␡
- 12.1 Functions as Values
- 12.2 Anonymous Functions
- 12.3 Parameters That Are Functions
- 12.4 Parameter Inference
- 12.5 Useful Higher-Order Functions
- 12.6 Closures
- 12.7 Interoperability with Lambda Expressions
- 12.8 Currying
- 12.9 Methods for Composing, Currying, and Tupling
- 12.10 Control Abstractions
- 12.11 Nonlocal Returns
- Exercises
This chapter is from the book
This chapter is from the book
This chapter is from the book
This chapter is from the book
12.7 Interoperability with Lambda Expressions
In Scala, you pass a function as a parameter whenever you want to tell another function what action to carry out. In Java, you use a lambda expression:
var button = new JButton("Increment"); // This is Java
button.addActionListener(event -> counter++);
In order to pass a lambda expression, the parameter type must be a “functional interface”—that is, any Java interface with a single abstract method.
You can pass a Scala function to a Java functional interface:
val button = JButton("Increment")
button.addActionListener(event => counter += 1)
Note that the conversion from a Scala function to a Java functional interface only works for function literals, not for variables holding functions. The following does not work:
val listener = (event: ActionEvent) => println(counter) button.addActionListener(listener) // Cannot convert a nonliteral function to a Java functional interface
The simplest remedy is to declare the variable holding the function as a Java functional interface:
val listener: ActionListener = event => println(counter) button.addActionListener(listener) // OK
Alternatively, you can turn a function variable into a literal expression:
val exit = (event: ActionEvent) => if counter > 9 then System.exit(0) button.addActionListener(exit(_))