Lambda Expressions and Functional Interfaces: Tips and Best Practices
转载自https://www.baeldung.com/java-8-lambda-expressions-tips
1. Overview
Now that Java 8 has reached wide usage, patterns, and best practices have begun to emerge for some of its headlining features. In this tutorial, we will take a closer look to functional interfaces and lambda expressions.
Further reading:
Why Do Local Variables Used in Lambdas Have to Be Final or Effectively Final?
Java 8 – Powerful Comparison with Lambdas
2. Prefer Standard Functional Interfaces
Functional interfaces, which are gathered in the java.util.function package, satisfy most developers' needs in providing target types for lambda expressions and method references. Each of these interfaces is general and abstract, making them easy to adapt to almost any lambda expression. Developers should explore this package before creating new functional interfaces.
Consider an interface Foo:
1
2
3
4
|
@FunctionalInterface public interface Foo { String method(String string); } |
and a method add() in some class UseFoo, which takes this interface as a parameter:
1
2
3
|
public String add(String string, Foo foo) { return foo.method(string); } |
To execute it, you would write:
1
2
|
Foo foo = parameter -> parameter + " from lambda" ; String result = useFoo.add( "Message " , foo); |
Look closer and you will see that Foo is nothing more than a function that accepts one argument and produces a result. Java 8 already provides such an interface in Function<T,R> from the java.util.function package.
Now we can remove interface Foo completely and change our code to:
1
2
3
|
public String add(String string, Function<String, String> fn) { return fn.apply(string); } |
To execute this, we can write:
1
2
3
|
Function<String, String> fn = parameter -> parameter + " from lambda" ; String result = useFoo.add( "Message " , fn); |
3. Use the @FunctionalInterface Annotation
Annotate your functional interfaces with @FunctionalInterface. At first, this annotation seems to be useless. Even without it, your interface will be treated as functional as long as it has just one abstract method.
But imagine a big project with several interfaces – it's hard to control everything manually. An interface, which was designed to be functional, could accidentally be changed by adding of other abstract method/methods, rendering it unusable as a functional interface.
But using the @FunctionalInterface annotation, the compiler will trigger an error in response to any attempt to break the predefined structure of a functional interface. It is also a very handy tool to make your application architecture easier to understand for other developers.
So, use this:
1
2
3
4
|
@FunctionalInterface public interface Foo { String method(); } |
instead of just:
1
2
3
|
public interface Foo { String method(); } |
4. Don't Overuse Default Methods in Functional Interfaces
You can easily add default methods to the functional interface. This is acceptable to the functional interface contract as long as there is only one abstract method declaration:
1
2
3
4
5
|
@FunctionalInterface public interface Foo { String method(); default void defaultMethod() {} } |
Functional interfaces can be extended by other functional interfaces if their abstract methods have the same signature. For example:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
|
@FunctionalInterface public interface FooExtended extends Baz, Bar {} @FunctionalInterface public interface Baz { String method(); default void defaultBaz() {} } @FunctionalInterface public interface Bar { String method(); default void defaultBar() {} } |
Just as with regular interfaces, extending different functional interfaces with the same default method can be problematic. For example, assume that interfaces Bar and Baz both have a default method defaultCommon(). In this case, you will get a compile-time error:
1
|
interface Foo inherits unrelated defaults for defaultCommon() from types Baz and Bar... |
To fix this, defaultCommon() method should be overridden in the Foo interface. You can, of course, provide a custom implementation of this method. But if you want to use one of the parent interfaces' implementations (for example, from the Baz interface), add following line of code to the defaultCommon() method's body:
1
|
Baz. super .defaultCommon(); |
But be careful. Adding too many default methods to the interface is not a very good architectural decision. It is should be viewed as a compromise, only to be used when required, for upgrading existing interfaces without breaking backward compatibility.
5. Instantiate Functional Interfaces with Lambda Expressions
The compiler will allow you to use an inner class to instantiate a functional interface. However, this can lead to very verbose code. You should prefer lambda expressions:
1
|
Foo foo = parameter -> parameter + " from Foo" ; |
over an inner class:
1
2
3
4
5
6
|
Foo fooByIC = new Foo() { @Override public String method(String string) { return string + " from Foo" ; } }; |
The lambda expression approach can be used for any suitable interface from old libraries. It is usable for interfaces like Runnable, Comparator, and so on. However, this doesn't mean that you should review your whole older codebase and change everything.
6. Avoid Overloading Methods with Functional Interfaces as Parameters
Use methods with different names to avoid collisions; let's look at an example:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
|
public interface Processor { String process(Callable<String> c) throws Exception; String process(Supplier<String> s); } public class ProcessorImpl implements Processor { @Override public String process(Callable<String> c) throws Exception { // implementation details } @Override public String process(Supplier<String> s) { // implementation details } } |
At first glance, this seems reasonable. But any attempt to execute either of the ProcessorImpl‘s methods:
1
|
String result = processor.process(() -> "abc" ); |
ends with an error with the following message:
1
2
3
4
5
|
reference to process is ambiguous both method process(java.util.concurrent.Callable<java.lang.String>) in com.baeldung.java8.lambda.tips.ProcessorImpl and method process(java.util.function.Supplier<java.lang.String>) in com.baeldung.java8.lambda.tips.ProcessorImpl match |
To solve this problem, we have two options. The first is to use methods with different names:
1
2
3
|
String processWithCallable(Callable<String> c) throws Exception; String processWithSupplier(Supplier<String> s); |
The second is to perform casting manually. This is not preferred.
1
|
String result = processor.process((Supplier<String>) () -> "abc" ); |
7. Don’t Treat Lambda Expressions as Inner Classes
Despite our previous example, where we essentially substituted inner class by a lambda expression, the two concepts are different in an important way: scope.
When you use an inner class, it creates a new scope. You can hide local variables from the enclosing scope by instantiating new local variables with the same names. You can also use the keyword this inside your inner class as a reference to its instance.
However, lambda expressions work with enclosing scope. You can’t hide variables from the enclosing scope inside the lambda’s body. In this case, the keyword this is a reference to an enclosing instance.
For example, in the class UseFoo you have an instance variable value:
1
|
private String value = "Enclosing scope value" ; |
Then in some method of this class place the following code and execute this method.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
|
public String scopeExperiment() { Foo fooIC = new Foo() { String value = "Inner class value" ; @Override public String method(String string) { return this .value; } }; String resultIC = fooIC.method( "" ); Foo fooLambda = parameter -> { String value = "Lambda value" ; return this .value; }; String resultLambda = fooLambda.method( "" ); return "Results: resultIC = " + resultIC + ", resultLambda = " + resultLambda; } |
If you execute the scopeExperiment() method, you will get the following result: Results: resultIC = Inner class value, resultLambda = Enclosing scope value
As you can see, by calling this.value in IC, you can access a local variable from its instance. But in the case of the lambda, this.value call gives you access to the variable value which is defined in the UseFoo class, but not to the variable value defined inside the lambda's body.
8. Keep Lambda Expressions Short And Self-explanatory
If possible, use one line constructions instead of a large block of code. Remember lambdas should be an expression, not a narrative. Despite its concise syntax, lambdas should precisely express the functionality they provide.
This is mainly stylistic advice, as performance will not change drastically. In general, however, it is much easier to understand and to work with such code.
This can be achieved in many ways – let's have a closer look.
8.1. Avoid Blocks of Code in Lambda's Body
In an ideal situation, lambdas should be written in one line of code. With this approach, the lambda is a self-explanatory construction, which declares what action should be executed with what data (in the case of lambdas with parameters).
If you have a large block of code, the lambda's functionality is not immediately clear.
With this in mind, do the following:
1
|
Foo foo = parameter -> buildString(parameter); |
1
2
3
4
5
|
private String buildString(String parameter) { String result = "Something " + parameter; //many lines of code return result; } |
instead of:
1
2
3
4
|
Foo foo = parameter -> { String result = "Something " + parameter; //many lines of code return result; }; |
However, please don't use this “one-line lambda” rule as dogma. If you have two or three lines in lambda's definition, it may not be valuable to extract that code into another method.
8.2. Avoid Specifying Parameter Types
A compiler in most cases is able to resolve the type of lambda parameters with the help of type inference. Therefore, adding a type to the parameters is optional and can be omitted.
Do this:
1
|
(a, b) -> a.toLowerCase() + b.toLowerCase(); |
instead of this:
1
|
( String a, String b) -> a.toLowerCase() + b.toLowerCase(); |
8.3. Avoid Parentheses Around a Single Parameter
Lambda syntax requires parentheses only around more than one parameter or when there is no parameter at all. That is why it is safe to make your code a little bit shorter and to exclude parentheses when there is only one parameter.
So, do this:
1
|
a -> a.toLowerCase(); |
instead of this:
1
|
(a) -> a.toLowerCase(); |
8.4. Avoid Return Statement and Braces
Braces and return statements are optional in one-line lambda bodies. This means, that they can be omitted for clarity and conciseness.
Do this:
1
|
a -> a.toLowerCase(); |
instead of this:
1
|
a -> { return a.toLowerCase()}; |
8.5. Use Method References
Very often, even in our previous examples, lambda expressions just call methods which are already implemented elsewhere. In this situation, it is very useful to use another Java 8 feature: method references.
So, the lambda expression:
1
|
a -> a.toLowerCase(); |
could be substituted by:
1
|
String::toLowerCase; |
This is not always shorter, but it makes the code more readable.
9. Use “Effectively Final” Variables
Accessing a non-final variable inside lambda expressions will cause the compile-time error. But it doesn’t mean that you should mark every target variable as final.
According to the “effectively final” concept, a compiler treats every variable as final, as long as it is assigned only once.
It is safe to use such variables inside lambdas because the compiler will control their state and trigger a compile-time error immediately after any attempt to change them.
For example, the following code will not compile:
1
2
3
4
5
6
7
|
public void method() { String localVariable = "Local" ; Foo foo = parameter -> { String localVariable = parameter; return localVariable; }; } |
The compiler will inform you that:
1
|
Variable 'localVariable' is already defined in the scope. |
This approach should simplify the process of making lambda execution thread-safe.
10. Protect Object Variables from Mutation
One of the main purposes of lambdas is use in parallel computing – which means that they're really helpful when it comes to thread-safety.
The “effectively final” paradigm helps a lot here, but not in every case. Lambdas can't change a value of an object from enclosing scope. But in the case of mutable object variables, a state could be changed inside lambda expressions.
Consider the following code:
1
2
3
|
int [] total = new int [ 1 ]; Runnable r = () -> total[ 0 ]++; r.run(); |
This code is legal, as total variable remains “effectively final”. But will the object it references to have the same state after execution of the lambda? No!
Keep this example as a reminder to avoid code that can cause unexpected mutations.
11. Conclusion
In this tutorial, we saw some best practices and pitfalls in Java 8's lambda expressions and functional interfaces. Despite the utility and power of these new features, they are just tools. Every developer should pay attention while using them.
The complete source code for the example is available in this GitHub project – this is a Maven and Eclipse project, so it can be imported and used as-is.
Lambda Expressions and Functional Interfaces: Tips and Best Practices的更多相关文章
- Functional Programming without Lambda - Part 1 Functional Composition
Functions in Java Prior to the introduction of Lambda Expressions feature in version 8, Java had lon ...
- jdk8新特性之lambda expressions
本文分两部分: 语法简单说明 lambda的使用 注:这两部分内容均以类+注释的方式进行说明,并且内容均来自官方教程(https://docs.oracle.com/javase/tutorial/j ...
- JAVA 8 Lambda表达式-Lambda Expressions
Lambda表达式介绍 Lambda表达式是在java规范提案JSR 335中定义的,Java 8 中引入了Lambda表达式,并被认为是Java 8最大的新特性,Lambda表达式促进了函数式编程, ...
- java语言中的匿名类与lambda表达式介绍与总结 (Anonymous Classes and Lambda Expressions)
2017/6/30 转载写明出处:http://www.cnblogs.com/daren-lin/p/anonymous-classes-and-lambda-expressions-in-java ...
- 1.8 新特性之 Lambda Expressions
Lambda expressions are allowed only at source level 1.8 or above The target type of this expression ...
- Hacking Lambda Expressions in Java
Hacking Lambda Expressions in Javahttps://dzone.com/articles/hacking-lambda-expressions-in-java At t ...
- C# Lambda Expressions 简介
C# Lambda Expressions 简介 原文http://msdn2.microsoft.com/en-us/library/bb397687.aspx 翻译:朱之光 (larry1zhu@ ...
- Starter Set of Functional Interfaces
Java Development Kit 8 has a number of functional interfaces. Here we review the starter set-the int ...
- lambda expressions are not supported at this language level
IDEA下报错:lambda expressions are not supported at this language level 解决: 1. File -> Project Struct ...
随机推荐
- SpringBoot整合Shiro实现权限控制
目录 1.SpringBoot整合Shiro 1.1.shiro简介 1.2.代码的具体实现 1.2.1.Maven的配置 1.2.2.整合需要实现的类 1.2.3.项目结构 1.2.4.ShiroC ...
- Hive——join的使用
Hive--join的使用 hive中常用的join有:inner join.left join .right join .full join.left semi join.cross join.mu ...
- 【开发工具】-- IDEA集成Git在实际项目中的运用
1.企业实际项目中Git的使用 在实际的企业项目开发中,我们一般Java的项目在公司都有自己的局域网代码仓库,仓库上存放着很多的项目.以我工作过的公司如华为的项目,一般是存放在企业内部的CodeHub ...
- 01_Nginx下载安装
一.简介 Nginx 是由俄罗斯人 Igor Sysoev 设计开发的,开发工作从2002 年开始,第一次公开发布在 2004 年 10 月 4 日. 官方网站为:http://nginx.org/ ...
- MySql查询上周(周一到周日)每天的日期!土方!
首先介绍一个函数:YEARWEEK(date[,mode]) 主要说明一下后面的可选参数mode,这个参数就是指定一周里面哪一天是第一天. 默认一周是从周日开始,这显然不太符合我们的要求.要指定每周从 ...
- Docker安装Kong API Gateway并使用
我最新最全的文章都在南瓜慢说 www.pkslow.com,文章更新也只在官网,欢迎大家来喝茶~~ 1 简介 Kong不是一个简单的产品,本文讲的Kong主要指的是Kong API Gateway,即 ...
- Hadoop 3.1.1 - 概述 - 总览
Apache Hadoop 3.1.1 和之前发布的 3.0.X 版本线相比,Apache Hadoop 3.1.1 吸收了许多重要的改进. 总览 建议用户阅读完整的版本说明.本文提供了对主要变动的总 ...
- DS1515+
DS1515+ 2021年05月02日 0 2.4k aahk DS1515+ 2021年05月02日 这篇文章用于记录.改进.提高.分享我在使用群晖DS1515+网络存储服务器过程中的具体操作. ...
- Java数组06——冒泡排序
冒泡排序 例子: package array; import java.util.Arrays; public class ArrayDemon08 { public static ...
- Android开发三年,面了八家公司终于腾讯上岸,含泪整理面经
在下2017年毕业,目前从事android开发工作已经3年多了,前段时间刚完成一次跳槽,面试了几家公司,将一些面试经验分享给大家,希望对大家有所帮助. 写在前面 这次跳槽总共是面了8家,(2小,4中, ...