New Features in Java 8 – Java 8的新功能

最后修改: 2016年 5月 31日


1. Overview


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

在本教程中,我们将快速浏览一下Java 8中一些最有趣的新功能。

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.

我们已经介绍了Java 8版本的一些特性–stream APIlambda表达式和功能接口–因为它们是全面的主题,值得单独看一下。

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.

在Java 8之前,接口只能有公共抽象方法。如果不强迫所有的实现类创建新方法的实现,就不可能为现有的接口添加新功能,也不可能创建带有实现的接口方法。

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

从Java 8开始,接口可以有staticdefault方法,这些方法尽管是在接口中声明的,但有一个定义的行为。

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<String> with the help of Stream API:

我们将尝试在Stream API的帮助下计算List<String>中的所有空字符串。

boolean isReal = -> 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.

让我们仔细看看anyMatch()方法中的lambda表达式。它只是对User类的静态方法isRealUser(User user)进行了调用。

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


boolean isReal =;

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.

对一个实例方法的引用持有语法 containingInstance::methodName

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

下面的代码调用了isLegalName(String string)类型User的方法,它验证了一个输入参数。

User user = new User();
boolean isLegalName =;

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


This reference method takes the syntax ContainingType::methodName.

这个参考方法的语法为 ContainingType::methodName

Let’s look at an example:


long count =;

3.4. Reference to a Constructor


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

对构造函数的引用采用的语法是 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 =;

4. Optional<T>


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.

在Java 8之前,开发人员必须仔细验证他们所引用的值,因为有可能抛出NullPointerException(NPE)。所有这些检查都需要一个相当烦人的、容易出错的模板代码。

Java 8 Optional<T> 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.

Java 8 Optional<T>类可以帮助处理有可能得到NPE的情况。它作为T类型对象的一个容器。如果这个值不是null,它可以返回这个对象的一个值。当这个容器中的值是null时,它允许做一些预定义的动作而不是抛出NPE

4.1. Creation of the Optional<T>


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<T> Usage

4.2. 可选<T> 使用方法

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


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

对于Java 8之前的代码,我们需要做这样的事情。

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:

使用Java 8,同样的功能可以用更短的代码实现。

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
  .orElse("not specified");

In this example, we used the map() method to convert results of calling the getAdress() to the Optional<Address> and getStreet() to Optional<String>. If any of these methods returned null, the map() method would return an empty Optional.

在这个例子中,我们使用map()方法将调用getAdress()的结果转换为Optional<Address> getStreet()转换为Optional<String> 。如果这些方法中的任何一个返回nullmap()方法将返回一个空的Optional

Now imagine that our getters return Optional<T>.


In this case, we should use the flatMap() method instead of the map():


Optional<OptionalUser> optionalUser = Optional.ofNullable(getOptionalUser());
String result = optionalUser
  .orElse("not specified");

Another use case of Optional is changing NPE with another exception.


So, as we did previously, let’s try to do this in pre-Java 8’s style:

所以,就像我们之前所做的那样,让我们尝试以Java 8之前的风格来做这件事。

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

And the answer is more readable and simpler if we use Optional<String>:


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

Notice that how to use Optional in our app and for what purpose is a serious and controversial design decision, and explanation of all its pros and cons is out of the scope of this article. But there are plenty of interesting articles devoted to this problem. This one and this one could be very helpful to dig deeper.


5. Conclusion


In this article, we briefly discussed some interesting new features in Java 8.

在这篇文章中,我们简要地讨论了Java 8中一些有趣的新功能。

There are of course many other additions and improvements spread across many Java 8 JDK packages and classes.

当然,还有许多其他的补充和改进,分布在许多Java 8 JDK包和类中。

But the information illustrated in this article is a good starting point for exploring and learning about some of these new features.


Finally, all the source code for the article is available over on GitHub.