jdk8StringJoiner

JAVA8——StringJoiner类
引言：在阅读项目代码是，突然看到了StringJoiner这个类的使用，感觉很有意思，对实际开发中也有用，实际上是运用了StringBuilder的一个拼接字符串的封装处理。

介绍
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。

StringJoiner sj = new StringJoiner(":", "[", "]");
sj.add("George").add("Sally").add("Fred");
String desiredString = sj.toString();
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代码输出为[George:Sally:Fred]

源码
这个类的源码很简单，大家很容易就可以看明白。

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());
}

}

参考链接：https://blog.csdn.net/diehuang3426/article/details/82229625