public static void main(String[] args) {
 
    //間隔符是,
    StringJoiner joiner1 = new StringJoiner(",");
    joiner1.add("1").add("2").add("3");
    System.out.println(joiner1.toString());
    //1,2,3
 
    //以[開頭中間的間隔符是,以]結(jié)尾
    StringJoiner joiner2 = new StringJoiner(",", "[", "]");
    joiner2.add("1").add("2").add("3");
    System.out.println(joiner2.toString());
    //[1,2,3]
 
  }

//前綴
private final String prefix;
//后綴
private final String delimiter;
//中間分隔符
private final String suffix;
 
 
public StringJoiner(CharSequence delimiter) {
    this(delimiter, "", "");
  }

public StringJoiner add(CharSequence newElement) {
    prepareBuilder().append(newElement);
    return this;
  }
 
private StringBuilder prepareBuilder() {
    if (value != null) {
      value.append(delimiter);
    } else {
      value = new StringBuilder().append(prefix);
    }
    return value;
  }

@Override
  public String toString() {
    if (value == null) {
      return emptyValue;
    } else {
      if (suffix.equals("")) {
        return value.toString();
      } else {
        int initialLength = value.length();
        String result = value.append(suffix).toString();
        // reset value to pre-append initialLength
        value.setLength(initialLength);
        return result;
      }
    }
  }