need assistance on compile error with basic java program

I am brand new to Java. I am having an issue compiling a basic java program, and I am trying to understand why. (note that the TextIO class in the code is used in book I am studying to simplify the IO process, I don't believe that is where the issue is) Here is my code:

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

        String      ln;
        String      tmp;
        int         i;
        int         noval;

        TextIO.readFile("sales.dat");

        while (TextIO.eof() == false){
            ln = TextIO.getln();
            for (i = 0; i < ln.length(); i++) {
                if (ln.charAt(i) == ':'){
                    tmp = ln.subString(i + 1);  
                }   
            } // end line for loop

            try {
                System.out.printf("%8.2f\n", Double(tmp.trim()));
            }
            catch (NumberFormatException e) {
                noval++;
            }
        } // end of file while loop

        System.out.printf("\nThere were a total of %d cities that didnt have data\n", noval); 

    } // end of main subroutine
} // end of ProcessSales class

The compile error I get is as follows:

[seldon@PrimeRadiant Exercises]$ javac ProcessSales.java
ProcessSales.java:15: cannot find symbol
symbol  : method subString(int)
location: class java.lang.String
                    tmp = ln.subString(i + 1);  
                 开发者_运维知识库           ^
ProcessSales.java:20: cannot find symbol
symbol  : method Double(java.lang.String)
location: class ProcessSales
                System.out.printf("%8.2f\n", Double(tmp.trim()));
                                             ^
2 errors

Ive declared ln as a String object. The subString method is straight out of the java api for a String object. I'm not understanding why I'm getting a cannot find symbol compile error, especially if it lists the method signature and location right below the error.

I marked the questions as homework, since I am working out of a textbook, and I am looking to understand the issue, rather than a flat solution. However it is self study, and not part of any actual class (right now).

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The great thing about the Java compiler is, it gives you alot of information to use in determining where problems exist in your code. For you, the first problem is here:

tmp = ln.subString(i + 1);

In this case you capitalized a letter that you shouldn't have. It should be:

tmp = ln.substring(i + 1);

Whenever you see compiler output saying 'cannot find symbol' its because the Java compiler could not find a method matching the outputted name, either due to a syntax error or missing dependency. For your second problem, you didn't post the appropriate code, but from the error message I can see you are missing the new keyword.

System.out.printf("%8.2f\n", Double(tmp.trim()));

Should be

System.out.printf("%8.2f\n", new Double(tmp.trim()));

If this is not your first programming language then I would recommend using an IDE like Eclipse, as it will give you auto-completion and syntax checking. It's a great tool for quickly learning the API's for the language. However if Java is your first programming language please do continue without hand-holding, as the hard knocks will cement in the lessons learned, faster.

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I haven't verified any of this, I just looked at the source and the error messages.

The first error seems to be a case error. The Java String class does not have a subString method, it has a substring method, note the lowercase s. Reference

The second error would probably be resolved if you used new Double or Double.valueof instead of Double. This is because you are probably trying to construct a new Double object and using new operator or the valueof method in the Double class do this for you. Reference

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In Java, method names are case sensitive. Check back with the String API specification for the correct "casing".

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Regarding your first error, you have a typo. It should be ln.substring(i+1) not ln.subString(i+1). All source code text in Java is case sensitive.

Regarding your second error, you need to use the new keyword to instantiate the Double object. Without new, the compiler is not looking for the constructor; instead it is looking for a method Double within your ProcessSales class and cannot find it.