Day 9 implementation in Java

java/src/main/java/day9/Main.java

@@ -0,0 +1,45 @@
+package day9;
+
+import io.vavr.collection.List;
+import util.FileReader;
+
+import java.util.ArrayList;
+
+public class Main {
+    private static final boolean IS_PREDICTING_PREVIOUS_VALUES = true;
+
+    public static void main(String[] args) {
+        String inputFilePath = System.getProperty("user.dir") + "\\java\\src\\main\\java\\day9\\input.txt";
+        FileReader reader = new FileReader(inputFilePath);
+        List<String> inputData = List.ofAll(reader.readContents());
+
+        Long predictionsSum = inputData
+            .map(string -> string.split(" "))
+            .map(splitString -> List.of(splitString).map(Long::parseLong))
+            .map(Main::predictValue)
+            .reduce(Long::sum);
+
+        System.out.println(predictionsSum);
+    }
+
+    private static Long predictValue(List<Long> values) {
+        if (values.filter(value -> value != 0L).isEmpty()) {
+            return 0L;
+        }
+
+        Integer predictionIndex = IS_PREDICTING_PREVIOUS_VALUES ? 0 : values.length() - 1;
+        Integer predictionSign = IS_PREDICTING_PREVIOUS_VALUES ? -1 : +1;
+
+        List<Long> reducedValues = getDifferencesList(values);
+
+        return values.get(predictionIndex) + (predictionSign * predictValue(reducedValues));
+    }
+
+    private static List<Long> getDifferencesList(List<Long> values) {
+        java.util.List<Long> differencesList = new ArrayList<>();
+        for (int i = 1; i < values.length(); i++) {
+            differencesList.add(values.get(i) - values.get(i-1));
+        }
+        return List.ofAll(differencesList);
+    }
+}