java中字符串常量的连接

In C++, the best canonical way to create a multiline string of which I am aware is to create adjacent strings and let the compiler concatenate them at compile time, like this:

string s = "This is a very long string ...
"
   " and it keeps on going...";

In Java, the only way I know to do this is with concatenation:

String s = "This is a very long string ...
" +
   " and it keeps on going...";

The question is, does this generate a single string at runtime, or does java actually concatenate at compile time also? The reason the question comes up is because of the following behavior:

String s1 = "abc";
String s2 = "abc";
System.out.println(s1 == s2); // this prints true, because the compiler
// generates only one "abc" object
String s3 = "a";
s3 += "bc";
System.out.println(s1 == s3); // false

解决方案

String s3 = "a";
s3 += "bc";

Is same as:

String s3 = "a";
s3 = new StringBuilder().append(s3).append("bc").toString();

So it creates a new instance of String.

You can even try:

String s = null;
s += null;
System.out.println(s); // prints "nullnull"