How to get words average length using Lambda Expre

问题:

I have a word list text file, I want to get min, max and average word lengths from that file.

I have a stream method:

public static Stream<String> readWords(String filename) {
    try {
        BufferedReader reader = new BufferedReader(new FileReader(filename));
        Stream<String> stringStream = reader.lines();
        return stringStream;
    } catch (IOException exn) {
        return Stream.<String>empty();
    }
}

In my main method for testing I'm printing max and min

System.out.println(readWords(filename)
        .min(Comparator.comparing(s -> s.length()))
        .get()
        .length()
);
System.out.println(readWords(filename)
        .max(Comparator.comparing(s -> s.length()))
        .get()
        .length()
);

it works as expected.

Questions:
Is it possible to get the average of the word length like I did in min and max? In both case yes or no, how to do that (only as Lambda Expression)?

回答1:

The lines() method will get you a stream of the lines, not the words. Once you have the Stream, call flatMap to replace the lines with the words, supplying the lambda expression to split out the words:

Stream<String> stringStream = reader.lines().flatMap( line -> 
    Stream.of(line.split("\\s+"))
);

This will correct your implementation of max and min. It also affects the correctness of any average calculation you wish to implement.

To obtain the average, you can call mapToInt to map the stream of words to their lengths (yielding an IntStream), then call average, which returns an OptionalDouble.

System.out.println(readWords(filename)
    .mapToInt( s -> s.length() )  // or .mapToInt(String::length)
    .average()
    .getAsDouble());

回答2:

Use IntSummaryStatistics to get the min, max and average in one pass.

IntSummaryStatistics summary = readWords(filename)
    .collect(Collectors.summarizingInt(String::length));
System.out.format("min = %d, max = %d, average = %.2f%n",
    summary.getMin(), summary.getMax(), summary.getAverage());

回答3:

Based on official documentation about reductions

System.out.println(readWords(filename)
                .mapToInt(String::length)
                .average()
                .getAsDouble()
);

Note that you can and probably should use method references like String::length