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Pattern to break up external API calls?

I am trying to wrap my mind around how I would go about implementing this specific problem. We have an external API that performs some logic and returns back the result of that logic. Unfortunately, the API returns a new object as the result as opposed to just the part that I am interested in. For example, the code would look something like this:
public class ComplexObject {
//lots of fields
}
public interface LogicApplier {
LogicResult applyLogic(ComplexObject obj);
}
public class LogicResult {
ComplexObject result;
public ComplexObject getResult();
}
public class FirstImplementation {
private LogicApplier _applier;
public Implementation(LogicApplier applier) {
_applier = applier;
}
public ComplexObject mainImplementation (ComplexObject apply) {
LogicResult logicResult = _applier.applyLogic(apply);
ComplexObject newComplexObject = logicResult.getResult();
//Do some other stuff with new ComplexObject
}
}
So question is: what would be the best way to put a limit on LogicApplier's "power" over FirstImplementation? For example, our motivation to call logic in the first place is to derive a missing field, let's say "name". This field could potentially be different in, say, SecondImplementation where that implementation is now looking to derive "street address" from LogicApplier API. However, there is nothing to stop LogicApplier from changing some other field, say "idNumber".
Is this best solved by a adding an interface for our specific implementations and manually mapping fields? Something like:
public interface SecondImplementationLogicApplier {
public String deriveAddress(ComplexObject o);
}
public class LimitedImplementationLogicApplier implements FirstImplementationLogicApplier, SecondImplementationLogicApplier {
LogicApplier _applier;
public LimitedImplementationLogicApplier(LogicApplier applier) {
_applier = applier;
}
public String deriveFirstName(ComplexObject o) {
LogicResult res = _applier.applyLogic(o);
return res.firstName;
}
public String deriveFirstName(ComplexObject o) {
LogicResult res = _applier.applyLogic(o);
return res.address;
}
}

I think you are on right track with your LimitedImplementationLogicApplier. You should guard objects in your domain from possible corruption from the outside. Only update fields that you need.
It looks like your ComplexObject is mutable. I'd consider hiding it behind the immutable interface (that don't have any setters or way to change the object exposed) and pass immutable interface into your LimitedImplementationLogicApplier so its got no chance of mutating ComplexObject.
If your API requires ComplexObject type and you can't change that, to prevent mutation you could:
Option 1
Create a clone of your base ComplexObject instance and pass it into the API. After you've got the result back, you update the needed fields on untouched base instance. This will work nicely if ComplexObject is "thing in itself" and changes in its state does not have side effects outside of the class instance, like changing databases or affecting other state.
If mutation of ComplexObject has side effects or may have them in future then its a real trouble.
Option 2
Inherit a ReadonlyComplexObject class from ComplexObject and pass that into the API. In ReadonlyComplexObject you will suppress all the behavior of the parent to prevent modification.
This is hacky in my opinion and will create more work later - if ComplexObject will be extended with new properties later you will need to make changes to ReadonlyComplexObject otherwise mutation will still occur.

Code repetition vs readablility

I have multiple services (in Spring MVC) that are children of a global Service. So I need to know about the best practice (or your opinions) with multiple methods with this example:
//Domain classes
public class MyParentObject{}
public class MyObj extends MyParentObject{}
//Services
public class MyParentObjectServiceImpl implements MyParentObjectService{
#Override
public MyParentObject findObjectByProp(String prop, String objectType){
//myCode (not abstract class)
}
}
public class MyObjServiceImpl extends MyParentObjectServiceImpl implements MyObjectService{
private myObjType = "MyObj";
#Override
public MyObj findMyObjByProp(String prop){
return (MyObj) super.findObjectByProp(prop, this.myObjType);
}
}
And in this approach, I use calls like this:
MyObj foo = myObjService.findMyObjByProp(prop);
So I need to know if this approach is "better" or more apropiate that calling directly the parent method with the second parameter. E.g:
MyObj foo = (MyObj)myParentObjectService.findObjectByProp(prop, "MyObj");
..and avoiding the creation of second methods, more specific. It is important to know that the children services will be created anyway, because we have lot of code that is specific of a domain objects.
I have the idea that the first approach is better, because is more readable, but I need to support that decision with some documents, blog, or opinions to discuss this designs with my colleagues.

This looks like a tagged class hierarchy. It's difficult to comment on the value of this design in general without knowing the details. However, a slightly different approach that I would recommend is to generify your base class to gain a little bit of type safety.
In particular:
public /* abstract */ class MyParentObjectServiceImpl<T extends MyParentObject>
implements MyParentObjectService{
MyParentObjectServiceImpl(Class<T> type) { this.type = type; }
private final Class<T> type; // subclasses provide this
#Override
public T findObjectByProp(String prop){
//you can use type for object specific stuff
}
}
public class MyObjServiceImpl extends MyParentObjectServiceImpl<MyObj>
// You might not need this interface anymore
// if the only method defined is findMyObjByProp
/* implements MyObjectService */ {
MyObjServiceImpl() {
super(MyObj.class);
}
#Override
public /* final */ MyObj findMyObjByProp(String prop) {
return (MyObj) super.findObjectByProp(prop, this.myObjType);
}
}
You definitely gain in type safety (casting will only appear in the base class), you get rid of the "tags" (the strings that identify the different objects) and possibly reduce the number of classes/interfaces required to implement the whole hierarchy. I successfully used this approach several times. Note that this works best if the base class is abstract. Food for thoughts.

Java - Alternatives to forcing subclass to have a static method

I often find I want to do something like this:
class Foo{
public static abstract String getParam();
}
To force a subclasses of Foo to return a parameter.
I know you can't do it and I know why you can't do it but the common alternative of:
class Foo{
public abstract String getParam();
}
Is unsatisfactory because it requires you to have an instance which is not helpful if you just want to know the value of the parameter and instantiating the class is expensive.
I'd be very interested to know of how people get around this without getting into using the "Constant Interface" anti pattern.
EDIT: I'll add some more detail about my specific problem, but this is just the current time when I've wanted to do something like this there are several others from the past.
My subclasses are all data processors and the superclass defines the common code between them which allows them to get the data, parse it and put it where it needs to go.
The processors each require certain parameters which are held in an SQL database. Each processor should be able to provide a list of parameters that it requires and the default values so the configuration database can be validated or initialised to defaults by checking the required parameters for each processor type.
Having it performed in the constructor of the processor is not acceptable because it only needs to be done once per class not once per object instance and should be done at system startup when an instance of each type of class may not yet be needed.

The best you can do here in a static context is something like one of the following:
a. Have a method you specifically look for, but is not part of any contract (and therefore you can't enforce anyone to implement) and look for that at runtime:
public static String getParam() { ... };
try {
Method m = clazz.getDeclaredMethod("getParam");
String param = (String) m.invoke(null);
}
catch (NoSuchMethodException e) {
// handle this error
}
b. Use an annotation, which suffers from the same issue in that you can't force people to put it on their classes.
#Target({TYPE})
#Retention(RUNTIME)
public #interface Param {
String value() default "";
}
#Param("foo")
public class MyClass { ... }
public static String getParam(Class<?> clazz) {
if (clazz.isAnnotationPresent(Param.class)) {
return clazz.getAnnotation(Param.class).value();
}
else {
// what to do if there is no annotation
}
}

I agree - I feel that this is a limitation of Java. Sure, they have made their case about the advantages of not allowing inherited static methods, so I get it, but the fact is I have run into cases where this would be useful. Consider this case:
I have a parent Condition class, and for each of its sub-classes, I want a getName() method that states the class' name. The name of the sub-class will not be the Java's class name, but will be some lower-case text string used for JSON purposes on a web front end. The getName() method will not change per instance, so it is safe to make it static. However, some of the sub-classes of the Condition class will not be allowed to have no-argument constructors - some of them I will need to require that some parameters are defined at instantiation.
I use the Reflections library to get all classes in a package at runtime. Now, I want a list of all the names of each Condition class that is in this package, so I can return it to a web front end for JavaScript parsing. I would go through the effort of just instantiating each class, but as I said, they do not all have no-argument constructors. I have designed the constructors of the sub-classes to throw an IllegalArgumentException if some of the parameters are not correctly defined, so I cannot merely pass in null arguments. This is why I want the getName() method to be static, but required for all sub-classes.
My current workaround is to do the following: In the Condition class (which is abstract), I have defined a method:
public String getName () {
throw new IllegalArugmentException ("Child class did not declare an overridden getName() method using a static getConditionName() method. This must be done in order for the class to be registerred with Condition.getAllConditions()");
}
So in each sub-class, I simply define:
#Override
public String getName () {
return getConditionName ();
}
And then I define a static getConditionName() method for each. This is not quite "forcing" each sub-class to do so, but I do it in a way where if getName() is ever inadvertently called, the programmer is instructed how to fix the problem.

It seems to me you want to solve the wrong problem with the wrong tool. If all subclasses define (can't really say inherit) your static method, you will still be unable to call it painlessly (To call the static method on a class not known at compile time would be via reflection or byte code manipulation).
And if the idea is to have a set of behaviors, why not just use instances that all implement the same interface? An instance with no specific state is cheap in terms of memory and construction time, and if there is no state you can always share one instance (flyweight pattern) for all callers.
If you just need to couple metadata with classes, you can build/use any metadata facility you like, the most basic (by hand) implementation is to use a Map where the class object is the key. If that suits your problem depends on your problem, which you don't really describe in detail.
EDIT: (Structural) Metadata would associate data with classes (thats only one flavor, but probably the more common one). Annotations can be used as very simple metadata facility (annotate the class with a parameter). There are countless other ways (and goals to achieve) to do it, on the complex side are frameworks that provide basically every bit of information designed into an UML model for access at runtime.
But what you describe (processors and parameters in database) is what I christened "set of behaviors". And the argument "parameters need to be loaded once per class" is moot, it completely ignores the idioms that can be used to solve this without needing anything 'static'. Namely, the flyweight pattern (for having only once instance) and lazy initialization (for doing work only once). Combine with factory as needed.

I'm having the same problem over and over again and it's hard for me to understand why Java 8 preferred to implement lambda instead of that.
Anyway, if your subclasses only implement retrieving a few parameters and doing rather simple tasks, you can use enumerations as they are very powerful in Java: you can basically consider it a fixed set of instances of an interface. They can have members, methods, etc. They just can't be instanciated (as they are "pre-instanciated").
public enum Processor {
PROC_IMAGE {
#Override
public String getParam() {
return "image";
}
},
PROC_TEXT {
#Override
public String getParam() {
return "text";
}
}
;
public abstract String getParam();
public boolean doProcessing() {
System.out.println(getParam());
}
}
The nice thing is that you can get all "instances" by calling Processor.values():
for (Processor p : Processorvalues()) {
System.out.println(String.format("Param %s: %s", p.name(), p.getParam()));
p.doProcessing();
}
If the processing is more complex, you can do it in other classes that are instanciated in the enum methods:
#Override
public String getParam() {
return new LookForParam("text").getParam();
}
You can then enrich the enumeration with any new processor you can think of.
The down side is that you can't use it if other people want to create new processors, as it means modifying the source file.

You can use the factory pattern to allow the system to create 'data' instances first, and create 'functional' instances later. The 'data' instances will contain the 'mandatory' getters that you wanted to have static. The 'functional' instances do complex parameter validation and/or expensive construction. Of course the parameter setter in the factory can also so preliminary validation.
public abstract class Processor { /*...*/ }
public interface ProcessorFactory {
String getName(); // The mandatory getter in this example
void setParameter(String parameter, String value);
/** #throws IllegalStateException when parameter validation fails */
Processor construct();
}
public class ProcessorA implements ProcessorFactory {
#Override
public String getName() { return "processor-a"; }
#Override
public void setParameter(String parameter, String value) {
Objects.requireNonNull(parameter, "parameter");
Objects.requireNonNull(value, "value");
switch (parameter) {
case "source": setSource(value); break;
/*...*/
default: throw new IllegalArgumentException("Unknown parameter: " + parameter);
}
}
private void setSource(String value) { /*...*/ }
#Override
public Processor construct() {
return new ProcessorAImpl();
}
// Doesn't have to be an inner class. It's up to you.
private class ProcessorAImpl extends Processor { /*...*/ }
}

Is my DAO strategy ok?

I'm using Hibernate. The question is at the bottom.
The current strategy
It's simple.
First of all, I have a basic Dao<T>.
public class Dao<T> {
private Class<T> persistentClass;
private Session session;
public Dao(Class<T> persistentClass) {
this.persistenClass = persistentClass;
this.session = HibernateUtil.getCurrentSession();
}
It's nice as a base class and it passes the most common methods up to its Session.
public T get(Serializable id) {
#SuppressWarnings("unchecked")
T t = (T) this.session.get(this.persistentClass, id);
return t;
}
protected Criteria getCriteria() {
return this.session.createCriteria(this.persistentClass);
}
When there's need to use queries on the model, it goes into a specific DAO for that piece of model, which inherits from Dao<T>.
public class DaoTask extends Dao<Task> {
public DaoTask() {
super(Task.class);
}
public List<Task> searchActiveTasks() {
#SuppressWarnings("unchecked")
List<Task> list = (List<Task>) this.getCriteria()
.add(Restrictions.eq("active", true))
.list();
return list;
}
}
This approach has always worked well.
However...
However, today I found that many times an instance needs reattachment to the Session and a line similar to the following ends up happening:
new Dao<Book>(Book.class).update(book);
... which I find to be bad, because
I don't like specifying the redundant Book.class
If ever a DaoBook arises, this construct will become obsolete.
So I turned Dao<T> into an abstract class, and went on to refactor the old code.
Question
In order to remove the Dao<T> references from the codebase, I thought of two approaches:
Create specific DAOs for every class that ever needs attachment, which would generate many almost empty DaoBooks and the sort.
Create a class that owns a Dao<Object> and exposes only the attachment methods (i.e. save(), update() etc).
I'm tending to go with #2, but I thought this "AttacherDao" pattern might be bad, so I'd like your opinion.
Any cons for #2? Also, do you find anything wrong with "the current strategy"?

Our approach is to have a DAO object (derived from a commonDao) for each persistent class. In fact we define interface for this DAO class and each DAO decides which interfaces are opened up.
Using the following code, user cannot delete the PersistentClass.
interface PersistentClassDao {
void save(PersistentClass persistentObject);
}
Class PersistentClassDaoImpl extends CommonDao implements PersistentClassDao {
void save(persistentObject) {
persist(persistentObject);
}
Even though it has some additional overhead, this approach helps in unit testing appropriate code before exposing an interface.

We've chosen an approach similar to lud0h's, with the following twist:
abstract class<T extends IModelObject> JdbcCrudDao<T>{
void create(T dbo){}
T findByFoo(String foo){}
void update(T dbo){}
void delete(T dbo){}
}
class BarDao extends JdbcCrudDao<Bar>{
}
But, the twist is that we selectively expose methods on the Dao through a facade and forward only those we absolutely must.
class BarController implements IController{
private static final BarDao dao;
// ...
void update( IBar bar ){
dao.update(bar);
}
}
The only short-coming in all this is it requires some casting about if you wish to hide your database keys behind an interface type (which we do), but it's a pretty minor inconvenience versus the alternative (database code outside of the Daos).

Couple of questions
Are you frequently creating your DAO to do a single task or are these long lived?
What about using a static function? Clearly your Book object can be bind the DAO function to without the Book.class reference...
Otherwise, I'm a little worried about keeping the session object around instead of fetching whatever the current session is - isn't it considered "bad" to have long lived session objects? I'm not a master of DAO, so maybe I'm missing something here.

Java: Design with Singletons and generics

I am using an interface called Predicate which is used for sifting through Collections. For example, I can define
public class BlackCatPredicate implements Predicate<Cat> {
public boolean evaluate( Cat c ) {
return c.isBlack();
}
}
and then use some utility findAll( Collection<T> coll, Predicate<T> pred) method to apply the predicate to a collection of Cats, and get just the black ones, etc.
My question is this: I'm finding black cats all over my code, so there is no need to keep instantiating the BlackCatPredicate over and over again. It should just have one instance. (A singleton?) But then, over the course of writing many predicates, I don't want to have to implement each one as a singleton. So -- what is the proper design here?

I'd use an anonymous class constant and put it with the class it operates on:
public class Cat{
public static final Predicate<Cat> BLACK_PREDICATE = new Predicate<Cat>(){
public boolean evaluate( Cat c ) {
return c.isBlack();
}
};
// Rest of the Cat class goes here
}
If the predicate has parameters, you can use a static factory method.
Edit: As was pointed out in the comments, depending on the usage patterns, it may result in clearer code to collect the predicate constants (and/or factory methods) in a separate class, either only those for Cat, or all of them. It depends mainly on their number, how much additional organization is helpful.

Something like this should work:
class Predicates
{
private static class BlackCatPredicate implements Predicate<Cat>
{
public boolean evaluate(final Cat c)
{
return c.isBlack();
}
}
private static final BlackCatPredicate = new BlackCatPredicate();
public static Predicate<Cat> getBlackCatPredicate()
{
return (blackCatPredicate);
}
}

You could make a generic factory that takes any predicate as a type arg - and then generates a single instance for a given predicate type.
Another more general approach would be to start using a dependency injection library - and do all of your object creation through it. Typically you can switch a type to be a singleton, if appropriate, with little change.

I wouldn't worry about creating extra BlackCatPredicate instances at all.
If you don't like writing new BlackCatPredicate() all over the place you can certainly add a static factory method so you can write BlackCatPredicate.getInstance() instead. Yet another option would be to create a separate class so you can write CatPredicates.getBlackCatPredicateInstance().
However this is only for abstracting the creation of the predicate from the client code, it has nothing to do with actual object creation. Dealing with short-lived objects is one of the things the JVM does best, so creating a bunch of extra BlackCatPredicate instances and discarding them immediately won't affect your performance in the slightest.