Item 43: Prefer method reference

The primary advantage of lambdas over anonymous classes is that they are more succinct. Java provides a way to generate function objects even more succinct than lambdas: method references. Here is a code snippet from a program that maintains a map from arbitrary keys to Integer values. If the value is interpreted as a count of the number of instances of the key, then the program is a multiset implementation. The function of the code snippet is to associate the number 1 with the key if it is not in the map and to increment the associated value if the key is already present:

lambda 表达式与匿名类相比，主要优势是更简洁。Java 提供了一种方法来生成比 lambda 表达式更简洁的函数对象：方法引用。下面是一个程序的代码片段，该程序维护从任意键到 Integer 类型值的映射。如果该值被解释为键实例数的计数，那么该程序就是一个多集实现。该代码段的功能是，如果数字 1 不在映射中，则将其与键关联，如果键已经存在，则将关联值递增：

map.merge(key, 1, (count, incr) -> count + incr);

Note that this code uses the merge method, which was added to the Map interface in Java 8. If no mapping is present for the given key, the method simply inserts the given value; if a mapping is already present, merge applies the given function to the current value and the given value and overwrites the current value with the result. This code represents a typical use case for the merge method.

注意，这段代码使用了 merge 方法，它是在 Java 8 中添加到 Map 接口的。如果给定键没有映射，则该方法只插入给定的值；如果已经存在映射，则 merge 将给定的函数应用于当前值和给定值，并用结果覆盖当前值。这段代码代表了 merge 方法的一个典型用例。

The code reads nicely, but there’s still some boilerplate. The parameters count and incr don’t add much value, and they take up a fair amount of space. Really, all the lambda tells you is that the function returns the sum of its two arguments. As of Java 8, Integer (and all the other boxed numerical primitive types) provides a static method sum that does exactly the same thing. We can simply pass a reference to this method and get the same result with less visual clutter:

代码读起来不错，但是仍然有一些刻板。参数计数和 incr 不会增加太多的价值，而且它们会占用相当大的空间。实际上，lambda 表达式告诉你的是函数返回两个参数的和。在 Java 8 中，Integer（和所有其他基本类型的包装类）提供了一个静态方法 sum，它的作用完全相同。我们可以简单地传递一个引用到这个方法，并得到相同的结果，同时减少视觉混乱：

map.merge(key, 1, Integer::sum);

The more parameters a method has, the more boilerplate you can eliminate with a method reference. In some lambdas, however, the parameter names you choose provide useful documentation, making the lambda more readable and maintainable than a method reference, even if the lambda is longer.

一个方法的参数越多，就可以通过一个方法引用消除越多的刻板模式。然而，在某些 lambda 表达式中，您选择的参数名提供了有用的文档，使得 lambda 表达式比方法引用更易于阅读和维护，即使 lambda 表达式更长。

There’s nothing you can do with a method reference that you can’t also do with a lambda (with one obscure exception—see JLS, 9.9-2 if you’re curious). That said, method references usually result in shorter, clearer code. They also give you an out if a lambda gets too long or complex: You can extract the code from the lambda into a new method and replace the lambda with a reference to that method. You can give the method a good name and document it to your heart’s content.

对于方法引用，没有什么是你不能对 lambda 表达式做的（只有一个模糊的例外，如果你好奇的话可参见 [JLS, 9.9-2]）。也就是说，方法引用通常会产生更短、更清晰的代码。如果 lambda 表达式太长或太复杂，它们还会给出一个输出：可以将代码从 lambda 表达式提取到一个新方法中，并以对该方法的引用替换 lambda 表达式。可以为该方法起一个好名字，并将其文档化以满足需要。

If you’re programming with an IDE, it will offer to replace a lambda with a method reference wherever it can. You should usually, but not always, take the IDE up on the offer. Occasionally, a lambda will be more succinct than a method reference. This happens most often when the method is in the same class as the lambda. For example, consider this snippet, which is presumed to occur in a class named GoshThisClassNameIsHumongous:

如果你使用 IDE 编程，它将在任何可能的地方建议用方法引用替换 lambda 表达式。通常应该（但不总是）接受 IDE 的建议。有时候，lambda 表达式会比方法引用更简洁。当方法与 lambda 表达式在同一个类中时，这种情况最常见。例如，考虑这段代码片段，它假定发生在一个名为 GoshThisClassNameIsHumongous 的类中：

service.execute(GoshThisClassNameIsHumongous::action);

The lambda equivalent looks like this:

使用 lambda 表达式是这样的：

service.execute(() -> action());

The snippet using the method reference is neither shorter nor clearer than the snippet using the lambda, so prefer the latter. Along similar lines, the Function interface provides a generic static factory method to return the identity function, Function.identity(). It’s typically shorter and cleaner not to use this method but to code the equivalent lambda inline: x -> x.

使用方法引用的代码片段并不比使用 lambda 表达式的代码片段短，也不清楚，所以选择后者。类似地，函数接口提供了一个通用静态工厂方法来返回标识函数 Function.identity()。不使用这个方法，而是一行中编写等价的 lambda 表达式：x -> x，通常更短，也更简洁。

Many method references refer to static methods, but there are four kinds that do not. Two of them are bound and unbound instance method references. In bound references, the receiving object is specified in the method reference. Bound references are similar in nature to static references: the function object takes the same arguments as the referenced method. In unbound references, the receiving object is specified when the function object is applied, via an additional parameter before the method’s declared parameters. Unbound references are often used as mapping and filter functions in stream pipelines (Item 45). Finally, there are two kinds of constructor references, for classes and arrays. Constructor references serve as factory objects. All five kinds of method references are summarized in the table below:

许多方法引用引用静态方法，但是有四种方法不引用静态方法。其中两个是绑定和非绑定实例方法引用。在绑定引用中，接收对象在方法引用中指定。绑定引用在本质上与静态引用相似：函数对象接受与引用方法相同的参数。在未绑定引用中，在应用函数对象时通过方法声明参数之前的附加参数指定接收对象。在流管道中，未绑定引用通常用作映射和筛选函数（Item-45）。最后，对于类和数组，有两种构造函数引用。构造函数引用用作工厂对象。五种方法参考文献汇总如下表：

Method Ref Type	Example	Lambda Equivalent
Static	Integer::parseInt	str ->
Bound	Instant.now()::isAfter	Instant then =Instant.now(); t ->then.isAfter(t)
Unbound	String::toLowerCase	str ->str.toLowerCase()
Class Constructor	TreeMap<K,V>::new	() -> new TreeMap<K,V>
Array Constructor	int[]::new	len -> new int[len]

In summary, method references often provide a more succinct alternative to lambdas. Where method references are shorter and clearer, use them; where they aren’t, stick with lambdas.

总之，方法引用通常为 lambda 表达式提供了一种更简洁的选择。如果方法引用更短、更清晰，则使用它们；如果没有，仍然使用 lambda 表达式。

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