为什么会新增这样一个string辅助类?
StringBuilder sb = new StringBuilder();
IntStream.range(1,10).forEach(i->{
sb.append(i+"");
if( i < 10){
sb.append(",")
}
});
StringJoiner sj = new StringJoiner(",");
IntStream.range(1,10).forEach(i->sj.add(i+""));
setEmptyValue, 默认情况下的emptyValue是前缀加后缀, 用户可自定义emptyValue merge(StringJoiner other),合并另外一个joiner length, 当前长度,为空看emptyValue的长度
让我实现StringJoiner,我会怎么办呢?
维护一个List,最后toString的时候join一下就好了。优势:实现非常方便 缺点:list太浪费空间(扩容时都是按照系数扩容的) 在StringBuilder基础上改造(jdk实现方式就是以组合的形式增强的StringBuilder
成员变量
private final String prefix;
private final String delimiter;
private final String suffix;
/*
* 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;
/*
* 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;
构造函数
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();
// !!!构造时就直接将emptyValue拼接好了。
this.emptyValue = this.prefix + this.suffix;
}
添加元素
public StringJoiner add(CharSequence newElement) {
prepareBuilder().append(newElement);
return this;
}
private StringBuilder prepareBuilder() {
// 从构造函数和类变量的声明可以看出,没有添加元素前stringbuilder是没有初始化的
if (value != null) {
// 已经有元素存在的情况下,添加元素前先将分隔符添加进去
value.append(delimiter);
} else {
// 没有元素存在的情况下先把前缀加进去
value = new StringBuilder().append(prefix);
}
return value;
}
关键的toString
public String toString() {
if (value == null) {
// 这里如果没有自定义空值就是前缀+后缀咯。。
return emptyValue;
} else {
// 为什么不直接value.toString()+suffix?????
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;
}
}
}
merge
public StringJoiner merge(StringJoiner other) {
Objects.requireNonNull(other);
if (other.value != null) {
final int length = other.value.length();
// 下面这段注释是说避免merge(this)时受影响,为什么?
// 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;
}
private StringBuilder prepareBuilder() {
if (value != null) {
value.append(delimiter);
} else {
value = new StringBuilder().append(prefix);
}
return value;
}
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());
}
总结
基于StringBuilder实现,add时就把prefix和分隔符给加上了,suffix永远都不加,知道toString和length调用时才加入计算。这样带来的merge操作实现的极大便利性!!!!!学到了,真的不错 emptyValue这个一定要构造时就生成吗?用户想有自己的默认值还需要先构造实例再注入吗。。。。这个觉得还是有点奇怪 Objects这个工具方法是返回的校验的值本身,不错。
public StringJoiner setEmptyValue(CharSequence emptyValue) {
// 注意这个Objects.requireNonNull方法是return的第一个参数。。。
this.emptyValue = Objects.requireNonNull(emptyValue,
"The empty value must not be null").toString();
return this;
}
来源:jianshu.com/p/469fe8fdd3be
end
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