Java string literal automatically interpreted as String object, but how?

Java actually does have a few overloaded operators. For example, + can be applied to String objects as well as to int, float, and other numerical types. In each case, the return type is defined by the type of the expression on each side of the +. However, all overloaded operators are predefined in the language; programmers have no mechanism in the language to define any others. See the language specification for details.

Basically the compiler rewrites your code.

String x = "Some String";

For concatenation, it is just syntactic sugar for StringBuffer append method, ie this:

z += " with some more in it";

is compiled as:

z = new StringBuffer(z).append(" with some more in it").toString();

Now for a single concat this is an object created and 2 method calls, so if you are building a very long string inside a loop, it is FAR more efficient to write:

StringBuilder buf = new StringBuilder(); // Not synchronized so quicker than StringBuffer
while ( condition is true )
    buf.append(...);
String z = buf.toString();

rather than:

String z = "";
while (condition is true)
   z += "...";

Edit: removed wrong code example...