When a function or method parameter of a Java API expects a single method interface type, you can pass a closure directly, as in:

# (...)
let button = JButton("Click me!")
button: addActionListener(|event| -> println("Clicked!"))

Note that this causes the creation of a method handle proxy object for each function or method invocation. For performance-sensitive contexts, we suggest that you use either asInterfaceInstance or the to conversion method described hereafter.