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Kiwi is a utility library. We really like Google's Guava, and also use Apache Commons. But if they don't have something we need, and we think it is useful, this is where we put it.

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package org.kiwiproject.json;

import com.fasterxml.jackson.annotation.JsonAnyGetter;
import com.fasterxml.jackson.annotation.JsonAnySetter;
import lombok.ToString;

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

/**
 * This class can be used as a base class for situations where there may be additional/unknown properties in
 * a JSON string that don't have a corresponding property in the class being de-serialized into. It uses Jackson's
 * {@link JsonAnyGetter} and {@link JsonAnySetter} feature to get and set these "extra fields" in a property
 * named {@code extraFields}.
 * 
 * The general use case for this "extra fields" is when a service or application proxies data from a canonical source
 * where we don't want or need to know about every individual field, but where we still want to capture those fields
 * so we can store, display, etc. them. For example, a remote service that has a large number of properties and
 * we only care about a few of them, but we might want to store all of them for analytics, debugging, etc.
 * 
 * This implementation intentionally does not define {@code equals} or {@code hashCode} methods, which means the
 * extra fields will not be included in equality checks unless subclasses choose to include them. It does, however,
 * include a default {@code toString} method that will print out the extra fields. Whether to include extra fields
 * in equality checks depends heavily on the use case, so subclasses must decide for themselves. In addition, many
 * times simple data holder objects don't need to be compared so don't need {@code equals} or {@code hashCode}.
 */
@ToString
public abstract class FlexibleJsonModel {

    protected Map extraFields = new HashMap<>();

    /**
     * Returns the "extra" fields that were not explicitly defined as properties when this object was de-serialized
     * from JSON.
     *
     * @return the extra fields
     */
    @JsonAnyGetter
    public Map getExtraFields() {
        return extraFields;
    }

    /**
     * Add an "extra" field when de-serializing from JSON.
     *
     * @param key   the extra field name
     * @param value the value of the extra field
     */
    @JsonAnySetter
    public void setExtraFields(String key, Object value) {
        extraFields.put(key, value);
    }
}