer>()).add(2);