Core Java

Strategy Pattern in Java 8

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;


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");


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

  public static void printDependingOnInput(String log) {

                   .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"))


   public static String addPrefix(String log) {

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


Nice and clean ;)

Reference: Strategy Pattern in Java 8 from our JCG partner Marco Castigliego at the Remove duplication and fix bad names blog.

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8 years ago

Sorry, but reread the strategy pattern… This is not a Strategy Pattern but a simple if-else-based output.

Using the strategy Pattern, you can “plug-in” a new strategy without having to change any existing code…

Cheers, Arndt

8 years ago
Reply to  Arndt

Well, ok it is a strategy pattern with addPrefix being the strategy (my fault) – just forget my post

8 years ago

This is not nice example, totally waste of time.

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