import java.text.MessageFormat;
import javafx.beans.property.StringProperty;
import javafx.beans.property.SimpleStringProperty;

public class StringFormatBug {
    public static void main(String[] args) {
        StringProperty prop = new SimpleStringProperty("");
        StringProperty otherProp = new SimpleStringProperty("1");
        prop.bindBidirectional(otherProp, new MessageFormat("DAY {0}")); 
        System.out.println("prop = " + prop.get());
    }
}