hangman.java

// Main

package com.first;


public class Main {

    public static void main(String[] args) {
        HangmanConsole console = new HangmanConsole();
        console.startGame();
    }
}

// Word Provider

package com.first;

import java.util.Arrays;
import java.util.List;
import java.util.Random;

public class WordProvider {
    static List<String> getWords(){
        return Arrays.asList("animals", "programming", "god", "animation", "characters", "universe", "looser");
    }

    static String getRandomWord() {
        List words = getWords();
        return (String) words.get(new Random().nextInt(words.size()));
    }
}

// HandgmanConsole

package com.first;

import java.util.Scanner;

public class HangmanConsole {
    final HangmanWord word;
    final Scanner in = new Scanner(System.in);

    public HangmanConsole() {
        this.word = new HangmanWord();
    }

    public void startGame() {
        printHeader();
        printGame();
    }

    public void printHeader() {
        System.out.println("The game is started");
        System.out.println("Check your challenge:");
        System.out.println(word.getWordProgression());
        printDivider();
    }

    public void printGame() {
        while (!word.wonGame() && !word.isGameOver()){
            System.out.print(word.getWordProgression());
            System.out.print("\t\t\t\t\t\t\t\t");
            System.out.print("Life left: ");
            System.out.println(word.getLifeLeft());

            String letter = "";
            while (letter.length() != 1) {
                System.out.print("Give me a letter (must be a single character) =>");
                letter = in.next();
            }

            word.selectLetter(letter);
        }

        if (word.wonGame()) {
            printDivider();
            System.out.print("The word is: ");
            System.out.println(word.getWordProgression());
            System.out.println("YOU WON!");
        }
        else{
            printDivider();
            System.out.print("The word is: ");
            System.out.println(word.getWord());
            System.out.println("YOU LOSE!");
        }
    }

    public void printDivider() {
        System.out.println("============================================");
    }

}

// HandmanWorld

package com.first;

import java.util.ArrayList;
import java.util.HashMap;

public class HangmanWord {
    private static final int ALPHABET_COUNT = 26;
    private static final int maxLifePoints = 100;
    private final int failCoefficient;
    private final String word;
    private final HashMap<String, ArrayList<Integer>> wordIndex = new HashMap<>();
    private ArrayList<String> selectedCharacters = new ArrayList<>();
    // This could be reactive but for sake of simplicity we keep it like that
    private StringBuilder wordProgression;

    public HangmanWord() {
        this.word = WordProvider.getRandomWord();
        processWord();
        this.failCoefficient = ALPHABET_COUNT / this.wordIndex.size() * 2;
        this.wordProgression = new StringBuilder(new String(new char[this.word.length()]).replace("\0", "_"));
    }

    String getWord() {return word;}

    public String getWordProgression() {
        return wordProgression.toString();
    }

    public int getLifeLeft() {
        return maxLifePoints - getWrongSelectedCharacters().size() * failCoefficient;
    }

    public boolean isGameOver() {
        return getWrongSelectedCharacters().size() * failCoefficient >= maxLifePoints;
    }

    public boolean wonGame() {
        return wordProgression.toString().equals(word);
    }

    public void selectLetter(String letter) {
        if (!isGameOver() && !selectedCharacters.contains(letter)){
            selectedCharacters.add(letter);
            if (wordIndex.containsKey(letter)) {
                for (int index : wordIndex.get(letter)) {
                    wordProgression.setCharAt(index, letter.charAt(0));
                }
            }
        }
    }

    private ArrayList<String> getWrongSelectedCharacters() {
        ArrayList<String> result = new ArrayList<>();
        for (String character : selectedCharacters){
            if (!wordIndex.containsKey(character)) {
                result.add(character);
            }
        }
        return result;
    }

    private void processWord() {
        String[] letters = word.split("");
        for (int i = 0; i < letters.length; i++) {
            ArrayList<Integer> letterIndex = wordIndex.getOrDefault(letters[i], new ArrayList<>());
            letterIndex.add(i);
            wordIndex.put(letters[i], letterIndex);
        }
    }
}