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Java, parent static method returning child type

I'm new to java and Generics so please bear with me. I don't even know if this is possible. I've looked around and although there seem to be a few posts about this here and there I haven't found one that addresses my specific case clearly enough for me to understand what to do.
I basically have a static method in the parent class and would like it to return various types based on which child calls it. The trick here is that said child class that is returned also needs to be instantiated within the method.
Here's what I mean:
public class Parent {
public String value;
public Parent(String value)
{
this.value = value;
}
public static <T extends Parent> T load(DataStorage data, String key)
{
T inst = new ???(data.getValue(key));
return inst;
}
}
public class Child extends Parent {
}
Child child = Child.load(dataStore, "key"); // random dataStore instance
I'm not sure where to go from here. What do I use in place of ??? which should be whichever child (or Parent) runs the load() ? Do I need to also do something along the lines of Child.<Child>load()?
I'm open to alternative designs if you feel I'm mistaking in trying to do things like this. I don't like the idea of having to play around with Reflection in this situation (feels a little hacky)
Thanks in advance, I really appreciate it.

I guess what you want would be possible if Java didn't have type erasure and had 'constructor with parameter constraint' for generic types(like .net, but it has constraint for parameterless constructors only).
Maybe those two suits your needs:
If type of Child based on some selection criteria(e.g. an enumaration) I would go with a factory pattern like:
public class ParentFactory {
Parent create(SomeEnum t, DataStore dataStore, String key) {
switch (t) {
case SomeEnum.Child1Related:
return new Child1(dataStore.get(key));
...
}
}
}
But if creation is completely irrelevant(which is not in most cases), you can just define an init method in Parent and have initializer code there:
abstract class Parent {
String value;
Parent() {}
protected void init(String s) { this.value = s; }
static <T extends Parent> void initialize(DataStore data, String key, T t) {
t.init(data.getValue(key));
}
Child c = new Child();
Parent.init(dataStore, key, c);
You can make init method as private if you want to prohibit childs to intercept that call.
Honestly, I favor first one much more. Second one is a little ugly :)

It sounds like the thing you're looking for is a strongly-typed key object, which can associate a string with the corresponding child type. What I've done in the past is simply write a class for this key type.
Assuming your datastore looks something like this:
public interface DataStore {
DataItem get(String key);
}
And your various child classes look like this:
public final class Child1 extends Parent {
public Child1(DataItem dataItem) {
...
}
...
}
Your Key type could look like this:
/**
* Represents a way to construct an object from a {#link DataItem}.
*
* #param <T> the type of the object to construct.
*/
public final class Key<T extends Parent> {
private final String key;
// Assuming Java 8 Function. If you're using Java 7 or older,
// you can define your own Function interface similarly.
private final Function<DataItem, T> factoryFunction;
public Key(String key, Function<String, T> factoryFunction) {
this.key = checkNotNull(key);
this.factoryFunction = checkNotNull(factoryFunction);
}
public String key() {
return this.key;
}
public T constructFrom(DataItem dataItem) {
if (!key.equals(dataItem.getKey())) {
throw new IllegalStateException(
"DataItem is not valid for key " + key);
}
return factoryFunction.apply(dataItem);
}
}
Then you'll probably want a collection of well-known keys:
/** Well-known {#link Key} instances. */
public final class Keys {
private Keys() {} // static class
/** Key for {#link Child1}. */
public static final Key<Child1> FIRST_CHILD
= new Key<>("child1", Child1::new);
/** Key for {#link Child2}. */
public static final Key<Child2> SECOND_CHILD
= new Key<>("child2", Child2::new);
// etc.
}
Then you can define classes that work with these strongly-typed key instances:
public final class Loader {
private final DataStore dataStore;
public Loader(DataStore dataStore) {
this.dataStore = checkNotNull(dataStore);
}
public <T extends Parent> T load(Key<T> dataKey) {
return key.constructFrom(dataStore.get(dataKey.key()));
}
...
}
Note that this example still works even if you don't have Java 8 -- you'll just need to use an anonymous inline class to construct the child, rather than a lambda expression:
public static final Key<Child1> FIRST_CHILD =
new Key<Child1>("child1", new Function<DataItem, Child1>() {
#Override public Child1 apply(DataItem dataItem) {
return new Child1(dataItem);
}
});
You could of course use reflection for this part if you want, but manually writing the supplier functions will be faster. (Or, if you want the best of both worlds, you could use something like cglib's FastClass.) If you wanted to, you could also make the Key class abstract, so that you would subclass it and override a factory method rather than using a Function.
If you want to, you can merge the Loader type into your Parent class, but I wouldn't, since I think that would violate the Single Responsibility Principle -- typically you want the job of loading domain objects from storage to be separate from the domain objects themselves.
Hopefully that helps!

How to use Java Enums being DRY with only a single parameter different between instantiations?

I'm trying to figure out if there is a clean way of doing this. I want to design an ENUM to maintain a list of constant values for different components in my application. Each enum would have the same configuration and same parameters, but would differ at the very least by component name.
In a normal Java class, I could build all the basic logic/code in a base abstract class, and have each component constants extend the abstract class and populate only its own pertinent information. However, Java enums do not allow extending existing classes.
Is there something I can do to avoid having to either push all my constants in a single Enum (ugggg!) or recreate the same enum class each time for each differing component? Definitely not DRY in that case, but I do not know how to avoid the issue.
For a quick use-case example off the top of my head. Say I want to keep a list of all my request mappings in an Enum for use elsewhere in my application. Fairly easy to design an enum that says:
public enum RequestMapping {
INDEX("index"),
GET_ALL_USERS( "getAllUsers");
private String requestMapping = "/users";
private String path;
RatesURI( String path ){
this.path = path;
}
public String getRequestMapping(){
return requestMapping;
}
public String getPath(){
return path;
}
public String getFullRequestPath(){
return requestMapping + "/" + path;
}
}
It becomes easy to use RequestMapping.GET_ALL_USERS.getFullRequestPath().
Now if I want to create this enum on a per-controller basis, I would have to recreate the entire Enum class and change the "requestMapping" value for each one. Granted, this enum has nearly no code in it, so duplicating it would not be difficult, but the concept still remains. The theoretical "clean" way of doing this would be to have an abstract AbstractRequestMapping type that contained all the methods, including an abstract getRequestMapping() method, and only have the extending Enums implement the controller-specific getReqeuestMapping(). Of course, since Enums cannot be extended, I can't think of a non DRY way of doing this.

Have you considered extending a class that takes Enum as a generic parameter? It is an amazingly flexible mechanism.
public class Entity<E extends Enum<E> & Entity.IE> {
// Set of all possible entries.
// Backed by an EnumSet so we have all the efficiency implied along with a defined order.
private final Set<E> all;
public Entity(Class<E> e) {
// Make a set of them.
this.all = Collections.unmodifiableSet(EnumSet.<E>allOf(e));
}
// Demonstration.
public E[] values() {
// Make a new one every time - like Enum.values.
E[] values = makeTArray(all.size());
int i = 0;
for (E it : all) {
values[i++] = it;
}
return values;
}
// Trick to make a T[] of any length.
// Do not pass any parameter for `dummy`.
// public because this is potentially re-useable.
public static <T> T[] makeTArray(int length, T... dummy) {
return Arrays.copyOf(dummy, length);
}
// Example interface to implement.
public interface IE {
#Override
public String toString();
}
}
class Thing extends Entity<Thing.Stuff> {
public Thing() {
super(Stuff.class);
}
enum Stuff implements Entity.IE {
One,
Two;
}
}
You can pass the nature of your implementation up to the parent class in many different ways - I use enum.class for simplicity.
You can even make the enum implement an interface as you can see.
The values method is for demonstration only. Once you have access to the Set<E> in the parent class you can provide all sorts of functionality just by extending Entity.

I will probably split the responsibilities into two parts:
Logic about how a request is structured, and put that into an immutable class.
Actual configurations of each request, stored in enums
The enum will then store an instance of that class, you can add new methods to the class, without modifying the different enums, as long as the constructor remains the same. Note that the class must be immutable, or your enum will not have a constant value.
You can use it like the:
ServiceRequest.INDEX.getRequest().getFullRequestPath()
With these classes:
public interface RequestType {
Request getRequest();
}
public class Request {
private final String requestMapping;
private final String path;
RatesURI(String requestMapping, String path){
this.requestMappint = requestMapping;
this.path = path;
}
public String getRequestMapping(){
return requestMapping;
}
public String getPath(){
return path;
}
public String getFullRequestPath(){
return requestMapping + "/" + path;
}
}
public enum ServiceRequest implements RequestType {
INDEX("index"),
GET_ALL_USERS( "getAllUsers");
private final Request;
ServiceRequest(String path) {
request = new Request("users/", path)
}
public String getRequest{
return request;
}
}

I think what you should be asking yourself is really why you want to use enums for this. First we can review some of the points that make Java enumerated types what they are.
Specifically
A Java enum is a class that extends java.lang.Enum.
Enum constants are static final instances of that class.
There is some special syntax to use them but that is all they boil down to. Because instantiating new Enum instances is disallowed outside of the special syntax (even with reflection, enum types return zero constructors) the following is also ensured to be true:
They can only be instantiated as static final members of the enclosing class.
The instances are therefore explicitly constant.
As a bonus, they are switchable.
What it really boils down to is what it is about the enums that makes them preferable over a simpler OOP design here. One can easily create a simple RequestMapping class:
/* compacted to save space */
public class RequestMapping {
private final String mapping, path;
public RequestMapping(String mapping, String path) {
this.mapping = mapping; this.path = path;
}
public String getMapping() {
return mapping; }
public String getPath() {
return path; }
public String getFullRequestPath() {
return mapping + "/" + path;
}
}
Which can easily be extended to break down the repeated code:
public class UserMapping extends RequestMapping {
public UserMapping(String path) {
super("/users", path);
}
}
/* where ever appropriate for the constants to appear */
public static final RequestMapping INDEX = new UserMapping("index"),
GET_ALL_USERS = new UserMapping("getAllUsers");
But I assume there is something about enums that is attractive to your design, such as the principle that instances of them are highly controlled. Enums cannot be created all willy-nilly like the above class can be. Perhaps it's important that there be no plausible way for spurious instances to be created. Of course anybody can come by and write in an enum with an invalid path but you can be pretty sure nobody will do it "by accident".
Following the Java "static instances of the outer class" enum design, an access modifier structure can be devised that generally abides by the same rule set as Enum. There are, however, two problems which we can't get around easily.
Two Problems
Protected modifier allows package access.
This can easily be surmounted initially by putting the Enum-analog in its own package. The problem becomes what to do when extending. Classes in the same package of the extended class will be able to access constructors again potentially anywhere.
Working with this depends on how stringent you want to be on creating new instances and, conversely, how clear the design ends up. Can't be a whole mess of scopes just so only a few places can do the wrong thing.
Static members are not polymorphic.
Enum surmounts this by not being extendable. Enum types have a static method values that appears "inherited" because the compiler inserts it for you. Being polymorphic, DRY and having some static features means you need instances of the subtype.
Defeating these two issues depends on how stringent you want your design to be and, conversely, how readable and stable you want your implementation to be. Trying to defy OOP principles will get you a design that's hard to break but totally explodes when you call that one method in a way you aren't supposed to (and can't prevent).
First Solution
This is almost identical to the Java enum model but can be extended:
/* 'M' is for 'Mapping' */
public abstract class ReturnMapping<M extends ReturnMapping> {
/* ridiculously long HashMap typing */
private static final HashMap <Class<? extends ReturnMapping>, List<ReturnMapping>>
VALUES = new HashMap<Class<? extends ReturnMapping>, List<ReturnMapping>>();
private final String mapping, path;
protected Mapping(String mapping, String path) {
this.mapping = mapping;
this.path = path;
List vals = VALUES.get(getClass());
if (vals == null) {
vals = new ArrayList<M>(2);
VALUES.put(getClass(), vals);
}
vals.add(this);
}
/* ~~ field getters here, make them final ~~ */
protected static <M extends ReturnMapping> List<M>(Class<M> rm) {
if (rm == ReturnMapping.class) {
throw new IllegalArgumentException(
"ReturnMapping.class is abstract");
}
List<M> vals = (List<M>)VALUES.get(rm);
if (vals == null) {
vals = new ArrayList<M>(2);
VALUES.put(rm, (List)vals);
}
return Collections.unmodifiableList(vals);
}
}
Now extending it:
public final class UserMapping extends ReturnMapping<UserMapping> {
public static final UserMapping INDEX = new UserMapping("index");
public static final UserMapping GET_ALL_USERS = new UserMapping("getAllUsers");
private UserMapping(String path) {
super("/users", path);
}
public static List<UserMapping> values() {
return values(UserMapping.class);
}
}
The huge static HashMap allows almost all of the values work to be done statically in the superclass. Since static members are not properly inherited this is the closest you can get to maintaining a list of values without doing it in the subclass.
Note there are two problems with the Map. The first is that you can call the values with ReturnMapping.class. The map should not contain that key (the class is abstract and the map is only added to in the constructor) so something needs to be done about it. Instead of throwing an exception you could also insert a "dummy" empty list for that key.
The other problem is that you can call values on the superclass before the instances of the subclass are instantiated. The HashMap will return null if this is done before the subclass is accessed. Static problem!
There is one other major problem with this design because the class can be instantiated externally. If it's a nested class, the outer class has private access. You can also extend it and make the constructor public. That leads to design #2.
Second Solution
In this model the constants are an inner class and the outer class is a factory for retrieving new constants.
/* no more generics--the constants are all the same type */
public abstract class ReturnMapping {
/* still need this HashMap if we want to manage our values in the super */
private static final HashMap <Class<? extends ReturnMapping>, List<Value>>
VALUES = new HashMap<Class<? extends ReturnMapping>, List<Value>>();
public ReturnMapping() {
if (!VALUES.containsKey(getClass())) {
VALUES.put(getClass(), new ArrayList<Value>(2));
}
}
public final List<Value> values() {
return Collections.unmodifiableList(VALUES.get(getClass()));
}
protected final Value newValue(String mapping, String path) {
return new Value(getClass(), mapping, path);
}
public final class Value {
private final String mapping, path;
private Value(
Class type,
String mapping,
String path) {
this.mapping = mapping;
this.path = path;
VALUES.get(type).add(this);
}
/* ~~ final class, field getters need not be ~~ */
}
}
Extending it:
public class UserMapping extends ReturnMapping {
public static final Value INDEX, GET_ALL_USERS;
static {
UserMapping factory = new UserMapping();
INDEX = factory.newValue("/users", "index");
GET_ALL_USERS = factory.newValue("/users", "getAllUsers");
}
}
The factory model is nice because it solves two problems:
Instances can only be created from within the extending class.
Anybody can create a new factory but only the class itself can access the newValue method. The constructor for Value is private so new constants can only be created by using this method.
new UserMapping().values() forces the values to be instantiated before returning them.
No more potential errors in this regard. And the ReturnMapping class is empty and instantiating new objects in Java is fast so I wouldn't worry about overhead. You can also easily create a static field for the list or add static methods such as in solution #1 (though this would deflate the design's uniformity).
There are a couple of downsides:
Can't return the subtyped values List.
Now that the constant values are not extended they are all the same class. Can't dip in to generics to return differently-typed Lists.
Can't easily distinguish what subtype a Value is a constant of.
But it's true this could be programmed in. You could add the owning class as a field. Still shaky.
Sum Of It
Bells and whistles can be added to both of these solutions, for example overriding toString so it returns the name of the instance. Java's enum does that for you but one of the first things I personally do is override this behavior so it returns something more meaningful (and formatted).
Both of these designs provide more encapsulation than a regular abstract class and most importantly are far more flexible than Enum. Trying to use Enum for polymorphism is an OOP square peg in a round hole. Less polymorphism is the price to pay for having enumerated types in Java.

Extending enum fields Java

I know that it isn't possible to extend enum in Java, but I am trying to find an elegant solution for the below
I am trying to model enums (or classes) which will contain http end points of various web services across regions, say I have service A and B, each will have 4 region specific end points in US, EU, JP or CN. (This is basically for some seperate debug code that I am writing, in production the end points will be picked from configuration)
I was hoping to do something like this (not compliant java code).
public enum IEndPoint {
NA_END_POINT,
EU_END_POINT,
JP_END_POINT,
CN_END_POINT,
}
public enum ServiceAEndPoint extends IEndPoint {
NA_END_POINT("http://A.com/");
EU_END_POINT("http://A-eu.com/");
JP_END_POINT("http://A-jp.com/");
CN_END_POINT("http://A-cn.com/");
}
I could do this using interfaces where I have a method for each region, but in my opinion the enum way is more expressive, is there any better way I could model this ? What I am looking for is if there is any better way to model the inheritence relation and also having the expressive power of enumerations.
ServiceAEndPoint.NA_END_POINT
vs
serviceAEndPoint.getNAEndPoint()

I'm assuming that you will also want a ServiceBEndPoint enum (and similar). In which case I don't think your model really makes that much sense.
IEndPoint is really an enumeration of the kind of environments/regions where a service might be running. It is not an enumeration of the services themselves. Each individual service (A, B or whatever) will have different addresses for each of the regions.
Therefore I would stick with just the IEndPoint enum, and then in some service-specific code have a lookup map that will give you the address for a given end-point. Something like this:
public enum IEndPoint {
NA_END_POINT,
EU_END_POINT,
JP_END_POINT,
CN_END_POINT,
}
public class ServiceABroker {
private static final Map<IEndPoint, String> addressesByEndPoint;
static {
addressesByEndPoint = new EnumMap<>();
addressesByEndPoint.put(NA_END_POINT, "http://A.com/");
addressesByEndPoint.put(EU_END_POINT, "http://A-eu.com/");
addressesByEndPoint.put(JP_END_POINT, "http://A-jp.com/");
addressesByEndPoint.put(CN_END_POINT, "http://A-cn.com/");
}
public String getAddressForEndPoint(IEndPoint ep) {
return addressesByEndPoint.get(ep);
}
}

If these are static final constants, then just put them in an interface. Name the interface something like IServiceAEndPointKeys, where the keys part is a convention.
Here's where I consider enums to be more appropriate and useful:
Example 1: File type. An enum containing jpg, pdf etc.
Example 2: Column definitions. If I have a table with 3 columns, I would write an enum declaring ID, Name, Description (for example), each one having parameters like column header name, column width and column ID.

Im not sure I understand you question, but you can add methods to an enum for example you could do something like the following:
public enum ServiceAEndPoint{
NA_END_POINT("http://A.com/");
EU_END_POINT("http://A-eu.com/");
JP_END_POINT("http://A-jp.com/");
CN_END_POINT("http://A-cn.com/");
private final String url;
private EndPoint(String url){
this.url=url;
}
public String getURL(){
return url;
}
}

Enums cannot be extended in such a manner, mostly because enums cannot be sub-classed or the constraints they must adhere to will not be possible to impose.
Instead leverage interfaces, like so
public interface IEndPoint;
public enum DefaultEndPoints implements IEndPoint {
NA_END_POINT,
EU_END_POINT,
JP_END_POINT,
CN_END_POINT,
}
public enum DefaultServiceEndPoints implements IEndPoint {
NA_END_POINT("http://A.com/");
EU_END_POINT("http://A-eu.com/");
JP_END_POINT("http://A-jp.com/");
CN_END_POINT("http://A-cn.com/");
}
public void doSomething(IEndPoint endpoint) {
...
}
The reason why one can't subclass in the manner you wish is related to the contract that enums will be both equal via .equals(object) and via ==. If you could subclass, would this make sense?
if ( (DefaultEndPoints)JP_END_POINT == (DefaultServiceEndPoints)JP_END_POINT) {
}
if you say "yes" then I would expect to be able to do this
DefaultEndPoint someEndpoint = DefaultServiceEndPoints.JP_END_POINT;
which would leave a door open for error, as there is no guarantee that a enum entry in one enum declaration is in the other enum declaration.
Could it be different? Perhaps, but it isn't, and changing it would definately introduce a lot of complications that would have to be thoroughly thought out (or it would open avenues to work around Java's strong static-type checking).

You may want to consider something like this:
public abstract class EndpointFactory {
public abstract String getNAEndPoint();
public abstract String getEUEndPoint();
}
public class ServiceAEndpointFactory extends EndpointFactory {
public static final String NA_END_POINT = "http://A.com/";
public static final String EU_END_POINT = "http://A-eu.com/";
public String getNAEndPoint() {
return ServiceAEndpointFactory.NA_END_POINT;
}
public String getEUEndPoint() {
return ServiceAEndpointFactory.EU_END_POINT;
}
}
public class ServiceBEndpointFactory extends EndpointFactory {
public static final String NA_END_POINT = "http://B.com/";
public static final String EU_END_POINT = "http://B-eu.com/";
public String getNAEndPoint() {
return ServiceAEndpointFactory.NA_END_POINT;
}
public String getEUEndPoint() {
return ServiceAEndpointFactory.EU_END_POINT;
}
}
Then you can refer to your strings directly like this:
ServiceAEndpointFactory.NA_END_POINT;
Or, you can use the base object if the type of service is not known until execution:
EndpointFactory ef1 = new ServiceAEndpointFactory();
String ep = ef1.getNAEndPoint();
The drawback of this is the redefinition of the get*Endpoint() functions in each sub-class. You could eliminate that by moving the static final variables to be not static in the base class and putting the getter/setter in the base class only one time. However, the drawback of that is you are not able to reference the values without instantiating an object (which essentially emulates what I find valuable with ENUMs).

How does a pattern like this appeal to you? I let the enum implement an interface and implement the interface in a Debug set and a Release set. The release set can then derive the property name from the enum name - which is neat.
public interface HasURL {
public String getURL();
}
public enum DebugEndPoints implements HasURL {
NA,
EU,
JP,
CN;
#Override
public String getURL() {
// Force debug to go to the same one always.
return "http://Debug.com/";
}
}
public enum NormalEndPoints implements HasURL {
NA,
EU,
JP,
CN;
final String url;
NormalEndPoints () {
// Grab the configured property connected to my name.
this.url = getProperty(this.name());
}
#Override
public String getURL() {
return url;
}
}

Can I declare enums within enums to specify/limit both keys and values in a map in Java?

I want to be able to specify a list of keys and allowed values for each key programatically so that the code can be checked at compile time for errors and in the hope of better performance.
Imagine I am representing word in a database and each word has a number of features:
public class Word {
public Map<Feature, FeatureValue> features = new EnumMap<Feature, FeatureValue>();
}
And I have an enum class:
public enum Feature {
TYPE("Type") {
enum Value {
NOUN("Noun"),
VERB("Verb");
}
#Override
public Value[] getValues() {
return new Value[]{Value.NOUN, Value.VERB};
}
},
PLURALITY("Plurality") {
enum Value {
SING("Singular"),
PL("Plural");
}
#Override
public Value[] getValues() {
return new Value[]{Value.SING, Value.PL};
}
},
}
I would at least want to be able to do something like:
word.features.put(TYPE, TYPE.Value.NOUN);
word.features.put(PLURALITY, PLURALITY.Value.PL);
So that it's easy to see that the values match the key, but the enum within enum syntax doesn't seem to be allowed.
I also tried this:
TYPE("Type") {
public String NOUN = "Noun";
public String VERB = "Verb";
but I couldn't reference TYPE.NOUN since they aren't allowed to be static for some reason.
Please is there someone who know a good pattern to specifying something like this? I'm just worried if use strings in my code like
word.features.put(TYPE, "Noun");
I am asking for trouble with typos etc.

You can't do it like that but you can do it like this:
// define a type values as an enum:
enum TypeValue {
Noun, Verb
}
// define an attribute class parametrized by an enum:
public class Attribute<E extends Enum<E>> {
// define your attribute types as static fields inside this class
public static Attribute<TypeValue> Type = new Attribute<TypeValue>();
}
// and now define your method like this:
<E extends Enum<E>, Feature extends Attribute<E>> void put(Feature feature, E value) {
}
// you will then have a compilation error when trying to invoke the method with improper associated parameters.
// eg if we define
enum OtherValue { X }
features.put(Attribute.Type, TypeValue.Noun); // ok
features.put(Attribute.Type, OtherValue.X); // Fails

Design pattern to handle settings in subclasses?

I have a small hierarchy of classes that all implement a common interface.
Each of the concrete class needs to receive a settings structure containing for instance only public fields. The problem is that the setting structure
has a part common to all classes
has another part that vary from one concrete class to another
I was wondering if you had in your mind any elegant design to handle this. I would like to build something like:
BaseFunc doer = new ConcreteImplementation1();
with ConcreteImplementation1 implements BaseFunc. And have something like
doer.setSettings(settings)
but have the ''settings'' object having a concrete implementation that would be suitable to ConcreteImplementation1.
How would you do that?

This may be a named design pattern, if it is, I don't know the name.
Declare an abstract class that implements the desired interface. The abstract class constructor should take an instance of your settings object from which it will extract the global settings. Derive one or more classes from the abstract class. The derived class constructor should take an instance of your settings object, pass it to the parent class constructor, then extract any local settings.
Below is an example:
class AbstractThing implements DesiredInterface
{
private String globalSettingValue1;
private String globalSettingValue2;
protected AbstractThing(Settings settings)
{
globalSettingValue1 = settings.getGlobalSettingsValue1();
globalSettingValue2 = settings.getGlobalSettingsValue2();
}
protected String getGlobalSettingValue1()
{
return globalSettingValue1;
}
protected String getGlobalSettingValue2()
{
return globalSettingValue2;
}
}
class DerivedThing extends AbstractThing
{
private String derivedThingSettingValue1;
private String derivedThingSettingValue2;
public DerivedThing(Settings settings)
{
super(settings);
derivedThingSettingValue1 = settings.getDerivedThingSettingsValue1();
derivedThingSettingValue2 = settings.getDerivedThingSettingsValue2();
}
}

Have a matching hierarchy of settings objects, use Factory to create the settings that match a specific class.

Sounds like you need a pretty standard Visitor pattern.
To put it simple, suppose, that all your properties are stored as key-value pairs in maps. And you have 3 classes in your hierarchy: A, B, C. They all implement some common interface CI.
Then you need to create a property holder like this:
public class PropertyHolder {
public Map<String, String> getCommonProperties () { ... }
public Map<String, String> getSpecialPropertiesFor (CI a) { return EMPTY_MAP; }
public Map<String, String> getSpecialPropertiesFor (A a) { ... }
public Map<String, String> getSpecialPropertiesFor (B b) { ... }
...
}
All your classes should implement 1 method getSpecialProperties which is declared in the interface CI. The implementation as simple as:
public Map<String, String> getSpecialProperties (PropertyHolder holder) {
return holder.getSpecialPropertiesFor (this);
}

I went down this route once. It worked, but after decided it wasn't worth it.
You can define a base class MyBean or something, and it has its own mergeSettings method. Every class you want to use this framework can extend MyBean, and provide its own implementation for mergeSettings which calls the superclasses mergeSettings. That way the common fields can be on the super class. If you want to get really fancy you can define and interface and abstract class to really make it pretty. And while your at it, maybe you could use reflection. anyway, mergeSettings would take a Map where the key is the property name. Each class would have its constants to related to the keys.
class MyBean extends AbstractMyBean ... {
public static final String FIELD1 = 'field1'
private String field1
public mergeSettings(Map<String, Object> incoming) {
this.field1 = incoming.get(FIELD1);
// and so on, also do validation here....maybe put it on the abstract class
}
}
Its a lot of work for setters though...

I started toying with a new pattern that I called "type-safe object map". It's like a Java Map but the values have type. That allows you to define the keys that each class wants to read and get the values in a type safe way (with no run-time cost!)
See my blog for details.
The nice thing about this is that it's still a map, so you can easily implement inheritance, notification, etc.

You could use Generics to define what kind of settings this instance need. Something like this:
public abstract class MySuperClass<T extends MySettingsGenericType>{
public MySuperClass(T settings){
//get your generic params here
}
}
public class MyEspecificClass extends MySuperClass<T extends MySettingsForThisType>{
public MySuperClass(T settings){
super(settings);
//Get your espefic params here.
}
}
//and you could use this
BaseFunc doer = new ConcreteImplementation1(ConcreteSettingsFor1);
//I dont compile this code and write in a rush. Sorry if have some error...