In Python, the defaultdict class provides a convenient way to create a mapping from key -> [list of values], in the following example,

from collections import defaultdict
d = defaultdict(list)
d[1].append(2)
d[1].append(3)
# d is now {1: [2, 3]}

Is there an equivalent to this in Java?

There is nothing that gives the behaviour of default dict out of the box. However creating your own default dict in Java would not be that difficult.

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

public class DefaultDict<K, V> extends HashMap<K, V> {

    Class<V> klass;
    public DefaultDict(Class klass) {
        this.klass = klass;    
    }

    @Override
    public V get(Object key) {
        V returnValue = super.get(key);
        if (returnValue == null) {
            try {
                returnValue = klass.newInstance();
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
            this.put((K) key, returnValue);
        }
        return returnValue;
    }    
}

This class could be used like below:

public static void main(String[] args) {
    DefaultDict<Integer, List<Integer>> dict =
        new DefaultDict<Integer, List<Integer>>(ArrayList.class);
    dict.get(1).add(2);
    dict.get(1).add(3);
    System.out.println(dict);
}

This code would print: {1=[2, 3]}