Java 8新特性(10) StringJoiner

原有的 StringBuilder 太死板，不支持分割，如果想让最终的字符串以逗号隔开，需要这样写

StringBuilder sb = new StringBuilder();
IntStream.range(1,10).forEach(i->{
    sb.append(i+"");
    if( i < 10){
        sb.append(",")
    }
});

是不是太死板了，不好用，StringJoiner 怎样写呢？

StringJoiner sj = new StringJoiner(",");
IntStream.range(1,10).forEach(i->sj.add(i+""));  // 1,2,3,4,5,6,7,8,9

// 还可以加前缀后缀
StringJoiner sj = new StringJoiner(",", "[", "]");
IntStream.range(1,10).forEach(i->sj.add(i+""));  // [1,2,3,4,5,6,7,8,9]

String 的新方法

String.join(":", "foobar", "foo", "bar");
// foobar:foo:bar

// 源码也是使用了 StringJoiner
public static String join(CharSequence delimiter, CharSequence... elements) {
    Objects.requireNonNull(delimiter);
    Objects.requireNonNull(elements);
    // Number of elements not likely worth Arrays.stream overhead.
    StringJoiner joiner = new StringJoiner(delimiter);
    for (CharSequence cs: elements) {
        joiner.add(cs);
    }
    return joiner.toString();
}

Collector.joining

String [] strArray = new String[] {"a", "b", "c"};
String s = Stream.of(strArray).collect(Collectors.joining(","));

// 实现原理也是借助了 StringJoiner
public static Collector<CharSequence, ?, String> joining(CharSequence delimiter,
                                                            CharSequence prefix,
                                                            CharSequence suffix) {
    return new CollectorImpl<>(
            () -> new StringJoiner(delimiter, prefix, suffix),
            StringJoiner::add, StringJoiner::merge,
            StringJoiner::toString, CH_NOID);
}

其他方法

• setEmptyValue：默认情况下的 emptyValue 是前缀加后缀， 用户可自定义 emptyValue
• merge(StringJoiner other)：合并另外一个 joiner
• length：当前长度，为空看 emptyValue 的长度

参考

https://www.jianshu.com/p/469fe8fdd3be