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Java Interfaces and Lambda Expressions

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Java supports function expressions that provide a convenient bridge between object-oriented and functional programming. In this chapter, you will learn about interfaces and lambda expressions.

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Java was designed as an object-oriented programming language in the 1990s when object-oriented programming was the principal paradigm for software development. Interfaces are a key feature of object-oriented programming: They let you specify what should be done, without having to provide an implementation.

Long before there was object-oriented programming, there were functional programming languages, such as Lisp, in which functions and not objects are the primary structuring mechanism. Recently, functional programming has risen in importance because it is well suited for concurrent and event-driven (or “reactive”) programming. Java supports function expressions that provide a convenient bridge between object-oriented and functional programming. In this chapter, you will learn about interfaces and lambda expressions.

The key points of this chapter are:

  1. An interface specifies a set of methods that an implementing class must provide.
  2. An interface is a supertype of any class that implements it. Therefore, one can assign instances of the class to variables of the interface type.
  3. An interface can contain static methods. All variables of an interface are automatically public, static, and final.
  4. An interface can contain default methods that an implementing class can inherit or override.
  5. An interface can contain private methods that cannot be called or overridden by implementing classes.
  6. The Comparable and Comparator interfaces are used for comparing objects.
  7. A functional interface is an interface with a single abstract method.
  8. A lambda expression denotes a block of code that can be executed at a later point in time.
  9. Lambda expressions are converted to functional interfaces.
  10. Method and constructor references refer to methods or constructors without invoking them.
  11. Lambda expressions and local classes can access effectively final variables from the enclosing scope.

3.1 Interfaces

An interface is a mechanism for spelling out a contract between two parties: the supplier of a service and the classes that want their objects to be usable with the service. In the following sections, you will see how to define and use interfaces in Java.

3.1.1 Declaring an Interface

Consider a service that works on sequences of integers, reporting the average of the first n values:

public static double average(IntSequence seq, int n)

Such sequences can take many forms. Here are some examples:

  • A sequence of integers supplied by a user
  • A sequence of random integers
  • The sequence of prime numbers
  • The sequence of elements in an integer array
  • The sequence of code points in a string
  • The sequence of digits in a number

We want to implement a single mechanism for dealing with all these kinds of sequences.

First, let us spell out what is common between integer sequences. At a minimum, one needs two methods for working with a sequence:

  • Test whether there is a next element
  • Get the next element

To declare an interface, you provide the method headers, like this:

public interface IntSequence {
    boolean hasNext();
    int next();

You need not implement these methods, but you can provide default implementations if you like—see Section 3.2.2, “Default Methods” (page 106). If no implementation is provided, we say that the method is abstract.

The methods in the interface suffice to implement the average method:

public static double average(IntSequence seq, int n) {
    int count = 0;
    double sum = 0;
    while (seq.hasNext() && count < n) {
        sum += seq.next();
    return count == 0 ? 0 : sum / count;

3.1.2 Implementing an Interface

Now let's look at the other side of the coin: the classes that want to be usable with the average method. They need to implement the IntSequence interface. Here is such a class:

public class SquareSequence implements IntSequence {
    private int i;

    public boolean hasNext() {
        return true;
    public int next() {
        return i * i;

There are infinitely many squares, and an object of this class delivers them all, one at a time. (To keep the example simple, we ignore integer overflow—see Exercise 6.)

The implements keyword indicates that the SquareSequence class intends to conform to the IntSequence interface.

This code gets the average of the first 100 squares:

SquareSequence squares = new SquareSequence();
double avg = average(squares, 100);

There are many classes that can implement the IntSequence interface. For example, this class yields a finite sequence, namely the digits of a positive integer starting with the least significant one:

public class DigitSequence implements IntSequence {
    private int number;

    public DigitSequence(int n) {
        number = n;

    public boolean hasNext() {
        return number != 0;

    public int next() {
        int result = number % 10;
        number /= 10;
        return result;

    public int rest() {
        return number;

An object new DigitSequence(1729) delivers the digits 9 2 7 1 before hasNext returns false.

3.1.3 Converting to an Interface Type

This code fragment computes the average of the digit sequence values:

IntSequence digits = new DigitSequence(1729);
double avg = average(digits, 100);
    // Will only look at the first four sequence values

Look at the digits variable. Its type is IntSequence, not DigitSequence. A variable of type IntSequence refers to an object of some class that implements the IntSequence interface. You can always assign an object to a variable whose type is an implemented interface, or pass it to a method expecting such an interface.

Here is a bit of useful terminology. A type S is a supertype of the type T (the subtype) when any value of the subtype can be assigned to a variable of the supertype without a conversion. For example, the IntSequence interface is a supertype of the DigitSequence class.

3.1.4 Casts and the instanceof Operator

Occasionally, you need the opposite conversion—from a supertype to a subtype. Then you use a cast. For example, if you happen to know that the object stored in an IntSequence is actually a DigitSequence, you can convert the type like this:

IntSequence sequence = ...;
DigitSequence digits = (DigitSequence) sequence;

In this scenario, the cast was necessary because rest is a method of DigitSequence but not IntSequence.

See Exercise 2 for a more compelling example.

You can only cast an object to its actual class or one of its supertypes. If you are wrong, a compile-time error or class cast exception will occur:

String digitString = (String) sequence;
    // Cannot possibly work—IntSequence is not a supertype of String
RandomSequence randoms = (RandomSequence) sequence;
    // Could work, throws a class cast exception if not

To avoid the exception, you can first test whether the object is of the desired type, using the instanceof operator. The expression

object instanceof Type

returns true if object is an instance of a class that has Type as a supertype. It is a good idea to make this check before using a cast.

if (sequence instanceof DigitSequence) {
    DigitSequence digits = (DigitSequence) sequence;

3.1.5 Extending Interfaces

An interface can extend another, requiring or providing additional methods on top of the original ones. For example, Closeable is an interface with a single method:

public interface Closeable {
    void close();

As you will see in Chapter 5, this is an important interface for closing resources when an exception occurs.

The Channel interface extends this interface:

public interface Channel extends Closeable {
    boolean isOpen();

A class that implements the Channel interface must provide both methods, and its objects can be converted to both interface types.

3.1.6 Implementing Multiple Interfaces

A class can implement any number of interfaces. For example, a FileSequence class that reads integers from a file can implement the Closeable interface in addition to IntSequence:

public class FileSequence implements IntSequence, Closeable {

Then the FileSequence class has both IntSequence and Closeable as supertypes.

3.1.7 Constants

Any variable defined in an interface is automatically public static final.

For example, the SwingConstants interface defines constants for compass directions:

public interface SwingConstants {
    int NORTH = 1;
    int NORTH_EAST = 2;
    int EAST = 3;

You can refer to them by their qualified name, SwingConstants.NORTH. If your class chooses to implement the SwingConstants interface, you can drop the SwingConstants qualifier and simply write NORTH. However, this is not a common idiom. It is far better to use enumerations for a set of constants; see Chapter 4.

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