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
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()); } }