Java8的StringJoiner

StringJoiner是java.util包下的一个工具类，jdk1.8出来的
作用是在构造字符串时，可以自动添加前缀、后缀及分隔符，而不需要自己去实现这些添加字符的逻辑

public void StringJoiner ()
    {
        StringJoiner sj1 = new StringJoiner(",");
        StringJoiner sj2 = new StringJoiner(",", "[", "]");

        System.out.println(sj1.add("张三").add("丽莎").add("c"));
        System.out.println(sj1.length());
        System.out.println(sj2.add("a").add("b").add("c"));
        System.out.println(sj1.merge(sj2));
        System.out.println(sj2.merge(sj1));

    }

StringJoiner实体类属性

public final class StringJoiner {
    private final String prefix;// 前缀
    private final String delimiter;//分隔符
    private final String suffix;//后缀

    /*
     * 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;//入参

    /*
     * By default, the string consisting of prefix+suffix, returned by
     * toString(), or properties of value, when no elements have yet been added,
     * i.e. when it is empty.  This may be overridden by the user to be some
     * other value including the empty String.
     */
    private String emptyValue;

    /**
     * Constructs a {@code StringJoiner} with no characters in it, with no
     * {@code prefix} or {@code suffix}, and a copy of the supplied
     * {@code delimiter}.
     * If no characters are added to the {@code StringJoiner} and methods
     * accessing the value of it are invoked, it will not return a
     * {@code prefix} or {@code suffix} (or properties thereof) in the result,
     * unless {@code setEmptyValue} has first been called.
     *
     * @param  delimiter the sequence of characters to be used between each
     *         element added to the {@code StringJoiner} value
     * @throws NullPointerException if {@code delimiter} is {@code null}
     */

SringJoiner构造方法
两个构造方法：
1：默人前缀和后缀为null，只需要传入分隔符即可
2：前缀、后缀、分隔符都不能为ull

 /**
     * Constructs a {@code StringJoiner} with no characters in it, with no
     * {@code prefix} or {@code suffix}, and a copy of the supplied
     * {@code delimiter}.
     * If no characters are added to the {@code StringJoiner} and methods
     * accessing the value of it are invoked, it will not return a
     * {@code prefix} or {@code suffix} (or properties thereof) in the result,
     * unless {@code setEmptyValue} has first been called.
     *
     * @param  delimiter the sequence of characters to be used between each
     *         element added to the {@code StringJoiner} value
     * @throws NullPointerException if {@code delimiter} is {@code null}
     */
    public StringJoiner(CharSequence delimiter) {
        this(delimiter, "", "");
    }

    /**
     * Constructs a {@code StringJoiner} with no characters in it using copies
     * of the supplied {@code prefix}, {@code delimiter} and {@code suffix}.
     * If no characters are added to the {@code StringJoiner} and methods
     * accessing the string value of it are invoked, it will return the
     * {@code prefix + suffix} (or properties thereof) in the result, unless
     * {@code setEmptyValue} has first been called.
     *
     * @param  delimiter the sequence of characters to be used between each
     *         element added to the {@code StringJoiner}
     * @param  prefix the sequence of characters to be used at the beginning
     * @param  suffix the sequence of characters to be used at the end
     * @throws NullPointerException if {@code prefix}, {@code delimiter}, or
     *         {@code suffix} is {@code null}
     */
    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;
    }

Demo

StringJoiner sj1 = new StringJoiner(",");//传入分隔符
StringJoiner sj2 = new StringJoiner(",", "[", "]");、//3个参数，分隔符、前缀、后缀

添加分割文本

public StringJoiner add(CharSequence newElement) {
        prepareBuilder().append(newElement);
        return this;
    }
//底层是调用 StringBuilder  进行字符串拼接的
  private StringBuilder prepareBuilder() {
        if (value != null) {
            value.append(delimiter);
        } else {
            value = new StringBuilder().append(prefix);
        }
        return value;
    }

StringJoiner的方法集合
1：add（）；//添加风格的字符文本
2：length（）；//StringJoiner的字符长度
3：merge（）；合并两个StringJoiner集
4：toString（）；打印字符合集

由于StringJoiner底层运用的是StringBuilder ，且StringBuilder 不是线程安全的，所以StringJoiner也是线程不安全的