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放弃 StringBuilder！用Java8的StringJoiner
（给ImportNew加星标，提高Java技能）
转自：炫迈哥
链接：jianshu.com/p/469fe8fdd3be
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;}- EOF -
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