map与flatMap

flatMap参数必须是一个个流，作用是将多个流合并成一个流

package chapter1;

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

public class IO {
    public static void main(String[] args) throws IOException {
        long uniqueWords = 0;
        try (Stream<String> lines = Files.lines(Paths.get("src/main/resources/data.txt"))) {
            /** map */
//            Stream<String[]> stream1 = lines.map(line -> line.split(" "));
            /** flatMap */
            Stream<String> stringStream = lines.flatMap(line -> {
                Stream<String> stream = Arrays.stream(line.split(" "));
                return stream;
            });
            uniqueWords = stringStream.distinct().count();
        }
        System.out.println(uniqueWords);//4
    }
}

data.txt:

a b b c
c
d a