如果编译器自动将字符串连接转换为StringBuilder，为什么要显式使用StringBuilder？ [英] Why use StringBuilder explicitly if the compiler converts string concatenation to a StringBuilder automatically?

问题描述

可能重复：

StringBuilder vs Java中toString（）的字符串连接

我很想知道，因为编译器内部使用StringBuilder在执行字符串连接时附加2个字符串，那么问题是什么？如果String连接已经为你完成了工作，我为什么要使用StringBuilder呢？还有其他具体原因吗？

I am wondering, since the compiler internally uses a StringBuilder to append 2 Strings when performing String concatenation, then what's the point and why should I bother using StringBuilder if String concatenation already did the job for you? Are there any other specific reasons?

推荐答案

如你所说，你不应该使用 StringBuilder 而不是简单的字符串连接表达式，例如 a +=+ b 。后者输入更快，更容易阅读，编译器内部仍会使用 StringBuilder ，因此重写它没有性能优势。

As you mention, you should not use StringBuilder instead of a simple string concatenation expression such as a + " = " + b. The latter is faster to type, easier to read, and the compiler will use a StringBuilder internally anyway so there is no performance advantage by rewriting it.

但是如果在循环中连接大量字符串，则 StringBuilder 非常有用。以下代码效率低下。它需要运行O（n ² ）时间并创建许多临时字符串。

However StringBuilder is useful if you are concatenating a large number of strings in a loop. The following code is inefficient. It requires O(n²) time to run and creates many temporary strings.

String result = "";
for (int i = 0; i < foo.length; ++i)
{
    result += bar(foo[i]);  // Bad
}

请改为尝试：

StringBuilder sb = new StringBuilder();
for (int i = 0; i < foo.length; ++i)
{
    sb.append(bar(foo[i]));
}
String result = sb.toString();

编译器只优化简单的 a + b + c 表达式。它无法自动优化上述代码。

The compiler optimises only simple a + b + c expressions. It cannot optimize the above code automatically.

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