【原创】大叔经验分享（17）编程实践对比Java vs Scala

scala

官方地址 https://www.scala-lang.org/

本文尽可能包含了一些主要的java和scala在编程实践时的显著差异，展现scala的代码的简洁优雅；scala通吃<面向对象编程Object Oriented Programming>和<函数式编程Functional Programming>，有很多开源组件都用scala开发（比如spark、kafka等），虽然java从8开始支持lambda表达式，有些方面已经接近scala，但是...

直接看代码对比：

一 Loop循环

1 for

取出0-10之间的偶数

scala

    for (i <- 0 to 10 if i % 2 == 0) println(i)
    //or
    for (i <- 0 to 5) println(i * 2)
    //or
    for (i <- 0 until 6) println(i * 2)
    //or
    Array.range(0, 10, 2).foreach(println)

java

        for (int i = 0; i <= 10; i++) {
            if (i % 2 == 0) {
                System.out.println(i);
            }
        }
        //or
        for (int i = 0; i <= 10; i += 2) {
            System.out.println(i);
        }
        //or
        for (int i = 0; i <= 5; i++) {
            System.out.println(i * 2);
        }

输出
0
2
4
6
8
10

2 foreach

迭代输出

scala

val list = List(2, 4, 1, 3, 5)
list.foreach(println)

java

        List<Integer> list = Arrays.asList(new Integer[]{2, 4, 1, 3, 5});
        for (Integer item : list) {
            System.out.println(item);
        }
        //or
        list.forEach((item) -> System.out.println(item));

输出
2
4
1
3
5

二 Functional函数式

函数式即将函数作为变量的一种，来看常见的包装应用

scala

    def wrapper(str : String, f : ((String) => String)): Unit = {
      println("wrapper start")
      println(f(str))
      println("wrapper end")
    }
    def say(str : String) : String = "hello " + str
    wrapper("test", say)
    //or
    wrapper("test", (str => "hello " + str))

java

    interface SayInterface {
        public String say(String str);
    }
    public void wrapper1(String str, SayInterface si) {
        System.out.println("wrapper start");
        System.out.println(si.say(str));
        System.out.println("wrapper end");
    }
    //or
    public void wrapper2(String str, Function<String, String> f) {
        System.out.println("wrapper start");
        System.out.println(f.apply(str));
        System.out.println("wrapper end");
    }

        this.wrapper1("test", new SayInterface() {
            @Override
            public String say(String str) {
                return "hello " + str;
            }
        });
        //or
        this.wrapper1("test", ((item) -> "hello " + item));
        //or
        this.wrapper2("test", ((item) -> "hello " + item));

在早期的Java中函数式等价于命令模式，需要定义一个命令接口，然后可以传入不同的接口实现，这样又形成策略模式，比如jdk的排序接口；

输出
wrapper start
hello test
wrapper end

三 Array Collection

1 数组相加

scala

    val arr1 = Array(1, 2)
    val arr2 = ArrayBuffer(1, 3)
    arr2 += (4)
    val arr3 = arr1 ++ arr2
    println(arr3.mkString(","))

java

        Integer[] arr1 = new Integer[]{1, 2};
        Integer[] arr2 = new Integer[]{1, 3};
        Integer[] arr3 = new Integer[arr1.length + arr2.length];
        for (int i = 0; i < arr1.length; i++) {
            arr3[i] = arr1[i];
        }
        for (int i = 0; i < arr2.length; i++) {
            arr3[arr1.length + i] = arr2[i];
        }
        for (Integer item : arr3) {
            System.out.println(item);
        }
        //or
        List<Integer> list = new ArrayList<>();
        list.addAll(Arrays.asList(arr2));
        list.addAll(Arrays.asList(arr3));
        list.add(4);
        list.forEach((item) -> System.out.println(item));

输出
1,2,1,3

2 创建Map及添加entry

scala

//    val map1 = Map("a" -> 1, "b" -> 2, "c" -> 3)
    var map1 = Map("a" -> 1, "b" -> 2)
    map1 += ("c" -> 3)
    map1.foreach(println)

java

        Map<String, Integer> map1 = new HashMap<>();
        map1.put("a", 1);
        map1.put("b", 2);
        map1.put("c", 3);
        map1.forEach((key, value) -> System.out.println(key + ", " + value));

输出
(a,1)
(b,2)
(c,3)

3 合并两个Map

两个Map中key相同时将value相加

scala

    val map1 = Map("a" -> 1, "b" -> 2)
    val map2 = Map("a" -> 1, "c" -> 3)
    Array.concat(map1.toArray, map2.toArray).groupBy(_._1).map(item => (item._1, item._2.map(_._2).reduce(_ + _))).foreach(println)

java

        Map<String, Integer> map1 = new HashMap<>();
        map1.put("a", 1);
        map1.put("b", 2);
        Map<String, Integer> map2 = new HashMap<>();
        map2.put("a", 1);
        map2.put("c", 3);
        //merge map1 and map2 to mapMerge
        Map<String, Integer> mapMerge = new HashMap<>();
        mapMerge.putAll(map1);
        for (String key : map2.keySet()) {
            if (mapMerge.containsKey(key)) {
                mapMerge.put(key, mapMerge.get(key) + map2.get(key));
            } else {
                mapMerge.put(key, map2.get(key));
            }
        }
        mapMerge.forEach((key, value) -> System.out.println(key + ", " + value));

输出
(b,2)
(a,2)
(c,3)

4 mkString

将集合元素逗号分隔输出

scala

    val arr = Array(2, 4, 1, 3, 5)
    println(arr.mkString(","))

java

    public String mkStringInJava(Object[] arr, String seperator) {
        StringBuffer buf = new StringBuffer();
        for (Object item : arr) {
            buf.append(seperator).append(item);
        }
        return buf.toString().substring(seperator.length());
    }

        Integer[] arr = new Integer[]{2, 4, 1, 3, 5};
        System.out.println(this.mkStringInJava(arr, ","));

输出
2,4,1,3,5

5 统计最小、最大、平均值

scala

    val arr = Array(2, 4, 1, 3, 5)
    println(arr.min + ", " + arr.max + ", " + arr.sum.toDouble / arr.length)

java

        Integer[] arr = new Integer[]{2, 4, 1, 3, 5};
        Integer min = null;
        Integer max = null;
        Integer sum = 0;
        for (Integer item : arr) {
            sum += item;
            if (min == null || min > item) {
                min = item;
            }
            if (max == null || max < item) {
                max = item;
            }
        }
        System.out.println(min + ", " + max + ", " + ((double)sum / arr.length));

输出
1, 5, 3.0

6 简单排序

正序、倒叙

scala

    val arr = Array(2, 4, 1, 3, 5)
    //
    println(arr.sorted.mkString(","))
    //
    println(arr.sorted.reverse.mkString(","))
    //
    println(arr.sortWith((v1 : Int, v2 : Int) => v2 < v1).mkString(","))

java

        Integer[] arr = new Integer[]{2, 4, 1, 3, 5};
        //
        Arrays.sort(arr);
        for (Integer item : arr) {
            System.out.println(item);
        }
        //or
        Arrays.asList(arr).forEach((v) -> System.out.println(v));
        //
        for (int i = arr.length - 1; i >= 0; i--) {
            System.out.println(arr[i]);
        }
        //
        Arrays.sort(arr, new Comparator<Integer>(){
            @Override
            public int compare(Integer o1, Integer o2) {
                return o2 - o1;
            }
        });
        //or
        Arrays.sort(arr, ((Integer v1, Integer v2) -> v2 - v1));
        for (Integer item : arr) {
            System.out.println(item);
        }

输出
1,2,3,4,5
5,4,3,2,1
5,4,3,2,1

7 flatMap distinct

将数组中每个元素拆成多个元素并去重

scala

    val arr = Array("1,2,3", "1,4,5")
    arr.flatMap(_.split(",")).distinct.foreach(println)

java

        String[] arr = new String[]{"1,2,3", "1,4,5"};
        Set<String> set = new HashSet<>();
        for (String item : arr) {
            set.addAll(Arrays.asList(item.split(",")));
        }
        set.forEach((item) -> System.out.println(item));

输出
1
2
3
4
5

8 foldLeft

将数组中值为奇数的元素求和

scala

    val arr = Array(2, 4, 1, 3, 5)
    println(arr.foldLeft(0)((result, item) => result + (if (item % 2 != 0) item else 0)))

java

        Integer[] arr = new Integer[]{2, 4, 1, 3, 5};
        Integer sumOdd = 0;
        for (Integer item : arr) {
            if (item % 2 == 1) {
                sumOdd += item;
            }
        }
        System.out.println(sumOdd);

输出
9

9 filter map collect

将数组中小于3的元素提取一个新的集合，同时将新集合中的每个元素*3

scala

    val arr = Array(2, 4, 1, 3, 5)
    arr.filter(_ < 3).map(_ * 3).foreach(println)
    //or
    arr.collect({case item : Int if item < 3 => item * 3}).foreach(println)

java

        Integer[] arr = new Integer[]{2, 4, 1, 3, 5};
        List<Integer> result = new ArrayList<>();
        for (Integer item : arr) {
            if (item < 3) {
                result.add(item * 3);
            }
        }
        result.forEach((item) -> System.out.println(item));

输出
6
3

三 Tuple及Bean

1 定义bean class

scala

    class Dummy(val name : String, val age : Int){
      override def toString = name + "," + age
    }

java

        class Dummy {
            public Dummy(String name, Integer age) {
                this.name = name;
                this.age = age;
            }
            @Override
            public String toString() {
                return this.name + "," + this.age;
            }
            private String name = null;
            private Integer age = null;
            public String getName() {
                return name;
            }
            public void setName(String name) {
                this.name = name;
            }
            public Integer getAge() {
                return age;
            }
            public void setAge(Integer age) {
                this.age = age;
            }
        }

熟悉的java bean class定义，field、getter、setter、constructor、toString等；

2 Bean复杂排序

scala

    val arr1 = Array(new Dummy("a", 1), new Dummy("b", 3), new Dummy("a", 2), new Dummy("b", 1))
    //
    println(arr1.sortBy(_.name).mkString("-"))
    //
    println(arr1.sortBy(_.age).mkString("-"))
    //
    println(arr1.sortWith((item1, item2) => if (item1.name.equals(item2.name)) item1.age < item2.age else item1.name.compareTo(item2.name) < 0).mkString("-"))

java

        Dummy[] arr = new Dummy[]{new Dummy("a", 1), new Dummy("b", 3), new Dummy("a", 2), new Dummy("b", 1)};
        //
        Arrays.sort(arr, new Comparator<Dummy>(){
            @Override
            public int compare(Dummy d1, Dummy d2) {
                return d1.getName().compareTo(d2.getName());
            }
        });
        //or
        Arrays.sort(arr, ((Dummy d1, Dummy d2) -> d1.getName().compareTo(d2.getName())));
        //or
        Arrays.sort(arr, Comparator.comparing(Dummy::getName));
        System.out.println(this.mkStringInJava(arr, "-"));
        //
        Arrays.sort(arr, Comparator.comparing(Dummy::getAge));
        System.out.println(this.mkStringInJava(arr, "-"));
        //
        Arrays.sort(arr, ((Dummy d1, Dummy d2) -> {
            Integer result = d1.getName().compareTo(d2.getName());
            return result != 0 ? result : d1.getAge() - d2.getAge();
        }));
        System.out.println(this.mkStringInJava(arr, "-"));

输出
a,1-a,2-b,3-b,1
a,1-b,1-a,2-b,3
a,1-a,2-b,1-b,3

3 Tuple

scala

    def statistics(arr : Array[Int]) : (Int, Int, Double) = (arr.min, arr.max, arr.sum.toDouble / arr.length)
    val arr = Array(2, 4, 1, 3, 5)
    val s = statistics(arr)
    println(s._1 + ", " + s._2 + ", " + s._3)

java

    class Statistics {
        private Integer min = null;
        private Integer max = null;
        private Double avg = null;
        public Integer getMin() {
            return min;
        }
        public void setMin(Integer min) {
            this.min = min;
        }
        public Integer getMax() {
            return max;
        }
        public void setMax(Integer max) {
            this.max = max;
        }
        public Double getAvg() {
            return avg;
        }
        public void setAvg(Double avg) {
            this.avg = avg;
        }
    }
    public Statistics statistics(Integer[] arr) {
        Statistics result = new Statistics();
        Integer min = null;
        Integer max = null;
        Integer sum = 0;
        for (Integer item : arr) {
            sum += item;
            if (min == null || min > item) {
                min = item;
            }
            if (max == null || max < item) {
                max = item;
            }
        }
        result.setMin(min);
        result.setMax(max);
        result.setAvg((double)sum / arr.length);
        return result;
    }

        Integer[] arr = new Integer[]{2, 4, 1, 3, 5};
        Statistics s = statistics(arr);
        System.out.println(s.getMin() + ", " + s.getMax() + ", " + s.getAvg());

java中方法返回多个值时通常需要借助class来实现（将多个值作为class的属性）；

输出
1, 5, 3.0

4 Tuple排序

scala

    val arr = Array(("a", 1), ("b", 3), ("a", 2), ("b", 1))
    println(arr.sortBy(_._1).mkString("-"))
    println(arr.sortBy(_._2).mkString("-"))
    println(arr.sortWith((item1, item2) => if (item1._1.equals(item2._1)) item1._2 < item2._2 else item1._1.compareTo(item2._1) < 0).mkString("-"))

java中需要借助第三方库才能支持数量有限的Tuple，这里不演示了；

输出
(a,1)-(a,2)-(b,3)-(b,1)
(a,1)-(b,1)-(a,2)-(b,3)
(a,1)-(a,2)-(b,1)-(b,3)

四 IO

1 read读文件

scala

    Source.fromFile("test.log").getLines().foreach(println)

java

        BufferedReader reader = null;
        try {
            reader = new BufferedReader(new FileReader(new File("test.log")));
            String line = null;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (reader != null) {
                try {
                    reader.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

2 write写文件

scala

    var writer : PrintWriter = null
    try {
      writer = new PrintWriter("test.log")
      writer.write("test")
      writer.flush()
    } catch {case e : Exception => e.printStackTrace}
    finally {try {if (writer != null) writer.close()} catch {case e : Exception => e.printStackTrace}}

java

        BufferedWriter writer = null;
        try {
            writer = new BufferedWriter(new FileWriter(new File("test.log")));
            writer.write("test");
            writer.flush();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (writer != null) {
                try {
                    writer.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

五 Actor 定时

akka.actor

1 Actor

scala

import akka.actor.{Actor, ActorSystem, Props}
import akka.util.Timeout
    class TestActor extends Actor {
      def receive = {
        case arg => {
          val result = "hello : " + arg
          println(result)
          Thread.sleep(3000)
          result
        }
      }
    }
    val system = ActorSystem("ActorSystem")
    val actor = system.actorOf(Props(new TestActor), "TestActor")
    actor ! "world async"
    implicit val timeout = Timeout(5, TimeUnit.SECONDS)
    import akka.pattern._
    val feature = actor ? "world async with feature"
    feature.foreach(result => println(result))

java

java.util.concurrent.LinkedBlockingQueue
java.util.concurrent.ThreadPoolExecutor
java.util.concurrent.Callable
java.util.concurrent.Future
实现效果：异步非阻塞，object接收消息放到queue中，同时内置ThreadPool不断处理消息；

2 定时

scala

    val system = ActorSystem("ActorSystem")
    import system.dispatcher
    system.scheduler.schedule(Duration.create(1000, TimeUnit. MILLISECONDS), Duration.create(1000, TimeUnit. MILLISECONDS))(
      println("hello")
    )

java

java.util.Timer
java.util.TimerTask
实现效果：定时执行task；

六 continue/break

scala

import scala.util.control.Breaks._

breakable {
  ...
  break
  ...
}

java 原生支持