Day 1 implementation in Java

java/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>org.example</groupId>
+    <artifactId>java</artifactId>
+    <version>1.0-SNAPSHOT</version>
+
+    <properties>
+        <maven.compiler.source>8</maven.compiler.source>
+        <maven.compiler.target>8</maven.compiler.target>
+    </properties>
+
+</project>

java/src/main/java/day1/Main.java

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+package day1;
+
+import util.FileReader;
+
+import java.util.List;
+import java.util.stream.Collectors;
+
+public class Main {
+    // Part 1 one of the problem has this flag set to false, while part 2 has it set to true
+    private static final boolean IS_NUMBER_SPELLING_ENABLED = true;
+
+    public static void main(String[] args) {
+        String inputFilePath = System.getProperty("user.dir") + "\\java\\src\\main\\java\\day1\\input.txt";
+        FileReader reader = new FileReader(inputFilePath);
+        List<String> inputData = reader.readContents();
+
+        Long result = inputData.stream()
+            .map(string -> IS_NUMBER_SPELLING_ENABLED ? replaceSpelledDigits(string) : string)
+            .map(string -> string.chars()
+                .mapToObj(encodedChar -> (char) encodedChar)
+                .collect(Collectors.toList()))
+            .map(charArray -> charArray.stream()
+                .filter(Character::isDigit)
+                .map(String::valueOf)
+                .map(Long::parseLong)
+                .collect(Collectors.toList()))
+            .map(longArray -> {
+                Long first = longArray.get(0);
+                Long last = longArray.get(longArray.size() - 1);
+                return first * 10 + last;
+            })
+            .reduce(Long::sum)
+            .orElseThrow(RuntimeException::new);
+
+        System.out.println("Result: " + result);
+    }
+
+    private static String replaceSpelledDigits(String string) {
+        return string
+            .replaceAll("oneight", "18")
+            .replaceAll("threeight", "38")
+            .replaceAll("fiveight", "58")
+            .replaceAll("nineight", "98")
+            .replaceAll("twone", "21")
+            .replaceAll("eightwo", "82")
+            .replaceAll("eighthree", "83")
+            .replaceAll("one", "1")
+            .replaceAll("two", "2")
+            .replaceAll("three", "3")
+            .replaceAll("four", "4")
+            .replaceAll("five", "5")
+            .replaceAll("six", "6")
+            .replaceAll("seven", "7")
+            .replaceAll("eight", "8")
+            .replaceAll("nine", "9");
+    }
+}
+

java/src/main/java/util/FileReader.java

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+package util;
+
+import java.io.File;
+import java.io.InputStream;
+import java.nio.file.Files;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Scanner;
+
+public class FileReader {
+    private final String filePath;
+
+    public FileReader(String filePath) {
+        this.filePath = filePath;
+    }
+
+    public List<String> readContents() {
+        File file = new File(filePath);
+        try (InputStream inputStream = Files.newInputStream(file.toPath())) {
+            Scanner scanner = new Scanner(inputStream);
+            scanner.useDelimiter("\n");
+            List<String> data = new ArrayList<>();
+            while (scanner.hasNext()) {
+                data.add(scanner.next());
+            }
+            return data;
+        } catch (Exception e) {
+            System.out.println("Error while parsing input data");
+            throw new RuntimeException(e);
+        }
+    }
+}