These are two examples on how to implement a Strategy pattern design using Java 8 functional style together with Cyclops pattern matching and Hamcrest libraries.

PrintDependingOnInput method is a strategy that will System.println some message based on the log passed.

AddPrefix is another strategy that will add a prefix to a message based on the message content.

package com.marco.patternmatching; 

 

import static org.hamcrest.Matchers.containsString;
import static org.hamcrest.Matchers.not;
import static org.hamcrest.Matchers.startsWith;
import static org.hamcrest.core.AllOf.allOf;
import java.util.ArrayList;
import java.util.List;
import com.aol.cyclops.matcher.builders.Matching; 

 

public class FunctionalStrategy {

 public static void main(String[] args) {
 List<String> toTest = new ArrayList<>();
 toTest.add("INFO everything is fine");
 toTest.add("WARN something weird happened");
 toTest.add("ERROR NullPointerException");
 toTest.add("ERROR IOException");
 
 toTest.stream().forEach(FunctionalStrategy::printDependingOnInput);

 System.out.println("--------------------");

 List<String> messages = new ArrayList<>();
 messages.add("everything is fine");
 messages.add("something weird happened");
 messages.add("NullPointerException");
 messages.add("IOException");

 messages.stream().map(FunctionalStrategy::addPrefix).forEach(System.out::println);
 }

 
 public static void printDependingOnInput(String log) {

 Matching
 .when().isMatch(startsWith("INFO"))
 .thenConsume(System.out::println)
 .when().isMatch(startsWith("WARN"))
 .thenConsume(message -> System.out.println("Found one warning : " + message))
 .when().isMatch(allOf(startsWith("ERROR"), containsString("NullPointerException")))
 .thenConsume(message -> System.err.println(message))
 .when().isMatch(allOf(startsWith("ERROR"), containsString("IOException")))
 .thenConsume(message -> System.err.println(message + " Retrying a couple of times"))
 .match(log);

 }

 public static String addPrefix(String log) {

 return Matching
 .when().isMatch(allOf(not(containsString("Exception")), not(containsString("weird"))))
 .thenApply(message -> "INFO " + message)
 .when().isMatch(containsString("weird"))
 .thenApply(message -> "WARN " + message)
 .when().isMatch(containsString("Exception"))
 .thenApply(message -> "ERROR " + message)
 .match(log).get();

 }
}

Nice and clean ;)