What’s the Difference Between These Two Button Action Lines in JavaFX?

You're spot on about the void type! Basically, line 2 is trying to set the button’s action to the result of your `reverse_visibility`, which is void. But in line 1, you're actually providing a function that will be called when an event occurs.

It's like this: line 1 passes a set of instructions for what to do when the button is pressed, while line 2 tries to hand over nothing, expecting it to work. The button wants to know what to do when clicked, not what to do right now!

A good way to think about it is the concept of event handlers. In your first line, `MouseEvent -> { Backend.reverse_visibility(list); }` defines a handler that JavaFX can call when the button is clicked. The second line is immediately invoking the method instead of merely referencing it. That's why you're running into the void type error. JavaFX is expecting a function to call later, not something that executes now and returns nothing!

The difference comes down to when each line is executed. In the second line, you're calling `Backend.reverse_visibility(list)` right away, which means it runs immediately and tries to set that result (which is void) to the button's action. That’s why you get the error.

In the first line, you’re defining a lambda expression (or an anonymous function) that tells the button to call `reverse_visibility` later, when the button is clicked. It creates a callback that executes your code only on the event, not before. So, while both lines may look similar, line one is setting up future behavior, and line two is trying to execute a method that doesn’t return anything clear for the button's action.