Why is the replacefirst method of the string class unavailable?

Today, we are using the string replace method to replace the following and. The result shows that it is not easy to use. Then we made a test,

CodeI:

String STR = new string ("and aae112 ");

Str. replacefirst ("and ","");
System. Out. println (STR );

And aae112

Code 2:

String STR = new string ("and aae112 ");

String out = Str. replacefirst ("and ","");
System. Out. println (out );

Aae112

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It can be seen that the result of Code 2 is correct. It can be seen that replaceall does not change the original STR, which is different from Pb and PL/SQL. The reason should be that STR points to an address.