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。省的我们开发人员再次通过StringBuffer或者StingBuilder拼接。
我们查看一下一下代码，试着猜一下。

1.简单的字符串拼接

　　输出：HelloWorld

 

2:可使用特定的分隔符或是前后缀来拼接字符串

注：当我们使用StringJoiner(CharSequence delimiter)初始化一个StringJoiner的时候，这个delimiter其实是分隔符，并不是字符串的初始值

　输出：[Hello,World]

 

3.实现原理

依赖StringBuilder实现，性能和StringBuilder差不多，同样也是非线程安全的

 

package java.util;public final class StringJoiner {private final String prefix;//前缀private final String delimiter;//间隔符private final String suffix;//后缀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) {//间隔符，前缀和后缀判断是否为null，null将抛出异常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"); // 成员变量赋值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nullpublic StringJoiner setEmptyValue(CharSequence emptyValue) {this.emptyValue = Objects.requireNonNull(emptyValue,"The empty value must not be null").toString();return this;}@Overridepublic 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 initialLengthvalue.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() {// Remember that we never actually append the suffix unless we return// the full (present) value or some sub-string or length of it, so that// we can add on more if we need to.//不忘添加后缀的长度return (value != null ? value.length() + suffix.length() :emptyValue.length());}
}