hangman.css

.word {
	-fx-font-family: sans-serif ;
	-fx-font-size: 32pt ;
	-fx-padding: 12 ;
	-fx-min-width: 300 ;
}

.controls {
	-fx-alignment: center ;
	-fx-padding: 12 ;
}

.buttons {
	-fx-hgap: 2;
	-fx-vgap: 2;
	-fx-padding: 12 ;
}

.letter-button {
	-fx-max-width: infinity ;
	-fx-max-height: infinity ;
}

.life-lost {
	-fx-background-color: -fx-color, black, -fx-color ;
	-fx-background-insets: 0, 2, 4 ;
	-fx-min-width: 36 ;
	-fx-min-height: 36 ;
	-fx-font-size: 24 ;
	-fx-font-family: "comic sans ms" ;
}

.life-lost .text {
	-fx-fill: red ;
}

Hangman.java

package hangman;

import java.util.function.Supplier;
import java.util.stream.Collectors;

import javafx.application.Application;
import javafx.beans.binding.Bindings;
import javafx.beans.binding.StringBinding;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.TilePane;
import javafx.stage.Stage;

public class Hangman extends Application {
    
    private StringBinding wordBinding;
    private StringBinding statusBinding;

	@Override
	public void start(Stage primaryStage) {
	    
	    Supplier<String> wordGenerator = new WordGenerator();
	    
	    HangmanGameState game = new HangmanGameState(wordGenerator.get());
	    
		BorderPane root = new BorderPane();
		
		Label word = new Label();
		
		wordBinding = Bindings.createStringBinding(() -> {
		    if (game.isWordGuessed() || game.isLost()) {
		        return game.getWord();
		    }
		    return game.getWord()
		        .chars()
		        .map(c -> game.getGuesses().contains((char)c) ? (char)c : '?')
		        .mapToObj(c -> Character.toString((char)c))
		        .collect(Collectors.joining());
		}, game.wordProperty(), game.getGuesses(), 
		   game.wordGuessedProperty(), game.lostProperty());
		
		
        word.textProperty().bind(wordBinding);
		
        HBox livesRemaining = new HBox();
        for (int i = 0 ; i < HangmanGameState.LIVES ; i++) {
            Label lifeLost = new Label("X");
            StackPane lifeLostHolder = new StackPane(lifeLost);
            lifeLostHolder.getStyleClass().add("life-lost");
            livesRemaining.getChildren().add(lifeLostHolder);
            lifeLost.visibleProperty().bind(
                    game.wrongGuessesProperty().greaterThan(i));
        }
		
		Label statusLabel = new Label();
		statusBinding = Bindings
		        .when(game.lostProperty())
		        .then("You lost!")
		        .otherwise("You won!");
        statusLabel.textProperty().bind(statusBinding);
        statusLabel.getStyleClass().add("status");
		
		Button playAgain = new Button("Play again");
		playAgain.setOnAction(e -> game.restartGame(wordGenerator.get()));
		
		HBox controls = new HBox(5, statusLabel, playAgain);
		controls.getStyleClass().add("controls");
		controls.visibleProperty().bind(
		        game.wordGuessedProperty().or(game.lostProperty()));
		
		TilePane buttons = new TilePane();
		buttons.getStyleClass().add("buttons");
		
		for (char c = 'A'; c <= 'Z'; c++) {
		    Character ch = new Character(c);
		    Button button = new Button(ch.toString());
		    button.disableProperty().bind(Bindings.createBooleanBinding(
		            () -> game.getGuesses().contains(ch), 
		            game.getGuesses()));
		    button.visibleProperty().bind(button.disableProperty().not());
		    button.setOnAction(e -> game.getGuesses().add(ch));
		    button.getStyleClass().add("letter-button");
		    buttons.getChildren().add(button);
		}
		
		StackPane wordHolder = new StackPane(word);
		wordHolder.getStyleClass().add("word");
		
		root.setCenter(wordHolder);
		root.setRight(buttons);
		root.setTop(livesRemaining);
		root.setBottom(controls);
		
		Scene scene = new Scene(root);
		scene.getStylesheets().add(
		        getClass().getResource("hangman.css").toExternalForm());
		primaryStage.setScene(scene);
		primaryStage.show();
	}

	public static void main(String[] args) {
		launch(args);
	}
}

HangmanGameState.java

package hangman;

import javafx.beans.binding.Bindings;
import javafx.beans.property.ReadOnlyBooleanProperty;
import javafx.beans.property.ReadOnlyBooleanWrapper;
import javafx.beans.property.ReadOnlyIntegerProperty;
import javafx.beans.property.ReadOnlyIntegerWrapper;
import javafx.beans.property.SimpleStringProperty;
import javafx.beans.property.StringProperty;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;

public class HangmanGameState {

    private final StringProperty word ;
    
    public final static int LIVES = 6 ;

    public final StringProperty wordProperty() {
        return this.word;
    }

    public final String getWord() {
        return this.wordProperty().get();
    }

    public final void setWord(final String word) {
        this.wordProperty().set(word);
    }
    
    private final ObservableList<Character> guesses = 
            FXCollections.observableArrayList();
    
    public ObservableList<Character> getGuesses() {
        return guesses ;
    }
    
    private final ReadOnlyIntegerWrapper wrongGuesses = 
            new ReadOnlyIntegerWrapper();

    public final ReadOnlyIntegerProperty wrongGuessesProperty() {
        return this.wrongGuesses.getReadOnlyProperty();
    }

    public final int getWrongGuesses() {
        return this.wrongGuessesProperty().get();
    }
    
    private final ReadOnlyBooleanWrapper wordGuessed = 
            new ReadOnlyBooleanWrapper() ;
    
    public final ReadOnlyBooleanProperty wordGuessedProperty() {
        return this.wordGuessed.getReadOnlyProperty();
    }

    public final boolean isWordGuessed() {
        return this.wordGuessedProperty().get();
    }    
    
    private final ReadOnlyBooleanWrapper lost = new ReadOnlyBooleanWrapper() ;
    
    public ReadOnlyBooleanProperty lostProperty() {
        return lost.getReadOnlyProperty() ;
    }
    
    public final boolean isLost() {
        return lostProperty().get();
    }

    public HangmanGameState(String initialWord) {
        
        word = new SimpleStringProperty(initialWord);
        
        wrongGuesses.bind(Bindings.createIntegerBinding(() -> {
            return (int) guesses.stream()
                    .map(c -> c.toString())
                    .filter(c -> ! word.get().contains(c))
                    .count();
        }, word, guesses));
        
        wordGuessed.bind(Bindings.createBooleanBinding(() -> {
            return word.get()
                    .chars()
                    .mapToObj((int c) -> Character.valueOf((char)c))
                    .allMatch(guesses::contains);
                    
        }, word, guesses));
        
        lost.bind(wrongGuesses.greaterThanOrEqualTo(LIVES));
        
    }
    
    public void restartGame(String newWord) {
        guesses.clear();
        word.set(newWord);
    }

    
}

WordGenerator.java

package hangman;

import java.util.Arrays;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.function.Supplier;

public class WordGenerator implements Supplier<String> {
    
    private final String allWords = "Accident agree arrive astronomy "
            + "atlas attention award aware Balance banner bare base "
            + "beach besides blast board bounce brain branch brave "
            + "bright Cage calf calm career center cheer chew claw "
            + "clear cliff club collect connect core corner couple crowd "
            + "curious Damp dangerous dash dawn deep demolish design "
            + "discard dive dome doubt dozen Earth enemy evening exactly "
            + "excess Factory fair famous feast field finally flap float "
            + "flood fold fresh frighten fuel Gap gaze gift gravity greedy "
            + "Harm herd Idea insect instrument invent island Leader leap "
            + "lizard local lonely luxury March mention motor Nervous net "
            + "nibble notice Ocean Pack pale parade past peak planet "
            + "present proof Reflect rumor Safe scholar seal search settle "
            + "share shelter shiver shy skill slight smooth soil stack "
            + "steady strand stream support Team telescope tiny tower "
            + "travel tremble Universe Village Warn weak wealthy whisper "
            + "wise wonder worry Yard Zigzag" ;
    
    private final List<String> words ;
    
    private Iterator<String> iterator ;
    
    public WordGenerator() {
        words = Arrays.asList(allWords.split(" ")) ;
        Collections.shuffle(words);
        iterator = words.iterator() ;
    }
        
    public String get() {
        if (! iterator.hasNext()) {
           Collections.shuffle(words);
           iterator = words.iterator() ;
        }
        return iterator.next().toUpperCase() ;
    }
}