Question

I would like to use characters from a string for many times and wonder if it is better to use string.charAt() everytime I need a character, or save the char array with string.toCharArray() and use index to access character in array. So I wrote a simple benchmark program and I observed a significant performance difference.

this.numRows = 0

and the result is

map

I wonder why charAt() is slower than direct access with array? I checked the implementation of chatAt() and I see no difference with array direct access method.

<div>
        <p>Number of rows = {this.numRows}</p>
        {this.state.members.map((member) => {
          if (member.display) {
            this.numRows++;
            return <p>{ member.name }</p>  
          }
        })}
      </div>

Answer 1

使用toCharArray（）会带来初始成本，即复制String的内部数组。

从那时起，它就是对数组的简单访问（在返回值时，charAt（）中也会发生隐式边界检查）。调用charAt（）会带来函数调用和重复边界检查的代价（抛出StringIndexOutOfBoundsException而不是ArrayIndexOutOfBoundsException）。

这种效果是众所周知的，并且已经在Java早期性能书中提到过。

简而言之：如果您只访问String中的单个字符，那么最好使用charAt（）。如果您访问更多或所有字符并且字符串可能更长，那么最好使用toCharArray（）并改为通过数组。

Better way to repeatedly use characters from a string in Java

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