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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。

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() {        // 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());    }}