江苏龙网

  1. 主页 > 生活百科 > >

放弃 StringBuilder！Java8的StringJoiner，真香

Java8


放弃 StringBuilder！Java8的StringJoiner，真香

文章插图
 
为什么会新增这样一个string辅助类？原有的stringbuilder太死板，不支持分割，如果想让最终的字符串以逗号隔开，需要这样写
StringBuilder sb = new StringBuilder();IntStream.range(1,10).forEach(i->{sb.Append(i+"");if( i < 10){sb.append(",")} });是不是太死板了，不好用，StringJoiner怎样写呢？
StringJoiner sj = new StringJoiner(",");IntStream.range(1,10).forEach(i->sj.add(i+""));【放弃 StringBuilder！Java8的StringJoiner，真香】有哪些平时用得还比较少的功能：
  • 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）
jdk实现的源码分析
  • 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;其实从成员变量的注释里就能看出他们的作用和需要注意的点了
  • 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 argumentsthis.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 = https://www.isolves.com/it/cxkf/yy/JAVA/2021-05-17/this.prefix + this.suffix;}为什么要一开始就构造好呢？如果我想直接自定义emptyValue直接用构造函数初始化不是更方便吗？是因为绝大多数场景下都不会自定义emptyValue的场景吗？不对啊，感觉这个场景非常必要啊 。。。
  • 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 = https://www.isolves.com/it/cxkf/yy/JAVA/2021-05-17/new StringBuilder().append(prefix);}return value;}可以看出在添加元素的过程中就已经把前缀和分割字符什么的都处理好了，全部都在stringbuilde中了，唯一没有处理的就是后缀 。为什么？这样做tostring什么的时候真的超级方便的有木有 。。。。。
  • 关键的toString
public String toString() {if (value =https://www.isolves.com/it/cxkf/yy/JAVA/2021-05-17/= 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 initialLengthvalue.setLength(initialLength);return result;}}}为什么不直接value.toString()+suffix？答案在merge方法
  • 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 = https://www.isolves.com/it/cxkf/yy/JAVA/2021-05-17/new StringBuilder().append(prefix);}return value;}


推荐阅读


上一篇：孙权追谥孙策,孙权对周瑜后代

下一篇：今日头条赚钱的6大经验分享