Is there a Java equivalent of Python's defaultdict?

Question

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

Answer 1

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 extends HashMap {

    Class 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> dict =
        new DefaultDict>(ArrayList.class);
    dict.get(1).add(2);
    dict.get(1).add(3);
    System.out.println(dict);
}

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