由于字符串在Java中是不可变的，因此在执行 + ， + = 或concat(String)，将生成一个新的String.字符串越大，花费的时间越长-要复制的内容更多，并且会产生更多的垃圾.

Since String is immutable in java, when you do a +, += or concat(String), a new String is generated. The bigger the String gets the longer it takes - there is more to copy and more garbage is produced.

例如，当今的Java编译器会优化您的字符串连接以使其达到***状态，例如

Today's java compilers optimizes your string concatenation to make it optimal, e.g.

System.out.println("x:"+x+" y:"+y);

编译器将其生成为:

System.out.println((new StringBuilder()).append("x:").append(x).append(" y:").append(y).toString());

我的建议是编写易于维护和阅读的代码.

My advice is to write code that's easier to maintain and read.

此链接显示 StringBuilder vs StringBuffer vs String.concat的性能-做对了