// 1 構(gòu)造 設(shè)置分隔符/前綴/后綴
StringJoiner joiner = new StringJoiner(",", "start", "end");
// 2 添加元素
List<String> elements = Arrays.asList("1","2","3","4")
for (String s: elements) {
    joiner.add(s);
}
// 3獲取拼接后結(jié)果
joiner.toString(); // "start1,2,3,4end"

public final class StringJoiner {
	// 前綴
    private final String prefix;
    // 分隔符
    private final String delimiter;
    // 后綴
    private final String suffix;
    // 拼接后的value，包含前綴、分隔符、和拼接元素，不包括后綴，后綴在toString返回前才會(huì)填充
    /*
     * StringBuilder value -- at any time, the characters constructed from the
     * prefix, the added element separated by the delimiter, but without the
     * suffix, so that we can more easily add elements without having to jigger
     * the suffix each time.
     */
    private StringBuilder value;
  	// 空值，默認(rèn)為prefix+suffix，支持自定義設(shè)置
    private String emptyValue;
}

public StringJoiner(CharSequence delimiter) {
    this(delimiter, "", "");
}

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");
    // make defensive copies of arguments
    this.prefix = prefix.toString();
    this.delimiter = delimiter.toString();
    this.suffix = suffix.toString();
    this.emptyValue = this.prefix + this.suffix;
}

this.emptyValue = this.prefix + this.suffix;

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;
}

joiner.add("1");
---
prepareBuilder()			//  value = "start"
	.append("1");			//  value = "start1"
=========================================================
joiner.add("2");
---
prepareBuilder()			//  value = "start1,"
	.append("2");			//  value = "start1,2"
=========================================================
joiner.add("3");
---
prepareBuilder()			//  value = "start1,2,"
	.append("3");			//  value = "start1,2,3"

public String toString() {
    // 沒有調(diào)用add方法添加元素，直接返回空值
    if (value == null) {
        return emptyValue;
    } else {
        // 沒有后綴，直接將value.toString()即可
        if (suffix.equals("")) {
            return value.toString();
        } else {
            // 追加后綴后返回
            int initialLength = value.length();
            String result = value.append(suffix).toString();
            // reset value to pre-append initialLength
            value.setLength(initialLength);
            return result;
        }
    }
}

String result = value.append(suffix).toString(); 

int initialLength = value.length(); // 獲取value長(zhǎng)度
String result = value.append(suffix).toString(); // value中追加后綴，得到最終結(jié)果
value.setLength(initialLength); // 長(zhǎng)度重置回追加后綴前的長(zhǎng)度，可以理解為將原長(zhǎng)度之后字符清空
return result;	// 返回拼接完成的結(jié)果

StringJoiner joiner = new StringJoiner(",", "start", "end");
List<String> elements = Arrays.asList("1","2","3","4")
for (String s: elements) {
    joiner.add(s);
}
joiner.toString(); // "start1,2,3,4end"
joiner.add("5"); 
joiner.toString(); // "start1,2,3,4end,5end"

//java.lang.AbstractStringBuilder#setLength
public void setLength(int newLength) {
    if (newLength < 0)
        throw new StringIndexOutOfBoundsException(newLength);
    ensureCapacityInternal(newLength);
    if (count < newLength) {
        Arrays.fill(value, count, newLength, '\0');
    }
    // 設(shè)置count為填充后綴之前的長(zhǎng)度值
    count = newLength;
}
// java.lang.StringBuilder#toString
public String toString() {
    // 使用value[0,count]位置的字符生成結(jié)果
    // Create a copy, don't share the array
    return new String(value, 0, count);
}