The Properties Pattern Based on http://steve-yegge.blogspot.com/2008/10/universal-design-pattern.html

Introduction Many names Bottom line Prototype Property List Properties Object Adaptive Object Model Bottom line Seems like a low-level hack Actually, a new way to look at the world

Map API get(key) put(key, value) has(key) remove(key) Also: keys()

Properties Pattern API Based on the Map API Enhancement: a parent() link Points at another Properties object Supplies additional properties Map method need to be adjusted

public class PropList { private PropList parent; private Map<String, Object> map = new HashMap<String, Object>(); public PropList(PropList parent) { this.parent = parent; } public Object get(String key) { if(map.containsKey(key)) return map.get(key); if(parent != null) return parent.get(key); return null; public void put(String key, Object value) { map.put(key, value);

public boolean has(String key) { return map.containsKey(key) || parent != null && parent.has(key); } public void remove(String key) { map.remove(key); public Collection<String> keys() { List<String> result = new ArrayList<String>(); result.addAll(map.keySet()); if(parent != null) result.addAll(parent.keys()); return result; public PropList parent() { return parent;

Motivation BigTables Flexible list of properties Relational DB are change resistant New storage facilities are based on triplets: key, attribute, value Property objects are the ideal in-memory counterpart Flexible list of properties Variable user content When the logic is not interested in specific properties It is interested in the relationship between the properties Examples: A movie list browser, protein-peptide browser

Motivation: Object-level inheritance Sometimes we want to extend an object! Here’s an existing instance I want the new instance to be essentially the same as the existing one Except for some additions, changes When the existing instance changes I want the new one to change as well “I want that one” “Inheritance by example”

Example: Object-level Inheritance On-line shop Inventory, customers, products, orders A basic order class Specifies customer, product, quantity Possible subclasses Presentation: HtmlFormattedOrder, TextFormattedOrder Logic: SpecialProductOrder, PremiumCustomerOrder We want the same object to be extended several times All extension should be built on top the same data

org.eclipse.jdt.core.dom.ASTNode Each AST node is capable of carrying an open-ended collection of client-defined properties. Newly created nodes have none. getProperty and setProperty are used to access these properties.

Implementation Representation of keys String (almost always) Arbitrary objects Instances of a Key class String + Numbers Representation of key-value pairs Linked list of pairs Hashtable

Deletion Three alternative Remove from current instance Remove also from its complete parent chain Remove replaces the value with a special “not here” value

public class PropList { private static final Object NOT_HERE = new Object(); public void remove(String key) { map.put(key, NOT_HERE); } public Object get(String key) { if(map.containsKey(key)) { Object o = map.get(key); if(o == NOT_HERE) o = null; return o; if(parent != null) return parent.get(key); return null; // Similar changes in has() method

Safety & Invariants A parent is notified of put() actions on any child Can veto the change

public class PropList { public interface Interceptor { public Object onPut(PropList p, String key, Object value); } private List<Interceptor> interceptors = new ArrayList<Interceptor>(); public void addInterceptor(Interceptor i) { interceptors.add(i); public void put(String key, Object value) { for(PropList p = this; p != null; p = p.parent) for(Interceptor i : interceptors) value = i.onPut(p, key, value); map.put(key, value);

Persistence Quite easy if all values are primitive (Otherwise) E.g.: One key/value pair per line (Otherwise) Maintain a unique id property (primitive or string) Saving If value is another PropList save its id Loading Build an id -> PropList map In each PropList replace properties that hold an id with the actual reference Maps nicely to a ternary tables on a relational DB: id, key, value