DefaultHashMap.java

import java.util.HashMap;

/**
 * Useful when you want to do something like map.get(key).add(value). In this case you would add
 * ArrayList::new and not have to check if there is already some ArrayList added as a key before
 * adding it to the pool.
 */
public class DefaultHashMap<K, V> extends HashMap<K, V> {
  private DefaultValueCreator<V> defaultValueCreator;

  public DefaultHashMap(DefaultValueCreator<V> defaultValueCreator) {
    this.defaultValueCreator = defaultValueCreator;
  }

  @Override
  public V get(Object key) {
    if (containsKey(key)) {
      return super.get(key);
    } else {
      V newDefault = defaultValueCreator.createDefaultValue();
      put((K) key, newDefault);
      return newDefault;
    }
  }

  public interface DefaultValueCreator<V> {
    V createDefaultValue();
  }
}