前言：

StringJoiner是Java8新出的一個類，用於構造由分隔符分隔的字元序列，並可選擇性地從提供的字首開始和以提供的字尾結尾。省的我們開發人員再次通過StringBuffer或者StingBuilder拼接。

用法示例：

StringJoiner sj = new StringJoiner(":", "[", "]");
sj.add("hu").add("jun").add("wei");
String desiredString = sj.toString();

輸出結果:

[hu:jun:wei]

原始碼分析：

    public String toString() {
        
 
if (value == null) {
            return emptyValue;//沒有值將返回空值或者後續設定的空值
        } else {
            if (suffix.equals("")) {
                return value.toString();//字尾為""直接返回字串，不用新增
            } else {
                //字尾不為""，新增字尾，然後直接返回字串，修改長度
                int initialLength = value.length();
                String result 
 
= value.append(suffix).toString();
                // reset value to pre-append initialLength
                value.setLength(initialLength);
                return result;
            }
        }
    }
    初始化，先新增字首，有了之後每次先新增間隔符，StringBuilder後續append字串
    public StringJoiner add(CharSequence newElement) {
        prepareBuilder().append(newElement);
        
 
return this;
    }
    //合併StringJoiner，注意後面StringJoiner 的字首就不要了，後面的appen進來
    public StringJoiner merge(StringJoiner other) {
        Objects.requireNonNull(other);
        if (other.value != null) {
            final int length = other.value.length();
            // lock the length so that we can seize the data to be appended
            // before initiate copying to avoid interference, especially when
            // merge 'this'
            StringBuilder builder = prepareBuilder();
            builder.append(other.value, other.prefix.length(), length);
        }
        return this;
    }
    //初始化，先新增字首，有了之後每次先新增間隔符
    private StringBuilder prepareBuilder() {
        if (value != null) {
            value.append(delimiter);
        } else {
            value = new StringBuilder().append(prefix);
        }
        return value;
    }

    public int length() {
        // Remember that we never actually append the suffix unless we return
        // the full (present) value or some sub-string or length of it, so that
        // we can add on more if we need to.
        //不忘新增字尾的長度
        return (value != null ? value.length() + suffix.length() :
                emptyValue.length());
    }
}