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()); } }
