Java 8 新特性

1. Overview

In this tutorial, we’ll have a quick look at some of the most interesting new features in Java 8.

We’ll talk about interface default and static methods, method reference and Optional.

We have already covered some the features of the Java 8 release — stream API, lambda expressions and functional interfaces — as they’re comprehensive topics that deserve a separate look.

2. Interface Default and Static Methods

Before Java 8, interfaces could have only public abstract methods. It was not possible to add new functionality to the existing interface without forcing all implementing classes to create an implementation of the new methods, nor was it possible to create interface methods with an implementation.

Starting with Java 8, interfaces can have static and default methods that, despite being declared in an interface, have a defined behavior.

2.1. Static Method

Consider this method of the interface (let’s call this interface Vehicle):

static String producer() {
    return "N&F Vehicles";
}

The static producer() method is available only through and inside of an interface. It can’t be overridden by an implementing class.

To call it outside the interface, the standard approach for static method call should be used:

String producer = Vehicle.producer();

2.2. Default Method

Default methods are declared using the new default keyword. These are accessible through the instance of the implementing class and can be overridden.

Let’s add a default method to our Vehicle interface, which will also make a call to the static method of this interface:

default String getOverview() {
    return "ATV made by " + producer();
}

Assume that this interface is implemented by the class VehicleImpl.

For executing the default method, an instance of this class should be created:

Vehicle vehicle = new VehicleImpl();
String overview = vehicle.getOverview();

3. Method References

Method reference can be used as a shorter and more readable alternative for a lambda expression that only calls an existing method. There are four variants of method references.

3.1. Reference to a Static Method

The reference to a static method holds the syntax ContainingClass::methodName.

We’ll try to count all empty strings in the List with the help of Stream API:

boolean isReal = list.stream().anyMatch(u -> User.isRealUser(u));

Let’s take a closer look at lambda expression in the anyMatch() method. It just makes a call to a static method isRealUser(User user) of the User class.

So, it can be substituted with a reference to a static method:

boolean isReal = list.stream().anyMatch(User::isRealUser);

This type of code looks much more informative.

3.2. Reference to an Instance Method

The reference to an instance method holds the syntax containingInstance::methodName.

The following code calls method isLegalName(String string) of type User, which validates an input parameter:

User user = new User();
boolean isLegalName = list.stream().anyMatch(user::isLegalName);

3.3. Reference to an Instance Method of an Object of a Particular Type

This reference method takes the syntax ContainingType::methodName.

Let’s look at an example:

long count = list.stream().filter(String::isEmpty).count();

3.4. Reference to a Constructor

A reference to a constructor takes the syntax ClassName::new.

As constructor in Java is a special method, method reference could be applied to it too, with the help of new as a method name:

Stream<User> stream = list.stream().map(User::new);

4. Optional

Before Java 8, developers had to carefully validate values they referred to because of the possibility of throwing the NullPointerException (NPE). All these checks demanded a pretty annoying and error-prone boilerplate code.

Java 8 Optional class can help to handle situations where there is a possibility of getting the NPE. It works as a container for the object of type T. It can return a value of this object if this value is not a null. When the value inside this container is null, it allows doing some predefined actions instead of throwing NPE.

4.1. Creation of the Optional

An instance of the Optional class can be created with the help of its static methods.

Let’s look at how to return an empty Optional:

Optional<String> optional = Optional.empty();

Next, we return an Optional that contains a non-null value:

String str = "value";
Optional<String> optional = Optional.of(str);

Finally, here’s how to return an Optional with a specific value or an empty Optional if the parameter is null:

Optional<String> optional = Optional.ofNullable(getString());

4.2. Optional Usage

Let’s say we expect to get a List, and in the case of null, we want to substitute it with a new instance of an ArrayList.

With pre-Java 8’s code, we need to do something like this:

List<String> list = getList();
List<String> listOpt = list != null ? list : new ArrayList<>();

With Java 8, the same functionality can be achieved with a much shorter code:

List<String> listOpt = getList().orElseGet(() -> new ArrayList<>());

There is even more boilerplate code when we need to reach some object’s field in the old way.

Assume we have an object of type User that has a field of type Address with a field street of type String, and we need to return a value of the street field if some exist or a default value if street is null:

User user = getUser();
if (user != null) {
    Address address = user.getAddress();
    if (address != null) {
        String street = address.getStreet();
        if (street != null) {
            return street;
        }
    }
}
return "not specified";

This can be simplified with Optional:

Optional<User> user = Optional.ofNullable(getUser());
String result = user
  .map(User::getAddress)
  .map(Address::getStreet)
  .orElse("not specified");

In this example, we used the map() method to convert results of calling the getAdress() to the Optional

Optional<OptionalUser> optionalUser = Optional.ofNullable(getOptionalUser());
String result = optionalUser
  .flatMap(OptionalUser::getAddress)
  .flatMap(OptionalAddress::getStreet)
  .orElse("not specified");

String value = null;
String result = "";
try {
    result = value.toUpperCase();
} catch (NullPointerException exception) {
    throw new CustomException();
}

String value = null;
Optional<String> valueOpt = Optional.ofNullable(value);
String result = valueOpt.orElseThrow(CustomException::new).toUpperCase();