Learning java, cannot find symbol

Question

I'm learning Java and stuck on a self test exercise writing a recursive function that prints a string backwards...

I understand the compiler error but I'm not sure what to do about it.

My code...

class Back {
    void Backwards(String s) {
            if (s.length = 0) { 
                    System.out.println();
                    return;
            }
            System.out.print(s.charAt(s.length));
            s = s.substring(0, s.length-1);
            Backwards(s);
    }
}

class RTest {
    public static void main(String args[]) {
            Back b;
            b.Backwards("A STRING");
    }

}

Compiler output...

john@fekete:~/javadev$ javac Recur.java 
Recur.java:3: error: cannot find symbol
    if (s.length = 0) { 
         ^
  symbol:   variable length
  location: variable s of type String
Recur.java:7: error: cannot find symbol
    System.out.print(s.charAt(s.length));
                               ^
  symbol:   variable length
  location: variable s of type String
Recur.java:8: error: cannot find symbol
    s = s.substring(0, s.length-1);
                        ^
  symbol:   variable length
  location: variable s of type String
3 errors

Finished code...

class Back {
    static void backwards(String s) {
            if (s.length() == 0) {
                    System.out.println();
                    return;
            }
            System.out.print(s.charAt(s.length()-1));
            s = s.substring(0, s.length()-1);
            backwards(s);
    }
}

class RTest {
    public static void main(String args[]) {

            Back.backwards("A STRING");
    }
}   

Answer 1

Write it like this:

s.length() == 0 // it's a method, not an attribute

Answer 2

Some general 'good coding' suggestions:

• Class names should represent a 'thing', usually a classname is a noun (e.g. "StringTool")
• Methods should represent an action, usually a methodname is a verb (e.g. "reverse")
• Parameter and variable names should be meaningful and describe what they represent.
• You should not re-assign method parameters because it can be misleading.
• A method should have precisely one responsability (so not reversing AND printing a string). This promotes clarity and reuse.

I have applied these suggestions to your finished code, see below:

public class StringTool {

    public static String reverse(String source) {

        // stop condition of the recursion
        if (source.isEmpty()) {
            return "";
        }

        int lastPosition = source.length() - 1;
        String lastCharacter = source.charAt(lastPosition);
        String restOfSource = source.substring(0, lastPosition);

        // place the last character at the beginning and reverse the rest 
        // of the source recursively
        return lastCharacter + reverse(restOfSource);
    }

    // test method
    public static void main(String args[]) {
        System.out.println(reverse("A STRING"));
    }

}