on java 8 Chapter XIV streaming programming

• 1 streaming support
• 2 stream creation
  • 2.1 random number stream
  • 2.2 int type range
  • 2.3 generate()
  • 2.4 iterate()
  • 2.5 flow pattern builder
  • 2.6 Arrays
  • 2.7 Regular Expressions
• 3 intermediate operation
  • 3.1 tracing and debugging
  • 3.2 Flow Elements ordering
  • 3.3 Removing elements
  • 3.4 application function to the elements
  • 3.5 map () of the combined stream
• 4 Optional Class
  • 4.1 Convenience functions
  • 4.2 Creating -optional
  • 4.3 optional- object manipulation
  • 4.4 optional- flow
• 5 terminal operation
  • 5.1 Array
  • 5.2 cycle
  • 5.3 collection
  • 5.4 Array
  • 5.5 match
  • Find 5.6
  • 5.7 Information
  • 5.8 Digital information flow
• 6 Summary

Optimization of the collection of stored objects, and the objects used in the processing stream.

---

• What is declarative programming (Declarative programming) declare to do, rather than how to do programming style

• Imperative programming imperative form

• Of explicitly writing the iteration mechanism called an external iterator

• Streaming programming using the internal iteration, which is one of the core features of streaming programming.

Flow is lazy loading, due to the delay calculation, flow allows us to express very large (even infinite) sequence, without the need to consider the memory problem

package stream;

import java.util.Random;

public class Randoms {
    public static void main(String[] args) {
        new Random(47)
                .ints(5,20)
                .distinct()
                .limit(7)
                .sorted()
                .forEach(System.out::println);
    }
}
/*
6
10
13
16
17
18
19
 */

package stream;

import java.util.Random;
import java.util.SortedSet;
import java.util.TreeSet;

public class ImperativeRandoms {
    public static void main(String[] args) {
        Random rand = new Random(47);
        SortedSet<Integer> rints = new TreeSet<>();
        while (rints.size() < 7){
            int r = rand.nextInt(20);
            if (r < 5)continue;
            rints.add(r);
        }
        System.out.println(rints);
    }
}
/*
[7, 8, 9, 11, 13, 15, 18]
 */

1 streaming support

Add the default (default) modifications at the interface. In this embodiment, a method designers can flow (stream) smoothly into existing classes.

There are three types of flow operations:
create a flow
modifying element stream (intermediate operation, Intermediate Operations),
consumption of stream elements (terminal operation, Terminal Operations).
The last type typically means having flow element (usually to the collection).

2 stream creation

The stream transformed into a set of elements

package stream;

import java.util.stream.Stream;

public class SteamOf {
    public static void main(String[] args) {
        Stream.of(new Bubble(1),new Bubble(2),new Bubble(3))
                .forEach(System.out::println);
        Stream.of("It's ","a ","wonderful ","day ","for ","pie! ")
                .forEach(System.out::print);
        System.out.println();
        Stream.of(3.14159,2.718,1.618)
                .forEach(System.out::println);
    }
}
/*
Bubble{i=1}
Bubble{i=2}
Bubble{i=3}
It's a wonderful day for pie! 
3.14159
2.718
1.618
 */

Each set may be generated by calling a flow stream () method

package stream;

import java.util.*;

public class CollectionToStream {
    public static void main(String[] args) {
        List<Bubble> bubbles = Arrays.asList(new Bubble(1),new Bubble(2),new Bubble(3));
        System.out.println(bubbles.stream()
        .mapToInt(b -> b.i)
        .sum());

        Set<String> w = new HashSet<>(Arrays.asList("It's a wonderful day for pie!".split(" ")));
        w.stream()
                .map(x -> x + " ")
                .forEach(System.out::print);
        System.out.println();

        Map<String,Double> m = new HashMap<>();
        m.put("pi",3.14159);
        m.put("e",2.718);
        m.put("phi",1.618);
        m.entrySet().stream()
                .map(e -> e.getKey() + ": " + e.getKey())
                .forEach(System.out::println);
    }
}
/*
6
a pie! It's for wonderful day 
phi: phi
e: e
pi: pi
 */

2.1 random number stream

Random class is a method of generating a set of enhancement stream

package stream;

import java.util.Random;
import java.util.stream.Stream;

public class RandomGenerators {
    public static <T> void show(Stream<T> stream){ // 基本指导原则：尽量使用泛型方法
        stream
        .limit(4)
        .forEach(System.out::println);
        System.out.println("+++++++");
    }

    public static void main(String[] args) {
        Random rand = new Random(47);
        show(rand.ints().boxed()); // boxed() 自动地把基本类型包装成为对应的装箱类型
        show(rand.longs().boxed());
        show(rand.doubles().boxed());
        //控制上限和下限
        show(rand.ints(10,20).boxed());
        show(rand.longs(10,20).boxed());
        show(rand.doubles(10,20).boxed());
        //控制流大小
        show(rand.ints(3,3,9).boxed());
        show(rand.longs(3,3,9).boxed());
        show(rand.doubles(3,3,9).boxed());
    }
}

Random collection created using any object as a Supplier

package stream;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.function.Supplier;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class RandomWords implements Supplier<String> {
    List<String> words = new ArrayList<>();
    Random rand = new Random(47);
    RandomWords(String fname) throws IOException { // 构造函数中循环体使用命令式编程（外部迭代）
        List<String> lines = Files.readAllLines(Paths.get(fname));//通过 File 类将 Cheese.dat 文件的所有行读取到 List<String>

        for (String line : lines.subList(1,lines.size())){// 略过第一行
            for (String word : line.split("[ .?,]+")) //前面的东西可以出现一次或者多次
                words.add(word.toLowerCase());
        }
    }
    @Override
    public String get() {
        return words.get(rand.nextInt(words.size()));
    }

    @Override
    public String toString() {
        return words.stream()
                .collect(Collectors.joining(" ")); // collect() 收集操作
    }

    public static void main(String[] args) throws IOException {
        System.out.println(
                //Stream.generate() 的用法，可以把任意 Supplier<T> 用于生成 T 类型的流。
                Stream.generate(new RandomWords("/Users/erin/JavaProject/thinking_in_java_example/src/main/java/stream/Cheese.dat"))
                .limit(10)
                .collect(Collectors.joining(" "))); // collect() 收集操作

    }
}
/*
it shop sir the much cheese by conclusion district is
 */

2.2 int type range

package stream;
import static java.util.stream.IntStream.*;

public class Ranges {
    public static void main(String[] args) {
        // IntStream 类提供 range() 方法用于生成整型序列的流。
        // 编写循环时，这个方法会更加便利
        int result = 0;
        for (int i = 10;i < 20; i++)
            result += i;
        System.out.println(result);

        // for-in循环
        result = 0;
        for (int i: range(10,20).toArray())
            result += i;
        System.out.println(result);

        // 使用流
        System.out.println(range(10,20).sum());
    }
}

Practical features repeat () can be used to replace the simple for loop

package stream;
import static java.util.stream.IntStream.*;
public class Repeat {
    public static void repeat(int n,Runnable action){
        range(0,n).forEach(i->action.run());
    }
}

package stream;

public class Looping {
    static void hi(){
        System.out.println("Hi!");
    }

    public static void main(String[] args) {
        Repeat.repeat(3,()-> System.out.println("Looping!"));
        Repeat.repeat(2,Looping::hi);
    }
}

2.3 generate()

package stream;

import java.util.Random;
import java.util.function.Supplier;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Generator implements Supplier<String> {
    Random rand = new Random(47);
    char[] letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray();

    @Override
    public String get(){
        return "" +letters[rand.nextInt(letters.length)];
    }

    public static void main(String[] args) {
        String word = Stream.generate(new Generator())
                .limit(30)
                .collect(Collectors.joining());
        System.out.println(word);
    }
}
/*
YNZBRNYGCFOWZNTCQRGSEGZMMJMROE
 */

Create a stream containing the same object, a lambda pass only those objects generated to generate () in

package stream;

import java.util.stream.Stream;

public class Duplicator {
    public static void main(String[] args) {
        Stream.generate(()->"duplicate")
                .limit(3)
                .forEach(System.out::println);
    }
}
/*
duplicate
duplicate
duplicate
 */

bubbler() 与 Supplier Interface is compatible

package stream;

import java.util.stream.Stream;

public class Bubbles {
    public static void main(String[] args) {
        Stream.generate(Bubble::bubbler)//创建单独工厂类（Separate Factory class）的另一种方式
                .limit(5)
                .forEach(System.out::println);
    }
}
/*
Bubble{i=0}
Bubble{i=1}
Bubble{i=2}
Bubble{i=3}
Bubble{i=4}
 */

2.4 iterate()

package stream;

import java.util.stream.Stream;

public class Fibonacci {
    int x = 1;

    Stream<Integer> numbers(){
        return Stream.iterate(0,i->{ // iterate() 只能记忆结果，因此我们需要利用一个变量 x 追踪另外一个元素。
            int result = x + i;
            x = i;
            return result;
        });
    }

    public static void main(String[] args) {
        new Fibonacci().numbers()
                .skip(20)//过滤前20个
                .limit(10)//取10个
                .forEach(System.out::println);
    }
}

2.5 flow pattern builder

Builder design mode (also known as the constructor mode)

package stream;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.stream.Stream;

public class FileToWordsBuilder {
    Stream.Builder<String> builder = Stream.builder();

    public FileToWordsBuilder(String filePath) throws IOException {//在建造者设计模式（也称构造器模式）中，首先创建一个 builder 对象，传递给它多个构造器信息，最后执行“构造”。
        Files.lines(Paths.get(filePath))
                .skip(1)
                .forEach(line -> {
                    for (String w : line.split("[ .?,]+"))
                        builder.add(w);
                });
    }

    Stream<String> stream(){//只要你不调用 stream() 方法，就可以继续向 builder 对象中添加单词
//        builder.add("lala");
//        Stream<String> t = builder.build();
//        builder.add("lala2");
//        //在 Stream.Builder 调用 build() 方法后继续尝试添加单词会产生一个异常。
//        // java.lang.IllegalStateException

        return builder.build();
    }

    public static void main(String[] args) throws IOException {
        new FileToWordsBuilder("/Users/erin/JavaProject/thinking_in_java_example/src/main/java/stream/Cheese.dat")
        .stream()
        .limit(7)
        .map(w ->w + " ")
        .forEach(System.out::print);
    }
}

2.6 Arrays

Arrays.stream array into stream

 package stream;


public interface Operations {
    void execute();

    static void runOps(Operations... ops){
        for (Operations op: ops){
            op.execute();
        }
    }
    static void show(String msg){
        System.out.println(msg);
    }
}

package stream;

import java.util.Arrays;

public class Machine2 {
    public static void main(String[] args) {
        Arrays.stream(new Operations[] {
                () -> Operations.show("Bing"), // 重写 execute
                () -> Operations.show("Crack"),
                () -> Operations.show("Twist"),
                () -> Operations.show("Pop")
        }).forEach(Operations::execute);
    }
}
/*
Bing
Crack
Twist
Pop
 */

package stream;

import java.util.Arrays;

public class ArrayStreams {
    public static void main(String[] args) {
        Arrays.stream(new double[]{3.14159,2.718,1.618})
                .forEach(n->System.out.format("%f ",n));
        System.out.println();

        Arrays.stream(new int[]{1,3,5}).forEach(n->System.out.format("%d ",n));
        System.out.println();

        Arrays.stream(new long[]{11,22,44,66}).forEach(n->System.out.format("%d ",n));
        System.out.println();

        //选择一个子域  开始/停止
        Arrays.stream(new int[]{1,3,5,7,15,28,37},3,6).forEach(n ->System.out.format("%d ",n));
    }
}
/*
3.141590 2.718000 1.618000 
1 3 5 
11 22 44 66 
7 15 28 
 */

2.7 Regular Expressions

package stream;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.regex.Pattern;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class FileToWordsRegexp {
    private String all;
    public FileToWordsRegexp(String filePath) throws IOException {
        all = Files.lines(Paths.get(filePath))
                .skip(1) //忽略第一行
                .collect(Collectors.joining(" "));
    }
    public Stream<String> stream(){
        return Pattern.compile("[ .,?]+").splitAsStream(all);
    }

    public static void main(String[] args) throws IOException {
        FileToWordsRegexp fw = new FileToWordsRegexp("/Users/erin/JavaProject/thinking_in_java_example/src/main/java/stream/Cheese.dat");
        fw.stream()
                .limit(7)
                .map(w -> w + " ")
                .forEach(System.out::print);
        fw.stream()
                .skip(7)
                .limit(2)
                .map(w->w + " ")
                .forEach(System.out::print);
    }
}
/*
Not much of a cheese shop really is it 
 */
多次调用 stream() 在已存储的字符串中创建一个新的流。这里有个限制，整个文件必须存储在内存中；在大多数情况下这并不是什么问题，但是这损失了流操作非常重要的优势：

流“不需要存储”。当然它们需要一些内部存储，但是这只是序列的一小部分，和持有整个序列并不相同。
它们是懒加载计算的。

3 intermediate operation

Intermediate operation for acquiring a stream from an object,

And another stream object as output from the rear end,

To connect to other operations.

3.1 tracing and debugging

The purpose peek () operation is to help debugging. It allows you to view the elements without modification of the stream

Stream<T> peek(Consumer<? super T> action)

package stream;

import java.io.IOException;

public class Peeking {
    public static void main(String[] args) throws IOException {
        FileToWords.stream("/Users/erin/JavaProject/thinking_in_java_example/src/main/java/stream/Cheese.dat")
                .skip(21)
                .limit(4)
                .map(w -> w + " ")
                .peek(System.out::print)
                .map(String::toUpperCase)
                .peek(System.out::print)
                .map(String::toLowerCase)
                .forEach(System.out::print);
    }
}
/*
Well WELL well it IT it s S s so SO so
 */

3.2 Flow Elements ordering

package stream;

import java.io.IOException;
import java.util.Comparator;

public class SortedComparator {
    public static void main(String[] args) throws IOException {
        FileToWords.stream("/Users/erin/JavaProject/thinking_in_java_example/src/main/java/stream/Cheese.dat")
                .skip(10)
                .limit(10)
                .sorted(Comparator.reverseOrder()) // sorted() 预设了一些默认的比较器。这里我们使用的是反转“自然排序”。当然你也可以把 Lambda 函数作为参数传递给 sorted()
                .map(w -> w + " ")
                .forEach(System.out::print);
    }
}
/*
you what to the that sir leads in district And
*/

3.3 Removing elements

package stream;
import java.util.stream.LongStream;

import static java.util.stream.LongStream.*;
public class Prime {
    public static Boolean isPrime(long n){ // 过滤器函数， 检测质数
        return rangeClosed(2,(long)Math.sqrt(n)).noneMatch(i -> n % i == 0);
    }
    public LongStream numbers(){
        return iterate(2,i-> i+1).filter(Prime::isPrime);
    }

    public static void main(String[] args) {
        new Prime().numbers()
                .limit(10)
                .forEach(n -> System.out.format("%d ",n));
        System.out.println();
        new Prime().numbers()
                .skip(99)
                .limit(10)
                .forEach(n-> System.out.format("%d ",n));
    }
}
/*
2 3 5 7 11 13 17 19 23 29 
541 547 557 563 569 571 577 587 593 599 
 */

3.4 application function to the elements

package stream;

import java.util.Arrays;
import java.util.function.Function;
import java.util.stream.Stream;

public class FunctionMap {
    static String[] elements ={"12","","23","45"};
    static Stream<String> testStream(){
        return Arrays.stream(elements);
    }
    static void test(String descr, Function<String,String> func){
        System.out.println(" ---(" + descr +" )---");
        testStream()
                .map(func)
                // map(Function)：将函数操作应用在输入流的元素中，并将返回值传递到输出流中。
                // map() 映射多种函数到一个字符串流中
                .forEach(System.out::println);
    }

    public static void main(String[] args) {
        test("add brackets",s->"[" + s +"]");
        test("Increment",s -> {
            try{
                return Integer.parseInt(s) + 1 + "";
            }catch (NumberFormatException e){
                //如果字符串不能转化成为整数就会抛出 NumberFormatException 异常，
                //我们只须回过头来将原始字符串放回到输出流中
                return s;
            }
        });
        test("Replace",s -> s.replace("2","9"));
        test("Take last digit",s-> s.length() > 0 ?
                s.charAt(s.length() - 1) + "":s );
    }
}
/*
 ---(add brackets )---
[12]
[]
[23]
[45]
 ---(Increment )---
13

24
46
 ---(Replace )---
19

93
45
 ---(Take last digit )---
2

3
5
 */

package stream;

import java.util.stream.Stream;

class Numbered{
    final int n;
    Numbered(int n){
        this.n = n;
    }

    @Override
    public String toString() {
        return "Numbered{" +
                "n=" + n +
                '}';
    }
}
public class FunctionMap2 {
    public static void main(String[] args) {
        Stream.of(15,7,9,11,13)
                .map(Numbered::new)
                .forEach(System.out::println);
    }
}
/*
将获取到的整数通过构造器 Numbered::new 转化成为 Numbered 类型
Numbered{n=15}
Numbered{n=7}
Numbered{n=9}
Numbered{n=11}
Numbered{n=13}
 */

package stream;

import java.util.stream.Stream;

public class FunctionMap3 {
    public static void main(String[] args) {
        Stream.of("5","7","9")
                .mapToInt(Integer::parseInt)
                .forEach( n -> System.out.format("%d ",n));
        System.out.println();
        Stream.of("17","19","23")
                .mapToLong(Long::parseLong)
                .forEach(n -> System.out.format("%d ",n));
        System.out.println();
        Stream.of("17","1.9",".23")
                .mapToDouble(Double::parseDouble)
                .forEach(n -> System.out.format("%f ",n));
    }
}
/*
5 7 9
17 19 23
17.000000 1.900000 0.230000
 */

3.5 map () of the combined stream

flatMap () does two things: the application flow generation function (the map () does the same) in each element, and then each stream into a flat element, thus eventually causing only element.

package stream;

import java.util.stream.Stream;

public class StreamOfStreams {
    public static void main(String[] args) {
        Stream.of(1,2,3)
                .map(i -> Stream.of("Gonzo","Kermit","Beaker"))
                .map(e -> e.getClass().getName())
                .forEach(System.out::println);
    }
}
/*
java.util.stream.ReferencePipeline$Head
java.util.stream.ReferencePipeline$Head
java.util.stream.ReferencePipeline$Head
 */

package stream;

import java.util.stream.Stream;

public class FlatMap {
    public static void main(String[] args) {
        Stream.of(1,2,3)
                .flatMap(i -> Stream.of("Gonzo","Fozzie","Beaker"))
                .forEach(System.out::println);
    }
}
/*
从映射返回的每个流都会自动扁平为组成它的字符串
Gonzo
Fozzie
Beaker
Gonzo
Fozzie
Beaker
Gonzo
Fozzie
Beaker
 */

package stream;

import java.util.Random;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class StreamOfRandoms {
    static Random rand = new Random(47);

    public static void main(String[] args) {
        Stream.of(1,2,3,4,5)
                .flatMapToInt(i -> IntStream.concat(// concat()，它以参数顺序组合两个流
                        rand.ints(0,100).limit(i),IntStream.of(-1)
                )).forEach(n -> System.out.format("%d ",n));
    }
}
/*
58 -1 55 93 -1 61 61 29 -1 68 0 22 7 -1 88 28 51 89 9 -1
 */

package stream;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.regex.Pattern;
import java.util.stream.Stream;

public class FileToWords {
    public static Stream<String> stream(String filePath) throws IOException {
        return Files.lines(Paths.get(filePath))
                .skip(1)
//                .flatMap(line -> Arrays.stream(line.split("\\W+")));
                .flatMap(line ->
                        Pattern.compile("\\W+").splitAsStream(line));
        // \\W+ 是一个正则表达式。他表示“非单词字符”，+ 表示“可以出现一次或者多次”。小写形式的 \\w 表示“单词字符”
    }

}

package stream;

import java.io.IOException;

public class FileToWordsTest {
    public static void main(String[] args) throws IOException {
        FileToWords.stream("/Users/erin/JavaProject/thinking_in_java_example/src/main/java/stream/Cheese.dat")
                .limit(7)
                .forEach(s -> System.out.format("%s ",s));
        System.out.println();
        FileToWords.stream("/Users/erin/JavaProject/thinking_in_java_example/src/main/java/stream/Cheese.dat")
                .skip(7)
                .limit(2)
                .forEach(s -> System.out.format("%s ",s));
    }
}
/*
Not much of a cheese shop really
is it
 */

4 Optional Class

对空流进行所有这些操作的简单测试
package stream;

import java.util.Optional;
import java.util.stream.Stream;

public class OptionalBasics {
    static void test(Optional<String> optString){
        if (optString.isPresent())
            System.out.println(optString.get());
        else
            System.out.println("Nothing inside");
    }

    public static void main(String[] args) {
        test(Stream.of("Epithets").findFirst());
        test(Stream.<String>empty().findFirst());//空流是通过 Stream.<String>empty() 创建的
//        Stream<String> s = Stream.empty();
    }
}
/*
Epithets
Nothing inside
 */

4.1 Convenience functions

package stream;

import javax.swing.text.html.Option;
import java.util.Optional;
import java.util.function.Consumer;
import java.util.stream.Stream;

public class Optionals {
    static void basics(Optional<String> optString){
        if (optString.isPresent())
            System.out.println(optString.get());
        else System.out.println("Nothing inside!");
    }
    static void ifPresent(Optional<String> optString){
        optString.ifPresent(System.out::println);
    }
    static void orElse(Optional<String> optString){
        System.out.println(optString.orElse("Nada"));
    }
    static void orElseGet(Optional<String> optString){
        System.out.println(optString.orElseGet(() -> "Generated"));
    }
    static void orElseThrow(Optional<String> optString){
        try{
            System.out.println(optString.orElseThrow(() -> new Exception("Supplied")));
        }catch (Exception e){
            System.out.println("Caught " + e);
        }
    }
    static void test(String testName, Consumer<Optional<String>> cos){
        System.out.println(" === " + testName + " === ");
        cos.accept(Stream.of("Epithets").findFirst());
        cos.accept(Stream.<String>empty().findFirst());
    }

    public static void main(String[] args) {
        test("basics",Optionals::basics);
        test("ifPresent",Optionals::ifPresent);
        test("orElse",Optionals::orElse);
        test("orElseGet",Optionals::orElseGet);
        test("orElseThrow",Optionals::orElseThrow);
    }
}
/*
ifPresent(Consumer)：当值存在时调用 Consumer，否则什么也不做。
orElse(otherObject)：如果值存在则直接返回，否则生成 otherObject。
orElseGet(Supplier)：如果值存在则直接返回，否则使用 Supplier 函数生成一个可替代对象。
orElseThrow(Supplier)：如果值存在直接返回，否则使用 Supplier 函数生成一个异常。

 === basics ===
Epithets
Nothing inside!
 === ifPresent ===
Epithets
 === orElse ===
Epithets
Nada
 === orElseGet ===
Epithets
Generated
 === orElseThrow ===
Epithets
Caught java.lang.Exception: Supplied
 */

4.2 Creating -optional

package stream;

import java.util.Optional;

public class CreatingOptionals {
    static void test(String testName, Optional<String> opt){
        System.out.println(" === " + testName + " === ");
        System.out.println(opt.orElse("Null"));
    }

    public static void main(String[] args) {
        test("empty",Optional.empty());
        test("of",Optional.of("Howdy"));
        try{
            test("of",Optional.of(null));//不能通过传递 null 到 of() 来创建 Optional 对象。
        }catch (Exception e){
            System.out.println(e);
        }
        test("ofNullable",Optional.ofNullable("Hi"));
        test("ofNullable",Optional.ofNullable(null));// 最安全的方法是， 使用 ofNullable() 来优雅地处理 null
    }
}
/*
 === empty ===
Null
 === of ===
Howdy
java.lang.NullPointerException
 === ofNullable ===
Hi
 === ofNullable ===
Null
 */

4.3 optional- object manipulation

filter(Predicate)：将 Predicate 应用于 Optional 中的内容并返回结果。当 Optional 不满足 Predicate 时返回空。如果 Optional 为空，则直接返回。

map(Function)：如果 Optional 不为空，应用 Function 于 Optional 中的内容，并返回结果。否则直接返回 Optional.empty。

flatMap(Function)：同 map()，但是提供的映射函数将结果包装在 Optional 对象中，因此 flatMap() 不会在最后进行任何包装。

package stream;

import java.util.Arrays;
import java.util.function.Predicate;
import java.util.stream.Stream;

public class OptionalFilter {
    static String[] elements = {"Foo", "", "Bar", "Baz", "Bingo"};
    static Stream<String> testStream(){
        return Arrays.stream(elements);
    }
    static void test(String descr, Predicate<String> pred){
        System.out.println(" ---(" + descr + " )---");
        for (int i = 0; i<= elements.length;i++){ // 最后一个元素实际上超出了流。将自动成为 Optional.empty
            System.out.println(testStream()
                    .skip(i)
                    .findFirst()// Optional<T> findFirst();
                    .filter(pred));
        }
    }

    public static void main(String[] args) {
        test("true", str -> true);
        test("false", str -> false);
        test("str != \"\"", str -> str !=  "");
        test("str.length() == 3",str -> str.length() == 3);
        test("startsWith(\"B\")",str -> str.startsWith("B"));
    }
}
/*
---(true )---
Optional[Foo]
Optional[]
Optional[Bar]
Optional[Baz]
Optional[Bingo]
Optional.empty
 ---(false )---
Optional.empty
Optional.empty
Optional.empty
Optional.empty
Optional.empty
Optional.empty
 ---(str != "" )---
Optional[Foo]
Optional.empty
Optional[Bar]
Optional[Baz]
Optional[Bingo]
Optional.empty
 ---(str.length() == 3 )---
Optional[Foo]
Optional.empty
Optional[Bar]
Optional[Baz]
Optional.empty
Optional.empty
 ---(startsWith("B") )---
Optional.empty
Optional.empty
Optional[Bar]
Optional[Baz]
Optional[Bingo]
Optional.empty
 */

package stream;

import java.util.Arrays;
import java.util.function.Function;
import java.util.stream.Stream;

public class OptionalMap {
    static String[] elements = {"12", "", "23", "45"};

    static Stream<String> testStream() {
        return Arrays.stream(elements);
    }

    static void test(String descr, Function<String, String> func) {
        System.out.println(" ---(" + descr + ")--- ");
        for (int i = 0; i <= elements.length; i++) {
            System.out.println(
                    testStream()
                            .skip(i)
                            .findFirst() //Optional
                            .map(func) //Optional 同 map() 一样 ， Optional.map() 应用于函数。它仅在 Optional 不为空时才应用映射函数，并将 Optional 的内容提取到映射函数
            );
        }
    }

    public static void main(String[] args) {
        // If Optional is not empty, map() first extracts
        // the contents which it then passes
        // to the function:
        test("Add branckets", s -> "[" + s + "]");
        test("Increment", s -> {
            try {
                return Integer.parseInt(s) + 1 + "";
            } catch (NumberFormatException e) {
                return s;
            }
        });
        test("Replace", s -> s.replace("2", "9"));
        test("Take last digit", s -> s.length() > 0 ?
                s.charAt(s.length() - 1) + "" :s);
        // After the function is finished, map() wraps the
        // result in an Optional before returning it:

    }
}
/*
 ---(Add branckets)---
Optional[[12]]
Optional[[]]
Optional[[23]]
Optional[[45]]
Optional.empty
 ---(Increment)---
Optional[13]
Optional[]
Optional[24]
Optional[46]
Optional.empty
 ---(Replace)---
Optional[19]
Optional[]
Optional[93]
Optional[45]
Optional.empty
 ---(Take last digit)---
Optional[2]
Optional[]
Optional[3]
Optional[5]
Optional.empty
 */

package stream;

import java.util.Arrays;
import java.util.Optional;
import java.util.function.Function;
import java.util.stream.Stream;

public class OptionalFlatMap {
    static String[] elements = {"12", "", "23", "45"};

    static Stream<String> testStream(){
        return Arrays.stream(elements);
    }

    static void test(String descr, Function<String, Optional<String>> func){
        System.out.println(" ---( " + descr + ")--- ");
        for (int i = 0; i <= elements.length; i++){
            System.out.println(
                    testStream()
                    .skip(i)
                    .findFirst()
                    .flatMap(func)
            );
        }
    }

    public static void main(String[] args) {
        test("Add brackets", s -> Optional.of("[" + s + "]"));
        test("Increment", s -> {
            try{
                return Optional.of(Integer.parseInt(s) + 1 + "");
            }catch (NumberFormatException e){
                return Optional.of(s);
            }
        });
        test("Replace", s -> Optional.of(s.replace("2", "9")));
        test("Take last digit", s -> Optional.of(s.length() > 0 ? s.charAt( s.length() -1) + "" : s));
    }

}

/*
Optional[[12]]
Optional[[]]
Optional[[23]]
Optional[[45]]
Optional.empty
 ---( Increment)---
Optional[13]
Optional[]
Optional[24]
Optional[46]
Optional.empty
 ---( Replace)---
Optional[19]
Optional[]
Optional[93]
Optional[45]
Optional.empty
 ---( Take last digit)---
Optional[2]
Optional[]
Optional[3]
Optional[5]
Optional.empty
 */

4.4 optional- flow

package stream;

import java.util.Optional;
import java.util.Random;
import java.util.stream.Stream;

public class Signal {
    private final String msg;
    public Signal(String msg){this.msg = msg;}

    public String getMsg() {
        return msg;
    }

    @Override
    public String toString() {
        return "Signal(" + msg +
                ')';
    }
    static Random rand = new Random(47);
    public static Signal morse(){
        switch (rand.nextInt(4)){
            case 1:return new Signal("dot");
            case 2:return new Signal("dash");
            default:return null;
        }
    }
    public static Stream<Optional<Signal>> stream(){
        return Stream.generate(Signal::morse)
                .map(signal -> Optional.ofNullable(signal));

    }
}

package stream;

import java.util.Optional;

public class StreamOfOptionals {
    public static void main(String[] args) {
        Signal.stream()
                .limit(10)
                .forEach(System.out::println);
        System.out.println(" ---");
        Signal.stream()
                .limit(10)
                .filter(Optional::isPresent) //使用 filter() 来保留那些非空 Optional
                .map(Optional::get)//使用 get() 获取元素
                .forEach(System.out::println);
    }
}
/*
Optional[Signal(dash)]
Optional[Signal(dot)]
Optional[Signal(dash)]
Optional.empty
Optional.empty
Optional[Signal(dash)]
Optional.empty
Optional[Signal(dot)]
Optional[Signal(dash)]
Optional[Signal(dash)]
 ---
Signal(dot)
Signal(dot)
Signal(dash)
Signal(dash)
 */

5 terminal operation

5.1 Array

toArray (): the flow into the appropriate array type.
toArray (generator): In special cases, to generate an array of custom types.

package stream;

import java.util.Arrays;
import java.util.Random;
import java.util.stream.IntStream;

public class RandInts {
    private static int[] rints = new Random(47).ints(0,1000).limit(100).toArray();
    public static IntStream rands(){
        return Arrays.stream(rints);
    }
}

5.2 cycle

forEach (Consumer) as common as Consumer System.out :: println function.
forEachOrdered (Consumer): Follow the original guarantee forEach stream order.

package stream;
import static stream.RandInts.*;

public class ForEach {
    static final int SZ = 14;

    public static void main(String[] args) {
        rands().limit(SZ)
                .forEach(n -> System.out.format("%d ",n));
        System.out.println();
        rands().limit(SZ)
                .parallel()
                .forEach(n -> System.out.format("%d ",n));
        System.out.println();
        rands().limit(SZ)
                .parallel()
                .forEachOrdered(n -> System.out.format("%d ",n)); // parallel() 和 forEachOrdered() 来强制保持原始流顺序

    }
}
/*
258 555 693 861 961 429 868 200 522 207 288 128 551 589 
551 589 861 258 868 555 429 288 961 128 522 693 207 200 
258 555 693 861 961 429 868 200 522 207 288 128 551 589 
 */

5.3 collection

collect (Collector): Collector collected using flow element into the result set.
collect (Supplier, BiConsumer, BiConsumer) : ibid., the first parameter Supplier a new result set is created, the second parameter BiConsumer the next element included in the results, for the third parameter BiConsumer combining two values .

In a word document to collect TreeSet collection

package stream;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.Set;
import java.util.TreeSet;
import java.util.stream.Collectors;

public class TreeSetOfWords {
    public static void main(String[] args) throws IOException {
        //转化成行流
        Set<String> words2 = Files.lines(Paths.get("/Users/erin/JavaProject/thinking_in_java_example/src/main/java/stream/TreeSetOfWords.java"))
                .flatMap( s -> Arrays.stream(s.split("\\W+")))//匹配一个或多个非单词字符分割 使用Arrays.stream转化为流，将结果展平映射为单词流
                .filter( s -> !s.matches("\\d+"))//查找并移除全数字字符串
                .map(String::trim)//去除单词两边空白
                .filter(s -> s.length() > 2)//过滤长度小于3的单词
                .limit(100)//只获取100个单词
                .collect(Collectors.toCollection(TreeSet::new));
        System.out.println(words2);

    }
}

Map generated in the stream

package stream;

import java.util.Iterator;
import java.util.Map;
import java.util.Random;
import java.util.stream.Collectors;
import java.util.stream.Stream;

class Pair {
    public final Character c;
    public final Integer i;

    Pair(Character c, Integer i) {
        this.c = c;
        this.i = i;
    }

    public Character getC() {
        return c;
    }

    public Integer getI() {
        return i;
    }

    @Override
    public String toString() {
        return "Pair{" +
                "c=" + c +
                ", i=" + i +
                '}';
    }
}
class RandomPair{
    Random rand = new Random(47);
    // An infinite iterator of random capital letters:
    Iterator<Character> capChars = rand.ints(65,91) // 整数流 随机生成大些字母迭代器
            .mapToObj(i -> (char)i)//转化为Pair流
            .iterator();
    public Stream<Pair> stream(){
        return rand.ints(100,1000).distinct().mapToObj(i -> new Pair(capChars.next(),i));
    }
}

public class MapCollector {
    public static void main(String[] args) {
        Map<Integer,Character> map = new RandomPair().stream()
                .limit(8)
                .collect(Collectors.toMap(Pair::getI,Pair::getC));
        //只使用最简单形式的 Collectors.toMap()，这个方法值需要一个可以从流中获取键值对的函数。还有其他重载形式，其中一种形式是在遇到键值冲突时，需要一个函数来处理这种情况。
        System.out.println(map);
    }
}

package stream;

import java.io.IOException;
import java.util.ArrayList;

public class SpecialCollector {
    public static void main(String[] args) throws IOException {
        ArrayList<String> words = FileToWords.stream("/Users/erin/JavaProject/thinking_in_java_example/src/main/java/stream/Cheese.dat")
                .collect(ArrayList::new,
                        ArrayList::add,
                        ArrayList::addAll);
        words.stream()
                .filter(s -> s.equals("cheese"))
                .forEach(System.out::println);
    }
}
/*
cheese
cheese
 */

5.4 portfolio

reduce (BinaryOperator): BinaryOperator to use combinations of elements found in the stream. Because the flow may be empty, its return value Optional.
reduce (identity, BinaryOperator): Function as above, but using its identity as an initial value combinations. Therefore, if the stream is empty, identity is the result.
reduce (identity, BiFunction, BinaryOperator) : the use of more complex forms (temporarily introduced), included it here because it can improve efficiency. Typically, we can explicitly composition map () and reduce () simpler to express it.

package stream;

import java.util.Random;
import java.util.stream.Stream;

class Frobnitz{
    int size;
    Frobnitz(int sz){size = sz;}

    @Override
    public String toString() {
        return "Frobnitz{" +
                "size=" + size +
                '}';
    }
    //Generator:
    static Random rand = new Random(47);
    static final int BOUND = 100;
    static Frobnitz supply(){
        return new Frobnitz(rand.nextInt(BOUND));
    }
}
public class Reduce {
    public static void main(String[] args) {
        Stream.generate(Frobnitz::supply) // supply() 的生成器；因为这个方法对于 Supplier<Frobnitz> 是签名兼容的，我们可以将其方法引用传递给 Stream.generate()（这种签名兼容性被称作结构一致性）
                .limit(10)
                .peek(System.out::println)
                .reduce((fr0,fr1) -> fr0.size < 50 ? fr0 : fr1) //Lambda 表达式中的第一个参数 fr0 是上一次调用 reduce() 的结果。而第二个参数 fr1 是从流传递过来的值。
                .ifPresent(System.out::println);//Optional.ifPresent() 只有在结果非空的时候才会调用 Consumer<Frobnitz>
    }
}
/*
Frobnitz{size=58}
Frobnitz{size=55}
Frobnitz{size=93}
Frobnitz{size=61}
Frobnitz{size=61}
Frobnitz{size=29}
Frobnitz{size=68}
Frobnitz{size=0}
Frobnitz{size=22}
Frobnitz{size=7}
Frobnitz{size=29}
 */

5.5 match

allMatch (Predicate): if each elementary stream according Predicate returns true provided the returned result is true. When the first false, then stopped to perform calculations.
anyMatch (Predicate): If any of the elements according to a stream of providing Predicate returns true, the result returned is true. The first stop was at a false perform calculations.
noneMatch (Predicate): If the stream according to each element are provided Predicate returns false, the result returned is true. Stop performing calculations at the first true.

package stream;

import java.util.function.BiPredicate;
import java.util.function.Predicate;
import java.util.stream.IntStream;
import java.util.stream.Stream;

interface Matcher extends BiPredicate<Stream<Integer>, Predicate<Integer>>{} //BiPredicate 是一个二元谓词
public class Matching {
    static void show(Matcher match,int val){
        System.out.println(
                match.test(IntStream.rangeClosed(1,9)
                        .boxed()
                        .peek(n -> System.out.format("%d ",n))
                        ,n -> n < val));
    }

    public static void main(String[] args) {
        show(Stream::allMatch,10);
        show(Stream::allMatch,4);
        show(Stream::anyMatch,2);
        show(Stream::anyMatch,0);
        show(Stream::noneMatch,5);
        show(Stream::noneMatch,0);
    }
}
/*
1 2 3 4 5 6 7 8 9 true
1 2 3 4 false
1 true
1 2 3 4 5 6 7 8 9 false
1 false
1 2 3 4 5 6 7 8 9 true
 */

Find 5.6

findFirst (): returns the first flow element Optional, if the stream is empty return Optional.empty.
findAny (: Optional contain any return flow element, if the stream is empty return Optional.empty.

package stream;
import static stream.RandInts.*;

public class SelectElement {
    public static void main(String[] args) {
        System.out.println(rands().findFirst().getAsInt());
        System.out.println(rands().parallel().findFirst().getAsInt());// findFirst() 无论流是否为并行化的，总是会选择流中的第一个元素
        System.out.println(rands().findAny().getAsInt());//非并行流，findAny()会选择流中的第一个元素（即使从定义上来看是选择任意元素）
        System.out.println(rands().parallel().findAny().getAsInt()); //使用 parallel() 来并行流从而引入 findAny() 选择非第一个流元素的可能性。
    }
}
/*
258
258
258
242
 */

package stream;

import java.util.Optional;
import java.util.OptionalInt;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class LastElement {
    public static void main(String[] args) {
        OptionalInt last = IntStream.range(10,20).reduce((n1,n2)-> n2);
        System.out.println(last.orElse(-1));
        // Non-numeric object:
        Optional<String> lastobj = Stream.of("one","two","three").reduce((n1,n2) -> n2);
        System.out.println(lastobj.orElse("Nothing there!"));
    }
}
/*
19
three
 */

5.7 Information

count (): the number of elements in the stream.
max (Comparator): "maximum" element from the passed Comparator determined.
min (Comparator): The "minimal" elements passed Comparator determined.

package stream;

import java.io.IOException;

public class Informational {
    public static void main(String[] args) throws IOException {
        System.out.println(FileToWords.stream("/Users/erin/JavaProject/thinking_in_java_example/src/main/java/stream/Cheese.dat").count());
        System.out.println(FileToWords.stream("/Users/erin/JavaProject/thinking_in_java_example/src/main/java/stream/Cheese.dat")
                .min(String.CASE_INSENSITIVE_ORDER) //min() 和 max() 的返回类型为 Optional，这需要我们使用 orElse()来解包
                .orElse("None")
        );
        System.out.println(FileToWords.stream("/Users/erin/JavaProject/thinking_in_java_example/src/main/java/stream/Cheese.dat")
                .max(String.CASE_INSENSITIVE_ORDER)
                .orElse("None")
        );
    }
}
/*
32
a
you
 */

5.8 Digital information flow

average (): obtaining the average flow element.
max () and min (): NUMERICAL operation without Comparator.
sum (): summing all flow elements.
summaryStatistics (): may generate useful data. Currently less clear necessity of this approach, because we can actually get the data you need in a more direct way.

package stream;
import static stream.RandInts.*;

public class NumericStreamInfo {
    public static void main(String[] args) {
        System.out.println(rands().average().getAsDouble());
        System.out.println(rands().max().getAsInt());
        System.out.println(rands().min().getAsInt());
        System.out.println(rands().sum());
        System.out.println(rands().summaryStatistics());
    }
}
/*
507.94
998
8
50794
IntSummaryStatistics{count=100, sum=50794, min=8, average=507.940000, max=998}
 */

6 Summary

Study reference https://lingcoder.github.io/OnJava8/#/book/14-Streams?id=%e4%bf%a1%e6%81%af