StringJoiner是Java8新出的一个类，用于构造由分隔符分隔的字符序列，并可选择性地从提供的前缀开始和以提供的后缀结尾。
StringJoiner nameJoiner = new StringJoiner(":", "[", "]");
nameJoiner.add("George").add("Sally").add("Fred");
System.out.println(nameJoiner.toString());说实话，为何要讲这个呢？主要是要从很频繁很常见的地方了解到你不知道的基本知识。要实现String Join需求一定也不复杂，google的guava也给出了实现：
System.out.println(Joiner.on(";").join(Lists.newArrayList("alex", "top")));
//源码也不复杂，主要是依赖StringBuilder
public class Joiner {
private final String separator;
public static Joiner on(String separator) {
return new Joiner(separator);
}
@CanIgnoreReturnValue
public final StringBuilder appendTo(StringBuilder builder, Iterator<?> parts) {
try {
this.appendTo((Appendable)builder, (Iterator)parts);
return builder;
} catch (IOException var4) {
throw new AssertionError(var4);
}
}
}Joiner更简洁，个人更喜欢，我猜想StringJoiner 也有它的影子（作者估计用过，看完整篇会懂）。
public final class StringJoiner {
private final String prefix;//前缀
private final String delimiter;//后缀
private final String suffix;//间隔符
private StringBuilder value;//值
private String emptyValue;//空值
public StringJoiner(CharSequence delimiter) {
this(delimiter, "", "");//默认前缀和后缀为"",重载调用
}
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");
this.prefix = prefix.toString();
this.delimiter = delimiter.toString();
this.suffix = suffix.toString();
this.emptyValue = this.prefix + this.suffix;//空值被设置为只有前后缀
}
//设置空值，检查是否为null
public StringJoiner setEmptyValue(CharSequence emptyValue) {
this.emptyValue = Objects.requireNonNull(emptyValue,
"The empty value must not be null").toString();
return this;
}
@Override
public String toString() {
if (value == null) {
return emptyValue;//没有值将返回空值或者后续设置的空值
} else {
if (suffix.equals("")) {
return value.toString();//后缀为""直接返回字符串，不用添加
} else {
//后缀不为""，添加后缀，然后直接返回字符串，修改长度
int initialLength = value.length();
String result = value.append(suffix).toString();
// reset value to pre-append initialLength
value.setLength(initialLength);
return result;
}
}
}
初始化，先添加前缀，有了之后每次先添加间隔符，StringBuilder后续append字符串
public StringJoiner add(CharSequence newElement) {
prepareBuilder().append(newElement);
return this;
}
//合并StringJoiner，注意后面StringJoiner 的前缀就不要了，后面的appen进来
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，先添加前缀，有了之后每次先添加间隔符
private StringBuilder prepareBuilder() {
if (value != null) {
value.append(delimiter);
} else {
value = new StringBuilder().append(prefix);
}
return value;
}
public int length() {
return (value != null ? value.length() + suffix.length() :
emptyValue.length());
}
}StringJoiner 的用法仅此而已，难道不支持List吗？感觉它有一般目的是为此而生，只不过它通过Stream来完成：
System.out.println(Lists.newArrayList("alex", "top").stream().collect(Collectors.joining(";")));
//源码
public final class Collectors {
public static Collector<CharSequence, ?, String> joining(CharSequence delimiter) {
return joining(delimiter, "", "");
}
public static Collector<CharSequence, ?, String> joining(CharSequence delimiter,
CharSequence prefix,
CharSequence suffix) {
return new CollectorImpl<>(
() -> new StringJoiner(delimiter, prefix, suffix),
StringJoiner::add, StringJoiner::merge,
StringJoiner::toString, CH_NOID);
}
/**它是一个可变的汇聚操作,它会在所有元素都处理完毕后，将累积的结果转换为一个最终的表示
* T：输入类型（流中每个元素的类型）
* A：表示中间结果容器的类型
* R：返回类型
**/
static class CollectorImpl<T, A, R> implements Collector<T, A, R> {
CollectorImpl(Supplier<A> supplier,
BiConsumer<A, T> accumulator,
BinaryOperator<A> combiner,
Function<A,R> finisher,
Set<Characteristics> characteristics) {
this.supplier = supplier; //延迟创建中间结果容器
this.accumulator = accumulator; //将流中的元素添加中间结果容器
this.combiner = combiner; //对于两个中间结果容器并成一个（非必然运行）
this.finisher = finisher;//可选的最终转换（中间结果与最终结果类型是否一致决定是否运行）
this.characteristics = characteristics;//标示中间结果是可以直接向最终结果进行强制类型转换
}
@Override
public BiConsumer<A, T> accumulator() {
return accumulator;
}
@Override
public Supplier<A> supplier() {
return supplier;
}
@Override
public BinaryOperator<A> combiner() {
return combiner;
}
@Override
public Function<A, R> finisher() {
return finisher;
}
@Override
public Set<Characteristics> characteristics() {
return characteristics;
}
}完！
