Java String return method does not return

Question

I have a method to return a string of the folder path. CSVFile method calls another method ConfigFilePath to extract the value from a config file and return it back to be used for another condition.

public CSVFile(ManagedElement_T[] meInfo) {
    String path = null;
    ConfigFilePath(path);
    system.out.print("CSV "+path);
}

public String ConfigFilePath(String path) {
    final String directory = System.getProperty("user.dir");
    final String filename = "config.properties";
    final File configFile = new File(directory+"/"+filename);

    final Properties prop = new Properties();
    InputStream input = null;

    try {
        input = new FileInputStream(configFile);
        prop.load(input);
        path = prop.getProperty("diretory_csv");
        System.out.println("PATH " + path);
    } catch (IOException ex) {
        ex.printStackTrace();
    } finally {
             // Close the file
    }

    File checkPath = new File(path.toString());
    System.out.println("CHECK " + checkPath);
    if (checkPath.exists()) {
        System.out.println("Directory already exists ...");
    } else {
           //mkdir if it does not exist
    }


    return path;
}

Problem is, it doesn't return any var path? just says null when I print it but inside the ConfigFilePath method it seems its getting the right values based from the prints in eclipse.

System.out.println("PATH " + path); = C:/opt/CORBA/input
System.out.println("CHECK " + checkPath); = C:\opt\CORBA\input
System.out.print("CSV "+path); = null

possible duplicate of [Is Java "pass-by-reference" or "pass-by-value"?](http://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value) — Oleg Estekhin, Jun 17 '14 at 08:17

score 6 · Answer 1 · answered Jun 17 '14 at 06:48

The method ConfigFilePath is returning a value. You have to assign that returned value to the variable path:

path = ConfigFilePath(path);

Note: Try to follow Java naming conventions. Use mixedCase for methods/variables and use CamelCase for classes/interfaces

score 2 · Accepted Answer · answered Jun 17 '14 at 06:52

Java is actually pass by value not pass by reference.

so when you initialized it here:

path = prop.getProperty("diretory_csv");

path wont actually reference that value but get destroyed when out of scope.

solution:

Get the returned value of the ConfigFilePath method

path = ConfigFilePath(path);

score 0 · Answer 3 · answered Jun 17 '14 at 06:50

0

ConfigFilePath is returning a value. You should get the result from it and put it in the path:

public CSVFile(ManagedElement_T[] meInfo) {
    String path = ConfigFilePath(path);
    system.out.print("CSV "+path);
}

answered Jun 17 '14 at 06:50

Alexandre Santos

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score 0 · Answer 4 · answered Jun 17 '14 at 07:01

The problem you face is that java is passing method-parameters by value, not by reference. It can be confusing, since when using object types the passed value is actually an object-reference.

Let's have a look at some lines of your code:

public String ConfigFilePath(String path)

Here your method gets passed a reference to some String and keeps it in a local variable called path.

path = prop.getProperty("diretory_csv");

Now you're changing the local variable path to point to a new String-object.

return path;

This is passing out the reference as a return-value. ConfigFilePath's local path-variable goes out of scope.

ConfigFilePath(path);

As the other answers said: in this line you're calling the method passing a reference to a String that is referenced by CSVFile's local path-variable. But since passing is by value this path-variable will not be changed by the method-call. Your result is passed via the return-value - and you are ignoring that instead of assigning it to your path-variable.

So to sum it up:

ConfigFilePath() doesn't need a parameter since it's not using the value that is passed in
You have to assign the returned value to some variable in your calling method for it to be of any use.

Java String return method does not return

4 Answers4