JDK-StringJoiner构造及添加元素源码分析

// 1 构造 设置分隔符/前缀/后缀
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获取拼接后结果
joiner.toString(); // "start1,2,3,4end"

public final class StringJoiner {
	// 前缀
    private final String prefix;
    // 分隔符
    private final String delimiter;
    // 后缀
    private final String suffix;
    // 拼接后的value，包含前缀、分隔符、和拼接元素，不包括后缀，后缀在toString返回前才会填充
    /*
     * 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，默认为prefix+suffix，支持自定义设置
    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() {
    // 没有调用add方法添加元素，直接返回空值
    if (value == null) {
        return emptyValue;
    } else {
        // 没有后缀，直接将value.toString()即可
        if (suffix.equals("")) {
            return value.toString();
        } else {
            // 追加后缀后返回
            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(); // 获取value长度
String result = value.append(suffix).toString(); // value中追加后缀，得到最终结果
value.setLength(initialLength); // 长度重置回追加后缀前的长度，可以理解为将原长度之后字符清空
return result;	// 返回拼接完成的结果

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');
    }
    // 设置count为填充后缀之前的长度值
    count = newLength;
}
// java.lang.StringBuilder#toString
public String toString() {
    // 使用value[0,count]位置的字符生成结果
    // Create a copy, don't share the array
    return new String(value, 0, count);
}