像写C#一样编写java代码

JDK8提供了非常多的便捷用法和语法糖，其编码效率几乎接近于C#开发，maven则是java目前为止最赞的jar包管理和build工具，这两部分内容都不算多，就合并到一起了。

愿编写java代码的过程如:Fast & Furious

鸟枪换炮咯，走起！J Java 7发布于2011年，Java 8发布于2014年，Java 9还远么？

在javase8中，lambda表达式的形式基本好C#中一致，Stream和LINQ类似，Future<T>和Task<T>类似，可以这样说，javase8大体达到了C#编码的便捷程度哈！帅帅哒

• Lambda（和.NET一样）

功能	示例
函数式接口	Arrays.sort(words, (first, second) -> Integer.compare(first.length(), second.length())); BiFunction<String, String, Integer> comp = (first, second) -> Integer.compare(first.length(), second.length()); 常见的函数式接口(推荐使用)，和C#Action<T>,Function<T, R>类似 Runnable,Supplier<T>,Consumer<T>， Function<T, R>, Predicate<T>
方法引用	Arrays.sort(words, String::compareToIgnoreCase);
构造器引用	list.stream().collect(Collectors.toList());
默认方法，接口中的静态方法	不太推荐
概念	所有的lambda表达式都符合闭包，且是延迟执行的

• Stream

功能	示例
创建Stream	Stream<String> song = Stream.of("Shanghai", "Beijing");
filter,map,flatmap方法	分别对应C#中Linq的where,select和selectMany
常见操作	去除重复: .distince() 排序.sorted(), 反向.reversed() 聚合方法 .max(), findFirst(), .findAny(), .anyMatch() 聚合操作 .reduce((x,y)->x+y) 分组和分片: .groupingBy(), mapping(), joining() 并行流: .parallel()
Optional类型	Optional<T>是对T类型封装，它不会返回null，使得引用更安全

• 时间日期

在1.8以前，主要使用joda-time库来处理一些比较复杂的时间日期操作，现在有官方的api了。

功能	示例
时间线Instant	Instant start = Instant.now(); Thread.sleep(1000); Instant end = Instant.now(); Duration timeElapsed = Duration.between(start, end); long millis = timeElapsed.toMillis();
本地日期LocalDate	LocalDate today = LocalDate.now(); LocalDate oneDay = LocalDate.of(2017, 7, 9); LocalDate addDay = LocalDate.now().plusDays(10);
日期校正器TemporalAdjueters	获取2017年5月的第一个周二 LocalDate time = LocalDate.of(2017, 5, 1) .with(TemporalAdjusters.nextOrSame(DayOfWeek.TUESDAY));
本地时间LocalTime	LocalTime time = LocalTime.of(16, 37, 11);
带时区的时间ZonedDateTime	ZonedDateTime applloTime = ZonedDateTime.of(2017, 5, 9, 16, 40, 11, 0, ZoneId.of("America/New_York"));
格式化和解析DateTimeFormatter	用于替代过去的DateFormat,SimpleDateFormat
遗留代码的互操作	Instant start = Instant.now();其他类型的操作类似 Date oldDate = Date.from(start);

• 并发

功能	示例
原子值	AtomicLong nextNumber = new AtomicLong(); nextNumber.incrementAndGet();
ConcurrentHashMap的改进	ConcurrentMap<String, Integer> map = new ConcurrentHashMap(); map.putIfAbsent("age", 100); map.compute("name", (k, v) -> v == null ? 1 : v + 1); map.merge("age", 1, (oldValue, newValue) -> oldValue + newValue); 批量数据操作：reduceValue, reduceKeys
并行数组操作	String[] people = new String[]{"xionger", "shuaishuaida"}; Arrays.parallelSort(people);
Future，和.NET的Task<T>类似	CompletableFuture<String> contents = CompletableFuture.supplyAsync(() -> getAsync()); 这部分知识之后再加强

• 杂项

功能	示例
字符串	String joined = String.join("/", "user", "local", "bin");
数字扩展	Long testValue = Integer.toUnsignedLong(Integer.MAX_VALUE);
新的数值函数	int result = Math.floorMod(Math.toIntExact(100L), 3);
新增集合方法	list.forEach((item) -> System.out.println(item)); Map: remove, putIfAbsent, computeIf, merge 排序：Arrays.sort(people, Comparator.comparing(Person::getAge));
使用文件	这儿看到try-with-resource，等价于using,java代码也可以和.net一样简介 try (Stream<String> lines = Files.lines(path)) { Optional<String> pwd = lines.filter(s -> s.contains("pwd")).findFirst();}
Base64编码	Base64.Encoder encoder = Base64.getMimeEncoder(); try (OutputStream output = Files.newOutputStream(encoderPath)) { Files.copy(originalPath, encoder.wrap(output));}
注解	通过在注解上添加@Repeatable，使得注解可多次使用 可以使用基于类型的注解 private @NonNull List<String> names = new ArrayList<>(); 方法参数反射，可以反射获取参数的名称
Java7	使用Path接口来代替File类 Path absolute = Paths.get("/", "home", "shanghai "); Files.write(absolute, content.getBytes(StandardCharsets.UTF_8));

Tip: demo项目

Maven使用起来相对比较简单，其配置均写在setting.xml中，主要的配置项包括镜像的选择和本地仓库的选择，如下所示。

 <?xml version="1.0" encoding="UTF-8"?>
 <settings xmlns="http://maven.apache.org/SETTINGS/1.0.0"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.0.0 http://maven.apache.org/xsd/settings-1.0.0.xsd">
 <localRepository>E:\javaAssist\maven\repository</localRepository>
 <pluginGroups>
 </pluginGroups>
 <proxies>
 </proxies>
 </servers>
 <mirrors>
 <!-- 阿里云仓库 -->
 <mirror>
 <id>alimaven</id>
 <mirrorOf>central</mirrorOf>
 <name>aliyun maven</name>
 <url>http://maven.aliyun.com/nexus/content/repositories/central/</url>
 </mirror>
 <!-- *仓库1 -->
 <mirror>
 <id>repo1</id>
 <mirrorOf>central</mirrorOf>
 <name>Human Readable Name for this Mirror.</name>
 <url>http://repo1.maven.org/maven2/</url>
 </mirror>
 <!-- *仓库2 -->
 <mirror>
 <id>repo2</id>
 <mirrorOf>central</mirrorOf>
 <name>Human Readable Name for this Mirror.</name>
 <url>http://repo2.maven.org/maven2/</url>
 </mirror>
 </mirrors>
 <profiles>
 </profiles>
 </settings>

其他常见的包括Eclipse中maven的配置，这部分只需要指定好Installations和UserSetting就好。

Maven编译的常见命令如下所示

命令	诠释
mvn package	生成target目录，编译、测试代码，生成测试报告，生成jar/war文件
mvn tomcat:run	运行项目于tomcat[jetty等server也OK]上
mvn compile	编译
mvn test	编译并测试
mvn clean	清空生成的文件
mvn install -X	想要查看完整的依赖踪迹，包含那些因为冲突或者其它原因而被拒绝引入的构件，打开 Maven 的调试标记运行
Maven的Scope
compile	编译范围，默认的范围
provided	已提供范围，比如有些web相关jar，tomcat中有，但本地没有时使用
runtime	运行时范围，比如编译时只需要slf4j-api，运行时才需要具体的实现jar
test	测试范围，例如junit,spring-test

Maven示例

 <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
     <modelVersion>4.0.0</modelVersion>
     <groupId>com.bjorktech.cayman</groupId>
     <artifactId>cm-web</artifactId>
     <packaging>war</packaging>

     <parent>
         <artifactId>cayman</artifactId>
         <groupId>com.bjorktech</groupId>
         <version>1.0.0-SNAPSHOT</version>
     </parent>

     <properties>
         <!-- Web -->
         <jsp.version>2.3.1</jsp.version>
         <jstl.version>1.2</jstl.version>
         <servlet.version>3.1.0</servlet.version>
         <!-- <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding> -->
     </properties>

     <dependencies>
         <dependency>
             <groupId>org.springframework</groupId>
             <artifactId>spring-webmvc</artifactId>
         </dependency>
         <!-- <dependency> <groupId>javax</groupId> <artifactId>javaee-web-api</artifactId>
             <version>7.0</version> <scope>provided</scope> </dependency> -->
         <dependency>
             <groupId>javax.servlet</groupId>
             <artifactId>jstl</artifactId>
             <version>${jstl.version}</version>
         </dependency>
         <dependency>
             <groupId>javax.servlet</groupId>
             <artifactId>javax.servlet-api</artifactId>
             <version>${servlet.version}</version>
             <scope>provided</scope>
         </dependency>
         <dependency>
             <groupId>javax.servlet.jsp</groupId>
             <artifactId>javax.servlet.jsp-api</artifactId>
             <version>${jsp.version}</version>
             <scope>provided</scope>
         </dependency>
         <!-- tx -->
         <dependency>
             <groupId>org.springframework</groupId>
             <artifactId>spring-tx</artifactId>
         </dependency>
         <!-- aop -->
         <dependency>
             <groupId>org.springframework</groupId>
             <artifactId>spring-aop</artifactId>
         </dependency>
         <dependency>
             <groupId>org.aspectj</groupId>
             <artifactId>aspectjweaver</artifactId>
         </dependency>
         <dependency>
             <groupId>org.aspectj</groupId>
             <artifactId>aspectjrt</artifactId>
             <version>1.8.5</version>
         </dependency>
         <!-- mybatis -->
         <dependency>
             <groupId>org.mybatis</groupId>
             <artifactId>mybatis</artifactId>
         </dependency>
         <dependency>
             <groupId>com.google.guava</groupId>
             <artifactId>guava</artifactId>
         </dependency>
         <dependency>
             <groupId>joda-time</groupId>
             <artifactId>joda-time</artifactId>
         </dependency>
         <!-- log -->
         <dependency>
             <groupId>org.slf4j</groupId>
             <artifactId>slf4j-api</artifactId>
         </dependency>
         <dependency>
             <groupId>ch.qos.logback</groupId>
             <artifactId>logback-access</artifactId>
         </dependency>
         <dependency>
             <groupId>ch.qos.logback</groupId>
             <artifactId>logback-classic</artifactId>
         </dependency>
         <dependency>
             <groupId>ch.qos.logback</groupId>
             <artifactId>logback-core</artifactId>
         </dependency>
         <!-- <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-api</artifactId>
             </dependency> <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-core</artifactId>
             </dependency> <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-slf4j-impl</artifactId>
             </dependency> -->
         <!-- 本地tomcat -->
         <!-- <dependency> <groupId>org.apache.tomcat</groupId> <artifactId>tomcat-servlet-api</artifactId>
             </dependency> -->
         <!-- json -->
         <dependency>
             <groupId>com.fasterxml.jackson.dataformat</groupId>
             <artifactId>jackson-dataformat-xml</artifactId>
             <version>2.5.3</version>
         </dependency>
         <dependency>
             <groupId>com.fasterxml.jackson.core</groupId>
             <artifactId>jackson-databind</artifactId>
             <version>2.5.3</version>
         </dependency>
         <!-- 文件上传 -->
         <dependency>
             <groupId>commons-fileupload</groupId>
             <artifactId>commons-fileupload</artifactId>
             <version>1.3.1</version>
         </dependency>
         <dependency>
             <groupId>commons-io</groupId>
             <artifactId>commons-io</artifactId>
             <version>2.3</version>
         </dependency>
         <!-- 老式soap -->
         <dependency>
             <groupId>javax.xml.rpc</groupId>
             <artifactId>javax.xml.rpc-api</artifactId>
             <version>1.1.1</version>
         </dependency>
         <dependency>
             <groupId>org.apache.axis</groupId>
             <artifactId>axis</artifactId>
             <version>1.4</version>
         </dependency>
         <!-- axis依赖包 -->
         <dependency>
             <groupId>commons-discovery</groupId>
             <artifactId>commons-discovery</artifactId>
             <version>0.2</version>
             <exclusions>
                 <exclusion>
                     <artifactId>commons-logging</artifactId>
                     <groupId>commons-logging</groupId>
                 </exclusion>
             </exclusions>
         </dependency>
         <dependency>
             <groupId>wsdl4j</groupId>
             <artifactId>wsdl4j</artifactId>
             <version>1.6.3</version>
         </dependency>
         <!-- xml解析 -->
         <dependency>
             <groupId>dom4j</groupId>
             <artifactId>dom4j</artifactId>
             <version>1.6.1</version>
         </dependency>
         <!-- 测试 -->
         <dependency>
             <groupId>org.springframework</groupId>
             <artifactId>spring-test</artifactId>
         </dependency>
         <dependency>
             <groupId>junit</groupId>
             <artifactId>junit</artifactId>
         </dependency>
     </dependencies>
     <build>
         <finalName>cm-web</finalName>
         <plugins>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-compiler-plugin</artifactId>
             </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-war-plugin</artifactId>
                 <configuration>
                     <!-- 无web.xml不报错 -->
                     <failOnMissingWebXml>false</failOnMissingWebXml>
                 </configuration>
             </plugin>
         </plugins>
     </build>
 </project>

参考资料

1. Cay, S, Horstmann. 写给大忙人看的JavaSE8[M]. 北京:电子工业出版社, 2015.