用Java以这种方式“（firstName.toLowerCase（））。charAt（0）”吗？ charAt（）方法仅应使用字符串引用变量？

Question

String firstName, middleName, lastName; 
char firstInitial, middleInitial, lastInitial;

firstName = "Huckle";
middleName = "Berry";
lastName = "Fin";

firstInitial= (firstName.toLowerCase()).charAt(0);
middleInitial = middleName.charAt(0);
lastInitial = lastName.charAt(0);

System.out.print(firstInitial);
System.out.print(middleInitial);
System.out.println(lastInitial);

Answer 1

...why is it possible to chain these methods. toLowerCase() and charAt() methods together?...

String class is immutable in java, so calling toLowerCase() will return another string with the result of that operation

..the charAt() method should only work with a string reference variables?...

you can use a literal string as well

firstInitial = "Huckle".toLowerCase().charAt(0);
middleInitial = middleName.charAt(0);
lastInitial = "Fin".charAt(0);

but as I said before, the method tolowerCase can be invoked on string objects and will return another string

you can (even that is not making much sense) call a sequence of method together

like:

firstInitial = firstName
        .toLowerCase()
        .toUpperCase()
        .substring(0)
        .toLowerCase()
        .toUpperCase()
        .trim()
        .charAt(0);