StringJoiner源码学习

 public StringJoiner(CharSequence delimiter,
                        CharSequence prefix,
                        CharSequence suffix) {
        Objects.requireNonNull(prefix, "The prefix must not be null");
        Objects.requireNonNull(delimiter, "The delimiter must not be null");
        Objects.requireNonNull(suffix, "The suffix must not be null");
        // 设置前缀
        this.prefix = prefix.toString();
        //设置切割符
        this.delimiter = delimiter.toString();
        //设置后缀
        this.suffix = suffix.toString();
        //设置空value
        this.emptyValue = this.prefix + this.suffix;
    }

    public StringJoiner add(CharSequence newElement) {
        prepareBuilder().append(newElement);
        return this;
    }
    //创建一个stringBuilder
    private StringBuilder prepareBuilder() {
        if (value != null) {
            //追加到在value之后添加一个分隔符
            value.append(delimiter);
        } else {
            //创建value，然后添加一个前缀
            value = new StringBuilder().append(prefix);
        }
        return value;
    }

    public String toString() {
        if (value == null) {
            return emptyValue;
        } else {
            if (suffix.equals("")) {
                //如果后缀为空，那么直接返回
                return value.toString();
            } else {
                int initialLength = value.length();
                //添加一个后缀
                String result = value.append(suffix).toString();
                // reset value to pre-append initialLength
                value.setLength(initialLength);
                return result;
            }
        }
    }

public class TestStringJoin {
    public static void main(String[] args) {
        StringJoiner stringJoiner=new StringJoiner(",","1 "," 2");
        stringJoiner.add("程序员记事本").add("test");
        System.out.println(stringJoiner.toString());
    }
}