StringJoiner介绍

如果需要拼接分隔符的字符串，建议使用 Java 8 中的这款拼接神器：StringJoiner，你值得拥有。

1、拼接

public static void main(String[] args) {
	StringJoiner stringJoiner = new StringJoiner(",");
	stringJoiner.add("hello");
	stringJoiner.add("guys");
	stringJoiner.add("欢迎关注Java后端指南");
	System.out.println(stringJoiner.toString());
}

结果：

hello,guys,欢迎关注Java后端指南

2、构造方法

源码可以看到两个构造方法

这个是必须带分割符

   public StringJoiner(CharSequence delimiter) {
        this(delimiter, "", "");
    }

这个是必须带分割符，前缀，后缀

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;
    }

公开方法：

• setEmptyValue：设置空值
• toString：转换成 String
• add：添加字符串
• merge：从另一个 StringJoiner 合并
• length：长度（包括前缀后缀）

setEmptyValue：

 public StringJoiner setEmptyValue(CharSequence emptyValue) {
        this.emptyValue = Objects.requireNonNull(emptyValue,
            "The empty value must not be null").toString();
        return this;
    }

toString：

@Override
    public String toString() {
        if (value == null) {
            return emptyValue;
        } else {
            if (suffix.equals("")) {
                return value.toString();
            } else {
                int initialLength = value.length();
                String result = value.append(suffix).toString();
                // reset value to pre-append initialLength
                value.setLength(initialLength);
                return result;
            }
        }
    }

add：

public StringJoiner add(CharSequence newElement) {
        prepareBuilder().append(newElement);
        return this;
    }

merge：

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;
    }

length：

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.
        return (value != null ? value.length() + suffix.length() :
                emptyValue.length());
    }

3、前后缀拼接

StringJoiner stringJoiner = new StringJoiner(",", "[", "]");
        stringJoiner.add("hello");
        stringJoiner.add("guys");
        stringJoiner.add("欢迎关注Java后端指南");

结果：

[hello,guys,欢迎关注Java后端指南]