How to pass a OUT String parameter to a function?

Question

How to pass a OUT String parameter to a function ? something like

public static Address getFromLocation (OUT string ErrorMsg) { ... }

If some errors are found in the getFromLocation then the function will update the ErrorMsg that i can use later.

If it's not possible, how in my example, can i retrieve the errorMsg ?

Answer 1

Java doesn't have "out" parameters.

There are a couple of standard(ish) ways to do this (report back an error) in Java:

1. Throw an exception, perhaps of a type defined specifically for this purpose, with the details in the exception.

2. Very much second best: Return an object with a property for the address and a property for the error string; in the successful case fill in the address and left the error string null, and in the error case fill in the error string and leave the address null.

3. Pass a List<String> or similar into the method (thanks a_horse_with_no_name for the reminder), and add to the list in the error case, returning null for the Address.

...or various variations on those themes.

#1 and #3 are vastly preferable to #2. #1 provides for structured exception handling. (For instance, the method calling getFromLocation may choose not to handle the exception and instead to document that it may throw that exception, leaving error handling to its caller.) #3 is particularly useful when you have a lot of methods like this and the error case is common (user input validation), and you want to present a series of errors.

Answer 2

One way in Java would be:

public void foo(List<String> bar) { ...

but, more reasonable:

public String foo() { ...

In other words:

• Java is pass by value
• so you can only pass a reference to a String object
• but String objects are immutable!
• so you can't OUT via parameter
• you can only return a string
• or (ugly hack) pass a "some sort of container" (for example a list of strings), and push your string into that container object.

In the end, the real answer is: a method should do one thing. If your method has to A) return some value and B) "out return" another value, then it is obviously doing two things, thus violating the Single Responsibility Principle.

Answer 3

There is no such thing as an out parameter in Java, so you have to imitate passing output parameters with pass-by-value by adding an extra level of indirection.

A very simple approach is passing a StringBuilder - a mutable object which could be modified by the caller:

public static Result getFromLocation(StringBuilder additionalInformation) {
    ...
    // Method modifies the mutable object
    additionalInformation.append("Some additional text");
    ...
}

The caller would have to provide a non-null StringBuilder, like this:

StringBuilder info = new StringBuilder();
Result res = getFromLocation(info);
// Caller sees information inserted by the method
System.out.println(info);

In situations when you receive back an error message, a better approach is to throw a checked exception. This way you would be able to provide the error message out of bounds, and cleanly separate error processing from the code for the main functionality of your system:

class AddressRetrievalExcepion extends Exception {
    ...
}
public static Address getFromLocation() throws AddressRetrievalExcepion {
    ...
    if (errorCondition) {
        throw new AddressRetrievalExcepion("Cannot get address");
    }
    ...
}

try {
    Address addr = getFromLocation(error);
} catch (AddressRetrievalExcepion ae) {
    System.out.println(ae.getMessage());
}