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