Convert a for loop to concat String into a lambda

问题:

I have the following for loop which iterates through a list of strings and stores the first character of each word in a StringBuilder. I would like to know how can I transform this to a lambda expression

StringBuilder chars = new StringBuilder();
for (String l : list) {
    chars.append(l.charAt(0));
}  

回答1:

Assuming you call toString() on the StringBuilder afterwards, I think you're just looking for Collectors.joining(), after mapping each string to a single-character substring:

String result = list
    .stream()
    .map(s -> s.substring(0, 1))
    .collect(Collectors.joining());

Sample code:

import java.util.*;
import java.util.stream.*;

public class Test {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("foo");
        list.add("bar");
        list.add("baz");
        String result = list
            .stream()
            .map(s -> s.substring(0, 1))
            .collect(Collectors.joining());
        System.out.println(result); // fbb
    }
}

Note the use of substring instead of charAt, so we still have a stream of strings to work with.

回答2:

Tons of ways to do this - the most simple option: stick to adding to a StringBuilder and do this:

    final StringBuilder chars = new StringBuilder();

    list.forEach(l -> chars.append(l.charAt(0)));

回答3:

Without creating many intermediate String objects you can do it like this:

StringBuilder sb = list.stream()
                       .mapToInt(l -> l.codePointAt(0))
                       .collect(StringBuilder::new, 
                                StringBuilder::appendCodePoint, 
                                StringBuilder::append);

Note that using codePointAt is much better than charAt as if your string starts with surrogate pair, using charAt you may have an unpredictable result.

回答4:

Here are three different solutions to this problem. Each solution filters empty strings first as otherwise StringIndexOutOfBoundsException may be thrown.

This solution is the same as the solution from Tagir with the added code for filtering empty strings. I included it here primarily to compare to the other two solutions I have provided.

List<String> list =
    Arrays.asList("the", "", "quick", "", "brown", "", "fox");
StringBuilder builder = list.stream()
    .filter(s -> !s.isEmpty())
    .mapToInt(s -> s.codePointAt(0))
    .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append);
String result = builder.toString();
Assert.assertEquals("tqbf", result);

The second solution uses Eclipse Collections, and leverages a relatively new container type called CodePointAdapter that was added in version 7.0.

MutableList<String> list =
    Lists.mutable.with("the", "", "quick", "", "brown", "", "fox");
LazyIntIterable iterable = list.asLazy()
    .reject(String::isEmpty)
    .collectInt(s -> s.codePointAt(0));
String result = CodePointAdapter.from(iterable).toString();
Assert.assertEquals("tqbf", result);

The third solution uses Eclipse Collections again, but with injectInto and StringBuilder instead of CodePointAdapter.

MutableList<String> list =
    Lists.mutable.with("the", "", "quick", "", "brown", "", "fox");
StringBuilder builder = list.asLazy()
    .reject(String::isEmpty)
    .collectInt(s -> s.codePointAt(0))
    .injectInto(new StringBuilder(), StringBuilder::appendCodePoint);
String result = builder.toString();
Assert.assertEquals("tqbf", result);

Note: I am a committer for Eclipse Collections.

回答5:

Simple way using method reference :

List<String> list = Arrays.asList("ABC", "CDE");
StringBuilder sb = new StringBuilder();

list.forEach(sb::append);

String concatString = sb.toString();