Initial structure

Author: rucek

src/main/java/org/kunicki/functional_java/Application.java

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+package org.kunicki.functional_java;
+
+import org.kunicki.functional_java.api.Api;
+import org.kunicki.functional_java.domain.Service;
+
+public class Application {
+
+    private void start() {
+        // load config
+        // create database connection pool
+
+        // create repository
+        final var service = new Service(/* repository */);
+        final var api = new Api(service);
+
+        // HTTP server
+        // ...
+    }
+
+    // @SpringBootApplication
+    public static void main(String[] args) {
+        // SpringApplication.run(Application.class, args);
+        new Application().start();
+    }
+}

src/main/java/org/kunicki/functional_java/Validation.java

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+package org.kunicki.functional_java;
+
+record Person(String name, int age) {
+}
+
+interface PersonValidator {
+
+    //region Common validation logic
+    default boolean isNameValid(String name) {
+        return name.startsWith("M");
+    }
+
+    default boolean isAgeValid(int age) {
+        return age == 38;
+    }
+    //endregion
+}
+
+class FailingFast implements PersonValidator {
+
+}
+
+class Accumulating implements PersonValidator {
+
+}

src/main/java/org/kunicki/functional_java/api/Api.java

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+package org.kunicki.functional_java.api;
+
+import org.kunicki.functional_java.domain.Service;
+
+public class Api {
+
+    private final Service service;
+
+    public Api(Service service) {
+        this.service = service;
+    }
+
+    public Response<?> findUserById(Long id) {
+        final var user = service.findUserById(id);
+        return user
+            .map(Response::ok)
+            .orElse(Response.notFound());
+
+        //region Questions
+            /*
+            Q: Can you tell whether the service can respond with expected/business/domain errors?
+             */
+            /*
+            Q: What happens to those errors?
+             */
+            /*
+            Q: Are GOTO statements good?
+             */
+            /*
+            Q: What if we represented expected errors as plain values?
+             */
+        //endregion
+    }
+}

src/main/java/org/kunicki/functional_java/api/Response.java

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+package org.kunicki.functional_java.api;
+
+sealed interface Response<T> {
+
+    record Ok<T>(T value) implements Response<T> {
+    }
+
+    record NotFound<T>() implements Response<T> {
+    }
+
+    record ServerError(String message) implements Response<String> {
+    }
+
+    //region Factory methods
+    static <T> Response<T> ok(T value) {
+        return new Ok<>(value);
+    }
+
+    static <T> Response<T> notFound() {
+        return new NotFound<>();
+    }
+
+    static Response<String> serverError(String message) {
+        return new ServerError(message);
+    }
+    //endregion
+}

src/main/java/org/kunicki/functional_java/common/Attempt.java

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+package org.kunicki.functional_java.common;
+
+public interface Attempt<A> {
+
+}

src/main/java/org/kunicki/functional_java/common/Or.java

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+package org.kunicki.functional_java.common;
+
+public sealed interface Or<L, R> {
+
+    record Left<L, R>(L value) implements Or<L, R> {
+    }
+
+    record Right<L, R>(R value) implements Or<L, R> {
+    }
+
+    //region Factory methods
+    static <L, R> Or<L, R> left(L value) {
+        return new Left<>(value);
+    }
+
+    static <L, B> Or<L, B> right(B value) {
+        return new Right<>(value);
+    }
+    //endregion
+}

src/main/java/org/kunicki/functional_java/domain/Data.java

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+package org.kunicki.functional_java.domain;
+
+class Data {
+
+    private Long id;
+
+    private String value;
+
+    public Data(Long id, String value) {
+        this.id = id;
+        this.value = value;
+    }
+
+    public Long getId() {
+        return id;
+    }
+
+    public void setId(Long id) {
+        this.id = id;
+    }
+
+    public String getValue() {
+        return value;
+    }
+
+    public void setValue(String value) {
+        this.value = value;
+    }
+
+    // equals, hashCode, toString
+}

src/main/java/org/kunicki/functional_java/domain/ImmutableData.java

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+package org.kunicki.functional_java.domain;
+
+//region Immutable Data
+
+class ImmutableData {
+
+}
+//endregion
+
+//region Record
+//endregion
+
+//region Record test
+
+class RecordTest {
+
+    public static void main(String[] args) {
+
+    }
+}
+//endregion

src/main/java/org/kunicki/functional_java/domain/Service.java

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+package org.kunicki.functional_java.domain;
+
+import java.util.Optional;
+
+public class Service {
+
+    //region Pure functions
+
+    //region Definition
+    /*
+     Q: What makes a function pure?
+        * the output only depends on the inputs
+        * no side effects
+     */
+    //endregion
+
+    long doStuff(final long n, final Data data) {
+        /*
+        P: mutates the argument
+         */
+        data.setValue("devoxxpl");
+
+        /*
+        P: interacts with I/O
+         */
+        System.out.println(n);
+
+        /*
+        P: depends on something external
+        P: the if statement does not have a value
+         */
+        if (System.currentTimeMillis() % 2 == 0) {
+            return n;
+        } else {
+            return n + 1;
+        }
+    }
+
+    //region Tips
+        /*
+        T: Use (immutable) values wherever possible
+         */
+        /*
+        T: Control the side effects
+        */
+        /*
+        T: Prefer immutability as a rule of thumb
+        */
+    //endregion
+
+    //endregion
+
+    //region Error handling
+    public Optional<User> findUserById(Long id) {
+        return Optional.of(new User(42L, "Jacek"));
+    }
+
+    //region Tips
+        /*
+        T: Return expected errors as plain values
+         */
+        /*
+        T: Keep a catch-all error handler for truly unexpected errors
+         */
+    //endregion
+    //endregion
+}

src/main/java/org/kunicki/functional_java/domain/User.java

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+package org.kunicki.functional_java.domain;
+
+public record User(Long id, String name) {
+}