问题描述
有人告诉我，使用 StringBuffer 连接Java中的字符串比使用 + 运算符更有效率字符串秒。当你这样做时会发生什么？ StringBuffer 做什么不同？
Someone told me it's more efficient to use StringBuffer to concatenate strings in Java than to use the + operator for Strings. What happens under the hood when you do that? What does StringBuffer do differently?
推荐答案
最好使用StringBuilder（它是一个不同步的版本;你什么时候并行构建字符串？）这几天，几乎在所有情况下，但这是发生的事情：
It's better to use StringBuilder (it's an unsynchronized version; when do you build strings in parallel?) these days, in almost every case, but here's what happens:
当你使用+两个字符串时，它编译如下代码：
When you use + with two strings, it compiles code like this:
String third = first + second;
对于这样的事情：
StringBuilder builder = new StringBuilder( first );
builder.append( second );
third = builder.toString();
因此，仅举几个例子，它通常没有什么区别。但是当你构建一个复杂的字符串时，你经常需要处理的事情比这更多;例如，您可能正在使用许多不同的附加语句，或者像这样的循环：
Therefore for just little examples, it usually doesn't make a difference. But when you're building a complex string, you've often got a lot more to deal with than this; for example, you might be using many different appending statements, or a loop like this:
for( String str : strings ) {
out += str;
}
在这种情况下，新的 StringBuilder instance，以及一个新的 String （ out的新值 - String每次迭代都需要 s是不可变的。这非常浪费。用一个 StringBuilder 替换它意味着你只能生成一个 String 而不是用<$ c填充堆$ c>字符串你不关心。
In this case, a new StringBuilder instance, and a new String (the new value of out - Strings are immutable) is required in each iteration. This is very wasteful. Replacing this with a single StringBuilder means you can just produce a single String and not fill up the heap with Strings you don't care about.
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