Higher-Order Functions

This chapter is from the book

Scala for the Impatient, 3rd Edition

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This chapter is from the book

This chapter is from the book 

Scala for the Impatient, 3rd Edition

Learn More Buy

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(_))