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Java - how to call a method of corresponding class based on string using enum/factory pattern

Yes, I read many examples in web, but I didn't find a way how to call a method based on string value. May be I am not searching in right way... I wrote all code, but don't know how to call the method.
fyi: I don't want to use if else or switch case
Here is what I want:
I get the card reader type as String from database. I have to call the corresponding class' method.
My code:
LoginPanel.java
public class LoginPanel {
public static void main(String args[]) {
String readerType = "Omnikey5427-CK"; // I get this ("Omnikey5427-CK" or "Omnikey5427-G2") from a database as String
// I WANT TO CALL getCardNumber() method of respective class
}
}
ISmartCardReader.java
public interface ISmartCardReader {
public Integer getCardNumber();
}
Omnikey5427G2.java
public class Omnikey5427G2 implements ISmartCardReader {
public Omnikey5427G2() {
System.out.println("G222222222222222...");
}
public Integer getCardNumber() {
return 222;
}
}
Omnikey5427CK.java
public class Omnikey5427CK implements ISmartCardReader {
public Omnikey5427CK() {
System.out.println("CKKKKKKKKKKKKKKK...");
}
public Integer getCardNumber() {
return 111;
}
}
SmacrtCardEnumFactory.java
public enum SmacrtCardEnumFactory {
OMNIKEY5427CK("Omnikey5427-CK") {
public ISmartCardReader geInstance() {
return new Omnikey5427CK();
}
},
OMNIKEY5427G2("Omnikey5427-G2") {
public ISmartCardReader geInstance() {
return new Omnikey5427G2();
}
};
private String cardReaderName;
private SmacrtCardEnumFactory(String cardReaderName) {
this.cardReaderName = cardReaderName;
}
public String cardReaderName() {
return cardReaderName;
}
}

You can use valueOf() function of enum provided your enum sonstant names match strings used to lookup (you may use cardName.toUpper()). You may also create objects for all the card types and store them in a hash map and then lookup them. You can also write some fatory method, but this will be if-then-else or switch inside

You could iterate over the factory's values() and get the one that matches the string:
public enum SmacrtCardEnumFactory {
// current code omitted for brevity
public static getSmartCardReader(String name) {
return Arrays.stream(values())
.filter(r -> r.cardReaderName().equals(name))
.map(SmacrtCardEnumFactory::getInstance();
.orElse(null);
}
}

Which pattern or method to use when calling a regex check method in multiple if statements?

I created a factory pattern in my class.
In this class I injected classes which implements Command interface based on incoming String parameter.
Factory class
#Component
#RequiredArgsConstructor
public class CommandFactory {
private final ACommand aCommand;
private final BCommand bCommand;
private final CCommand cCommand;
private final DCommand dCommand;
private final ECommand eCommand;
private final FCommand fCommand;
public Command createCommand(String content) {
if (aCommand.isMatching(content)) {
return aCommand;
} else if (bCommand.isMatching(content)) {
return bCommand;
} else if (cCommand.isMatching(content)) {
return cCommand;
} else if (dCommand.isMatching(content)) {
return dCommand;
} else if (eCommand.isMatching(content)) {
return eCommand;
} else if (fCommand.isMatching(content)) {
return fCommand;
} else {
return null;
}
}
In isMatching() method there are different regex'es and I try to figure out how this incoming String should be processed.
I am looking for a cleaner way to get rid of these sequential if statements. Because whenever I create a new class into this factory I add another if statement.

Maybe Stream can help?
Stream<Command> stream = Stream.of(aCommand, bCommand, cCommand ...);
return stream.filter(x -> x.isMatching(content)).findFirst().orElse(null);
Now every time you add a new class, you just add a new object to the first line.

If you want to get rid of the sequential if statements you can use streams (like user Sweeper suggested) or loops and I would also suggest to return and optional which makes null handling clearer for the client.
Here are two suggested options to get rid of if else repetitions one with loops another with streams:
import java.util.Arrays;
import java.util.List;
import java.util.Optional;
public class CommandPatternExample {
private List<Command> candidates = Arrays.asList(new ACommand(), new BCommand(), new CCommand());
public Optional<Command> createCommand(String content) {
for(Command command : candidates) {
if(command.isMatching(content)) {
return Optional.of(command);
}
}
return Optional.empty();
}
public Optional<Command> createCommandStream(String content) {
return candidates.stream().filter(c -> c.isMatching(content)).findFirst();
}
}
interface Command<T> {
void execute(T obj);
boolean isMatching(String s);
}
class ACommand implements Command<String> {
#Override
public void execute(String obj) {
}
#Override
public boolean isMatching(String s) {
return "A".equals(s);
}
}
class BCommand implements Command<String> {
#Override
public void execute(String obj) {
}
#Override
public boolean isMatching(String s) {
return "B".equals(s);
}
}
class CCommand implements Command<String> {
#Override
public void execute(String obj) {
}
#Override
public boolean isMatching(String s) {
return "C".equals(s);
}
}

Map might be a good idea. Meaning if you place your command instances into a map as values where your key would be something that you could match against incoming String. Then instead of sequential search with Efficiency O(n) you can get much better performance O(1). This is a short answer.
Besides that There is an open source java library MgntUtils (wriiten by me) that contains some utility called "Self-instantiating factories" Basically it manages and the Factory for you. All you will need to do is to create a class that implements a certain interface and the utility will add it for you into a map based factory. It might be useful to you. Here is the link to an article that explains about the utilities in the library as well as where to get the library (Github and Maven central). In the article look for the paragraph "Lifecycle management (Self-instantiating factories)". Also library comes with a detailed written javadoc and code example for that feature.

enum singleton, referenced by interface, instance by string name

Thanks for viewing my question, which I have not successfully found an answer for in my searches/books. I'm learning java by writing a roguelike, but I think this question is more java-related than game-related. Feel free to educate me if I'm wrong.
I have similar classes that I want to each have specific abilities. The abilities are enum singletons with a set of standard method names that I would pass the Actor to - I wanted to avoid implementing methods from an interface in every Actor class, and just really liked the envisioned use of this approach as I go forward. I come from a shell/perl background and can't tell if I'm just not thinking OOP, or if I'm on to something useful and don't have the skills yet to pull it off.
addAbility(String) in StdActor is where it finally broke in this experiment.
Question is - am I doing something wrongheaded here? If not, how could I implement this?
interface for manipulating abilities:
public interface ActorAbility {
// doesn't work, but need something to enable
// instance retrieval for addAbility...
public ActorAbility getInstance();
public void act(Actor actor);
public boolean isTickable();
}
sanitized implementation of interface:
public enum ActorMove implements ActorAbility {
INSTANCE;
private ActorMove() {
}
public ActorAbility getInstance() {
return INSTANCE;
}
public void act(Actor actor) {
log.debug("Move");
}
public boolean isTickable() {
return true;
}
}
sanitized use of the ability. trial and error run amock. addAbility(String) broken, copy/paste from SO and elsewhere. it probably needs to be nuked from orbit.
public class StdActor implements Actor {
private HashSet<ActorAbility> abilities = new HashSet<>();
// this whole method is wrecked
public void addAbility(String ability) {
// Class<? extends ActorAbility> action; // in a maze of twisty passages...
ActorAbility actionInstance = null;
try {
// action = Class.forName("game3.Actors.Abilities." + ability);
actionInstance = ActorAbility.valueOf("game3.Actors.Abilities."
+ ability);
} catch (Exception e) {
e.printStackTrace();
}
this.abilities.add(actionInstance);
}
}
use case:
public class StdCharClass extends StdActor {
public StdCharClass() {
// I like this because it's clean and easily
// changeable
addAbility("ActorMove");
}
}
future planned use:
HashSet<ActorAbility> abilities = actor.getAbilities();
for (ActorAbility ability : abilities) {
if (ability.isTickable()) {
ability.act(actor);
}
}
Thanks!
EDIT:
Thanks for such a quick comment, JB. I tried what you suggested and it appears to do what I was hoping. It appears I was just off in the weeds and needed to be pulled back.

new class:
public enum Ability {
MOVE(ActorMove.INSTANCE), FIGHT(ActorFight.INSTANCE);
private ActorAbility ability;
private Ability(ActorAbility abilityClass) {
this.ability = abilityClass;
}
public ActorAbility getAbility() {
return this.ability;
}
}
StdActor:
public class StdActor implements Actor {
private HashSet<Ability> abilities = new HashSet<>();
public void addAbility(Ability ability) {
this.abilities.add(ability);
}
subclass:
public class StdCharClass extends StdActor {
public StdCharClass() {
addAbility(Ability.MOVE);
}
}
and finally, usage:
HashSet<Ability> abilities = bob.getAbilities();
for (Ability ability : abilities) {
ActorAbility abilityClass = ability.getAbility();
if (abilityClass.isTickable()) {
abilityClass.act(bob);
}
}
output!
12:44:15.835 [main] DEBUG ActorMove - Move

Enum's toString method and Java good practice

I'd like to ask about good coding practice in Java. I want to create a enumeratoin of some properties and override toString() to use it in the following way (JSF 1.2 is used in order to retrieve localized message):
package ua.com.winforce.casino.email.util;
import java.text.MessageFormat;
import java.util.Locale;
import java.util.MissingResourceException;
import java.util.ResourceBundle;
import javax.faces.context.FacesContext;
public enum StatisticItems {
MESSAGES,
VIEWS;
private static String BUNDLE_NAME = "messages";
public String toString(){
switch (this) {
case MESSAGES:
return getLocalizedMsg("messages.title");
case VIEWS:
return getLocalizedMsg("views.title");
default:
return null;
}
}
private static String getLocalizedMsg(String key, Object... arguments) {
Locale locale = FacesContext.getCurrentInstance().getViewRoot().getLocale();
String resourceString;
try {
ResourceBundle bundle = ResourceBundle.getBundle(BUNDLE_NAME, locale);
resourceString = bundle.getString(key);
} catch (MissingResourceException e) {
return key;
}
if (arguments == null) {
return resourceString;
}
MessageFormat format = new MessageFormat(resourceString, locale);
return format.format(arguments);
}
}
My question is about good practice. Is it considered good to put all such methods within a enum definition? If not, I'd like to understand why and of course how to do it better.

There are two points to be made here:
If the default case (returning null in your code) is a runtime error, then using a switch is kind of error prone. There are two better alternatives in my opinion:
Use a field localizationKey, initialized in the constructor of the enum instances, and refer to this key in the toString method
Or, (for more complicated cases) make the toString abstract and force each instance to override with proper implementation. See this question for example.
Many people argue that toString is intended either for really obvious implementations, otherwise only for debugging. (See this question for a good elaboration.) My recommendation: Come up with a more descriptive method name and don't reuse toString just for convenience.
Update: Zooming out from Java semantics a bit: This logic belongs to the view and not the model, as pointed out by BalusC in the comments below.

I would ensure all of that complex logic is done once only at initialise time.
public enum StatisticItems {
MESSAGES("messages"),
VIEWS("views");
final String asString;
StatisticItems(String localised) {
asString = getLocalizedMsg(localised + ".title");
}
#Override
public String toString () {
return asString;
}
private static String BUNDLE_NAME = "messages";
private static final Locale locale = FacesContext.getCurrentInstance().getViewRoot().getLocale();
private static final ResourceBundle bundle = ResourceBundle.getBundle(BUNDLE_NAME, locale);
private static String getLocalizedMsg(String key) {
String resourceString;
try {
resourceString = bundle.getString(key);
} catch (MissingResourceException e) {
return key;
}
if (arguments == null) {
return resourceString;
}
MessageFormat format = new MessageFormat(resourceString, locale);
return format.format(arguments);
}
}

Replacing if else statement with pattern

I have a if else statement which might grow in the near future.
public void decide(String someCondition){
if(someCondition.equals("conditionOne")){
//
someMethod("someParameter");
}else if(someCondition.equals("conditionTwo")){
//
someMethod("anotherParameter");
}
.
.
else{
someMethod("elseParameter");
}
}
Since, this is already looking messy, I think it would be better if I can apply any design patterns here. I looked into Strategy pattern but I am not sure if that will reduce if else condition here. Any suggestions?

This is a classic Replace Condition dispatcher with Command in the Refactoring to Patterns book.
Basically you make a Command object for each of the blocks of code in your old if/else group and then make a Map of those commands where the keys are your condition Strings
interface Handler{
void handle( myObject o);
}
Map<String, Handler> commandMap = new HashMap<>();
//feel free to factor these out to their own class or
//if using Java 8 use the new Lambda syntax
commandMap.put("conditionOne", new Handler(){
void handle(MyObject o){
//get desired parameters from MyObject and do stuff
}
});
...
Then instead of your if/else code it is instead:
commandMap.get(someCondition).handle(this);
Now if you need to later add new commands, you just add to the hash.
If you want to handle a default case, you can use the Null Object pattern to handle the case where a condition isn't in the Map.
Handler defaultHandler = ...
if(commandMap.containsKey(someCondition)){
commandMap.get(someCondition).handle(this);
}else{
defaultHandler.handle(this);
}

Let's assume that we have such code (which is the same as yours):
public void decide(String someCondition) {
if(someCondition.equals("conditionOne")) {
someMethod("someParameter");
}
else if(someCondition.equals("conditionTwo")) {
someMethod("anotherParameter");
}
else {
someMethod("elseParameter");
}
}
Assuming that you don't want to refactor other parts of the application and you don't want to change method signature there are possible ways in which it could be refactored:
Warning - You should use generic versions of mentioned patterns.
I showed non generic ones because it is easier to read them.
Strategy + Factory Method
We can use Strategy and Factory Method patterns. We also take advantage of polymorphism.
private final StrategyConditionFactory strategyConditionFactory = new StrategyConditionFactory();
public void decide(String someCondition) {
Strategy strategy = strategyConditionFactory.getStrategy(someCondition)
.orElseThrow(() -> new IllegalArgumentException("Wrong condition"));
strategy.apply();
}
It would be better to design it in a way that else condition is included in the factory, and developer calls it on purpose. In such case we throw exception when condition is not meet. Alternatively we could write it exactly as it was in question. If you want so instead of .orElseThrow(() -> new IllegalArgumentException("Wrong condition")); put .orElse(new ElseStrategy());
StrategyConditionFactory (factory method):
public class StrategyConditionFactory {
private Map<String, Strategy> conditions = new HashMap<>();
public StrategyConditionFactory() {
conditions.put("conditionOne", new ConditionOneStrategy());
conditions.put("conditionTwo", new ConditionTwoStrategy());
//It is better to call else condition on purpose than to have it in the conditional method
conditions.put("conditionElse", new ElseStrategy());
//...
}
public Optional<Strategy> getStrategy(String condition) {
return Optional.ofNullable(conditions.get(condition));
}
}
Strategy interface:
public interface Strategy {
void apply();
}
Implementations:
public class ConditionOneStrategy implements Strategy {
#Override
public void apply() {
//someMethod("someParameter");
}
}
public class ConditionTwoStrategy implements Strategy {
#Override
public void apply() {
//someMethod("anotherParameter")
}
}
public class ElseStrategy implements Strategy {
#Override
public void apply() {
//someMethod("elseParameter")
}
}
Usage (simplified):
public void strategyFactoryApp() {
//...
decide("conditionOne");
decide("conditionTwo");
decide("conditionElse");
//...
}
Strategy + Factory Method - this particular case (where only parameter changes)
We can use the fact that in this case we always call the same method, only parameter changes
We change our base strategy interface to abstract class with getParameter() method and we make new implementations of this abstract class. Other code remains the same.
public abstract class Strategy {
public abstract String getParameter();
public void apply() {
someMethod(getParameter());
}
private void someMethod(String parameter) {
//someAction
}
}
Implementations:
public class CondtionOneStrategy extends Strategy {
#Override
public String getParameter() {
return "someParameter";
}
}
public class CondtionTwoStrategy extends Strategy {
#Override
public String getParameter() {
return "anotherParameter";
}
}
public class ElseStrategy extends Strategy {
#Override
public String getParameter() {
return "elseParameter";
}
}
Enum + enum kinda "factory"
We might use Enum to implement strategy and instead of factory method we can use valueOf() from enum.
public void decide(String someCondition) {
ConditionEnum conditionEnum = ConditionEnum.valueOf(someCondition);
conditionEnum.apply();
}
Condition enum:
public enum ConditionEnum {
CONDITION_ONE {
#Override
public void apply() {
//someMethod("someParameter");
}
},
CONDITION_TWO {
#Override
public void apply() {
//someMethod("anotherParameter");
}
},
CONDITION_ELSE {
#Override
public void apply() {
//someMethod("elseParameter");
}
};
//...more conditions
public abstract void apply();
}
Usage (simplified):
public void enumFactoryApp() {
//...
decide("CONDITION_ONE");
decide("CONDITION_TWO");
decide("CONDITION_ELSE");
//...
}
Notice that you will get IllegalArgumentException when enum type has no constant with the specified name.
Command + Factory
The difference between strategy and command is that command holds also state, so if you have for example compute(int a, int b, String someCondition) and you want to refactor it with strategy including it's signature change you can reduce it to compute(int a, int b, ComputeStrategy computeStrategy) with command you can reduce it to one argument compute(ComputeCommand computeCommand). In this case we also take advantage of polymorphism similarly to strategy pattern case.
CommandConditionFactory commandConditionFactory = new CommandConditionFactory();
public void decide(String someCondition) {
Command command = commandConditionFactory.getCommand(someCondition)
.orElseThrow(() -> new IllegalArgumentException("Wrong condition"));
command.apply();
}
It would be better to design it in a way that else condition is included in the factory, and developer calls it on purpose. In such case we throw exception when condition is not meet. Alternatively we could write it exactly as it was in question. If you want so instead of .orElseThrow(() -> new IllegalArgumentException("Wrong condition")); put .orElse(new ElseCommand());
CommandConditionFactory (factory method):
public class CommandConditionFactory {
private Map<String, Command> conditions = new HashMap<>();
public CommandConditionFactory() {
conditions.put("conditionOne", new ConditionOneCommand("someParameter"));
conditions.put("conditionTwo", new ConditionTwoCommand("anotherParameter"));
//It is better to call else condition on purpose than to have it in the conditional method
conditions.put("conditionElse", new ElseCommand("elseParameter"));
//...
}
public Optional<Command> getCommand(String condition) {
return Optional.ofNullable(conditions.get(condition));
}
}
Command interface:
public interface Command {
void apply();
}
Implementations (there is some redundancy, but It is there to show how command should look in more general case where instead of someMethod() we have three different methods):
public class ConditionOneCommand implements Command {
private final String parameter;
public ConditionOneCommand(String parameter) {
this.parameter = parameter;
}
#Override
public void apply() {
//someMethod(parameter);
}
}
public class ConditionTwoCommand implements Command {
private final String parameter;
public ConditionTwoCommand(String parameter) {
this.parameter = parameter;
}
#Override
public void apply() {
//someMethod(parameter);
}
}
public class ElseCommand implements Command {
private final String parameter;
public ElseCommand(String parameter) {
this.parameter = parameter;
}
#Override
public void apply() {
//someMethod(parameter);
}
}
Usage (simplified):
public void commandFactoryApp() {
//...
decide("conditionOne");
decide("conditionTwo");
decide("conditionElse");
//...
}
Command + Factory - This particular case.
This in fact isn't a real command pattern just a derivative. It takes advantage of the fact that in this case we are always calling the same method someMethod(parameter) and only the parameter changes.
Abstract class:
public abstract class Command {
abstract void apply();
protected void someMethod(String parameter) {
//someAction
}
}
Implementation (the same for all 3 conditional cases):
public class CommandImpl extends Command {
private final String parameter;
public CommandImpl (String parameter) {
this.parameter = parameter;
}
#Override
public void apply(){
someMethod(parameter);
}
}
Factory, please notice that there is only one command implementation, only parameter changes:
public class CommandConditionFactory {
Map<String, Command> conditions = new HashMap<>();
public CommandConditionFactory() {
conditions.put("conditionOne", new CommandImpl("someParameter"));
conditions.put("conditionTwo", new CommandImpl("anotherParameter"));
//It is better to call else condition on purpose than to have it in the conditional method
conditions.put("conditionElse", new CommandImpl("elseParameter"));
//...
}
public Optional<Command> getCommand(String condition) {
return Optional.ofNullable(conditions.get(condition));
}
}
Nested if's
Note that even if you have nested ifs sometimes it is possible to refactor them and use one of the mentioned techniques.
Lets say that we have following code:
public void decide2(String someCondition, String nestedCondition) {
if(someCondition.equals("conditionOne")) {
if(nestedCondition.equals("nestedConditionOne")){
someLogic1();
}
else if(nestedCondition.equals("nestedConditionTwo")){
someLogic2();
}
}
else if(someCondition.equals("conditionTwo")) {
if(nestedCondition.equals("nestedConditionThree")){
someLogic3();
}
else if(nestedCondition.equals("nestedConditionFour")){
someLogic4();
}
}
}
You could refactor it using mathematical logic rules:
public void decide2(String someCondition, String nestedCondition) {
if(someCondition.equals("conditionOne")
&& nestedCondition.equals("nestedConditionOne")) {
someLogic1();
}
else if(someCondition.equals("conditionOne")
&& nestedCondition.equals("nestedConditionTwo")) {
someLogic2();
}
else if(someCondition.equals("conditionTwo")
&& nestedCondition.equals("nestedConditionThree")) {
someLogic3();
}
else if(someCondition.equals("conditionTwo")
&& nestedCondition.equals("nestedConditionFour")) {
someLogic4();
}
}
and then you can use strategy, enum or command. You just have a pair of Strings <String, String> instead of single String.
Decision Tables
When you have nested ifs that couldn't be refactored as mentioned you can implement your own decision tables or use some ready to go decision tables solution. I won't give the implementation there.
Rules Engine
When you have nested ifs that couldn't be refactored as mentioned you can also implement your own simple rules engine. You should use it only if you have many nested ifs, otherwise it is triumph of form over content.
For very complicated Business Logic there are professional Rule Engines like Drools.
I won't give the implementation there.
One more thing
In the example that you gave there is a high possibility that someone introduced these ifs, but they are totally redundant. And we can check it by trying to refactor decide method signature to make it take some other argument and to refactor surrounding code that is calling our method. By doing so we are getting rid of our Factory Method. There are examples that present how the code might look when it occurs that these ifs were redundant.
Strategy
Decide method:
public void decide(Strategy strategy) {
strategy.apply();
}
Usage (simplified):
public void strategyApp() {
//...
decide(new ConditionOneStrategy());
decide(new ConditionTwoStrategy());
decide(new ElseStrategy());
//...
}
Enum
Decide method:
public void decide(ConditionEnum conditionEnum) {
conditionEnum.apply();
}
Usage (simplified):
public void enumApp() {
//...
decide(ConditionEnum.CONDITION_ONE);
decide(ConditionEnum.CONDITION_TWO);
decide(ConditionEnum.CONDITION_ELSE);
//...
}
Command
Decide method:
public void decide(Command command) {
command.apply();
}
Usage (simplified):
public void commandApp() {
//...
decide(new ConditionOneCommand("someParameter"));
decide(new ConditionTwoCommand("anotherParameter"));
decide(new ElseCommand("elseParameter"));
//...
}
In fact it is quite specific case, there are cases in which for example we have to use simple type like String, because it comes from the external system or condition is based on integer from input so we can't refactor the code so easily.

The general recommendation by Martin Fowler is to
Replace Conditional with Polymorphism.
In terms of design patterns this would often be the Strategy Pattern
Replace Conditional Logic with Strategy.
If you have a small, finite set of conditions, I recommend to use an enum to implement the Strategy Pattern (provide an abstract method in the enum and override it for each constant).
public enum SomeCondition{
CONDITION_ONE{
public void someMethod(MyClass myClass){
//...
}
},
CONDITION_TWO{
public void someMethod(MyClass myClass){
}
}
public abstract void someMethod(MyClass myClass);
}
public class MyClass{
//...
public void decide(SomeCondition someCondition){
someCondition.someMethod(this);
}
}
If it's really just a parameter you want to pick, then you could define the enum like this instead:
public enum SomeCondition{
CONDITION_ONE("parameterOne"),
CONDITION_TWO("parameterTwo");
private final String parameter;
private SomeCondition(String parameter){
this.parameter = parameter;
}
public String getParameter(){
return parameter;
}
}
public class MyClass{
//...
public void decide(SomeCondition someCondition){
someMethod(someCondition.getParameter());
}
}

Another way to solve the current problem is to use Factory Pattern. This provides functionality to extract a factory method that returns an object of a given type and performs the operation based on the concrete object behavior.
public interface Operation {
String process(String a, String b);
}
The method takes two string as input and returns the result.
public class Concatenation implements Operation {
#Override
public String process(String a, String b) {
return a.concat(b);
}
}
public class Join implements Operation {
#Override
public String process(String a, String b) {
return String.join(", ", a, b);
}
}
And then we should define a factory class which returns instances of Operation based on the given operator:
public class OperatorFactory {
static Map<String, Operation> operationMap = new HashMap<>();
static {
operationMap.put("concatenation", new Concatenation());
operationMap.put("join", new Join());
// more operators
}
public static Optional<Operation> getOperation(String operator) {
return Optional.ofNullable(operationMap.get(operator));
}
}
And now we can use it:
public class SomeServiceClass {
public String processUsingFactory(String a, String b, String operationName) {
Operation operation = OperatorFactory
.getOperation(operationName)
.orElseThrow(() -> new IllegalArgumentException("Invalid Operation"));
return operation.process(a, b);
}
}

I guess you must have already considered it, but if you are using JDK 7 or above, you can switch on strings. That way your code can look cleaner than a bunch of if-else statements.