The performance of a concatenation operation for a String or StringBuilder object depends on how often a memory allocation occurs. A String concatenation operation always allocates memory, whereas a StringBuilder concatenation operation only allocates memory if the StringBuilder object buffer is too small to accommodate the new data. Consequently, the String class is preferable for a concatenation operation if a fixed number of String objects are concatenated. In that case, the individual concatenation operations might even be combined into a single operation by the compiler. A StringBuilder object is preferable for a concatenation operation if an arbitrary number of strings are concatenated; for example, if a loop concatenates a random number of strings of user input.

来源:MSDN

字符串连接将花费更多。 在Java中，您可以根据需要使用StringBuffer或StringBuilder。 如果您想要一个同步的、线程安全的实现，请使用StringBuffer。这将比字符串连接更快。

如果你不需要同步的或者线程安全的实现，选择StringBuilder。 这将比String连接更快，也比StringBuffer更快，因为它们没有同步开销。

StringBuilder is probably preferable. The reason is that it allocates more space than currently needed (you set the number of characters) to leave room for future appends. Then those future appends that fit in the current buffer don't require any memory allocation or garbage collection, which can be expensive. In general, I use StringBuilder for complex string concatentation or multiple formatting, then convert to a normal String when the data is complete, and I want an immutable object again.

除了前面的答案，当我想到这样的问题时，我总是做的第一件事是创建一个小型测试应用程序。在这个应用程序中，为这两种情况执行一些计时测试，看看哪个更快。

恕我直言，添加500+字符串条目绝对应该使用StringBuilder。

一个简单的例子来演示使用String连接和StringBuilder在速度上的差异:

System.Diagnostics.Stopwatch time = new Stopwatch();
string test = string.Empty;
time.Start();
for (int i = 0; i < 100000; i++)
{
    test += i;
}
time.Stop();
System.Console.WriteLine("Using String concatenation: " + time.ElapsedMilliseconds + " milliseconds");

结果:

使用字符串串联:15423毫秒

StringBuilder test1 = new StringBuilder();
time.Reset();
time.Start();
for (int i = 0; i < 100000; i++)
{
    test1.Append(i);
}
time.Stop();
System.Console.WriteLine("Using StringBuilder: " + time.ElapsedMilliseconds + " milliseconds");

结果:

使用StringBuilder: 10毫秒

结果，第一次迭代花费了15423毫秒，而使用StringBuilder的第二次迭代花费了10毫秒。

在我看来，使用StringBuilder更快，快得多。

如果你要做很多字符串连接，使用StringBuilder。当您连接一个String时，每次都会创建一个新的String，这会占用更多的内存。

Alex