java - 如何使用lambda拼接字符串？

Question

我现在有一个List<String> 我想使用lambda把这个list中的字符串全部循环拼接到一个字符串上，lambda可以实现么？该怎么做呢？

Answer 1

Using foreach has side effects on StringBuilder, which is not in line with functional programming. It can be like this

String result = list.stream().collect(Collectors.joining(""));

Answer 2

StringBuilder str = new StringBuilder();
list.forEach(item -> {
    str.append(item);
});

Supplementary answer: Why can't str be changed to String type and string splicing can be achieved through str = str + item?

The above code is actually equivalent to:

final StringBuilder str = new StringBuilder();
list.forEach(new Consumer<String>() {

    @Override
    public void accept(String item) {
        str.append(item);
    }
});

That is, the essence of Lambda is actually an anonymous inner class, so str must be a final type (but the final in the code can be omitted) and cannot be reassigned.

Answer 3

I just saw the invitation email, and I’m sorry for the late reply. In fact, Brother Chacha above was very, very professional and accurate, but since I was invited, I felt sorry for not writing a few sentences in this email, so I’ll just write it in my usual way. Listing them out, I feel like I have entered the pedantic realm of writing the word "hui" in several ways. . . I hope you won’t scold me:

    public static void join1(){
        List<String> list = Arrays.asList("11","22","23");
        
        //最传统写法：
        StringBuilder sb = new StringBuilder();
        for(String s : list){
            sb.append(s);
        }

        System.out.println(sb.toString());

        //如果想要加个分隔符，比如逗号，传统写法：
        sb = new StringBuilder();
        for(int i = 0; i < list.size(); i++){
            sb.append(list.get(i));

            if(i < list.size() - 1){
                sb.append(",");
            }
        }
        System.out.println(sb.toString());

        //使用commons-lang库写法, 其实这个已经够简单了，就这个功能而言，我很喜欢，而且最最常用：
        System.out.println(StringUtils.join(list.toArray(), ","));

        //进入jdk8时代：
        System.out.println(list.stream().collect(Collectors.joining()));
        //jdk8时代，加个分隔符：
        System.out.println(list.stream().collect(Collectors.joining(",")));
    }

Answer 4

$this->

Answer 5

I’ll use rxJava as an example. It’s the same with java8’s Streaming:

StringBuilder sb = new StringBuilder();
rx.Observable.from(list).subscribe(sb::append);

If you want to add a separator between each item, use java8's Streaming this time:

String separator = ",";
StringBuilder sb = new StringBuilder();
Stream.of(list).forEach(item -> sb.append(item).append(separator));
sb.deleteCharAt(sb.length() - sepLen);