JavaでもReactivePropertyしたいだけの人生だった ref: https://qiita.com/YSRKEN/items/723ebdc067dd8dd3ab42

ReactiveProperty.java

package com.github.jrper;

import javafx.beans.InvalidationListener;
import javafx.beans.property.Property;
import javafx.beans.property.SimpleObjectProperty;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;

import java.util.function.BiFunction;
import java.util.function.Consumer;
import java.util.function.Function;

public class ReactiveProperty<T> implements Property<T> {
    private Property<T> value;
    // コンストラクタ
    public ReactiveProperty(){
        value = new SimpleObjectProperty<>();
    }
    public ReactiveProperty(T t){
        value = new SimpleObjectProperty<>(t);
    }

    // Propertyインターフェースを継承するので……
    @Override
    public void addListener(ChangeListener listener) {value.addListener(listener);}
    @Override
    public void removeListener(ChangeListener listener) {value.removeListener(listener);}
    @Override
    public void setValue(Object value){this.value.setValue((T)value);}
    @Override
    public void bind(ObservableValue observable) {value.bind(observable);}
    @Override
    public void unbind() {value.unbind();}
    @Override
    public boolean isBound() {return value.isBound();}
    @Override
    public void bindBidirectional(Property other) {value.bindBidirectional(other);}
    @Override
    public void unbindBidirectional(Property other) {value.unbindBidirectional(other);}
    @Override
    public Object getBean() {return value.getBean();}
    @Override
    public String getName() {return value.getName();}
    @Override
    public void addListener(InvalidationListener listener) {value.addListener(listener);}
    @Override
    public void removeListener(InvalidationListener listener) {value.removeListener(listener);}

    // getter
    public T getValue(){return value.getValue();}

    // ReactivePropertyからReactivePropertyを作成
    public <R> ReactiveProperty<R> select(Function<T, R> func){
        ReactiveProperty<R> retVal = new ReactiveProperty<>(func.apply(this.getValue()));
        this.addListener((sb, oldValue, newValue)->{retVal.setValue(func.apply((T)newValue));});
        return retVal;
    }
    // 2つのReactivePropertyからReactivePropertyを作成
    public <U, R> ReactiveProperty<R> combineLatest(ReactiveProperty<U> u, BiFunction<T, U, R> func){
        ReactiveProperty<R> retVal = new ReactiveProperty<>(func.apply(this.getValue(), u.getValue()));
        this.addListener((sb, oldValue, newValue)->{retVal.setValue(func.apply((T)newValue, u.getValue()));});
        u.addListener((sb, oldValue, newValue)->{retVal.setValue(func.apply(this.getValue(), (U)newValue));});
        return retVal;
    }
    // ◯◯PropertyからReactivePropertyを作成
    public static <R> ReactiveProperty<R> ToReactiveProperty(Property<R> r){
        ReactiveProperty<R> retVal = new ReactiveProperty<>(r.getValue());
        r.addListener((sb, oldValue, newValue)->{retVal.setValue(r.getValue());});
        return retVal;
    }
    // 変更通知機能
    public void subscribe(Consumer<T> cons){
        this.addListener((sb, oldValue, newValue) -> {cons.accept(this.getValue());});
    }
}

sample1.java

public class ReactiveProperty<T> implements Property<T> {
    private Property<T> value;
    // コンストラクタ
    public ReactiveProperty(){
        value = new SimpleObjectProperty<>();
    }
    public ReactiveProperty(T t){
        value = new SimpleObjectProperty<>(t);
    }
}

sample2.java

public class ReactiveProperty<T> implements Property<T> {
    // ReactivePropertyからReactivePropertyを作成
    public <R> ReactiveProperty<R> select(Function<T, R> func){
        ReactiveProperty<R> retVal = new ReactiveProperty<>(func.apply(this.getValue()));
        this.addListener((sb, oldValue, newValue)->{retVal.setValue(func.apply((T)newValue));});
        return retVal;
    }
    // 2つのReactivePropertyからReactivePropertyを作成
    public <U, R> ReactiveProperty<R> combineLatest(ReactiveProperty<U> u, BiFunction<T, U, R> func){
        ReactiveProperty<R> retVal = new ReactiveProperty<>(func.apply(this.getValue(), u.getValue()));
        this.addListener((sb, oldValue, newValue)->{retVal.setValue(func.apply((T)newValue, u.getValue()));});
        u.addListener((sb, oldValue, newValue)->{retVal.setValue(func.apply(this.getValue(), (U)newValue));});
        return retVal;
    }
    // ◯◯PropertyからReactivePropertyを作成
    public static <R> ReactiveProperty<R> ToReactiveProperty(Property<R> r){
        ReactiveProperty<R> retVal = new ReactiveProperty<>(r.getValue());
        r.addListener((sb, oldValue, newValue)->{retVal.setValue(r.getValue());});
        return retVal;
    }
    // 変更通知機能
    public void subscribe(Consumer<T> cons){
        this.addListener((sb, oldValue, newValue) -> {cons.accept(this.getValue());});
    }
}