How to write Java function that returns values of multiple data types?

Question

For example, I want to create a function that can return any number (negative, zero, or positive).

However, based on certain exceptions, I\'d like the function to re

Answer 1

IEEE Floating Point

Most languages will handle your specific example without Exceptions or union types because IEEE floating point includes a representation for NaN. In Java, use Double.NaN:

public static double quadratic(double a, double b, double c, int polarity) {
    double x = b*b - 4*a*c;

    // When x < 0, Math.sqrt(x) retruns NaN
    if (x < 0) {
        return Double.NaN;
    }
    return (-b + Math.sqrt(x) * polarity) / (2*a);
}

That produces your exact output that you wanted:

x = 4.0
x = 3.0
x = NaN
x = NaN

Exceptions

Exceptions are The Old Java Way of solving similar problems:

public static double quadratic(double a, double b, double c, int polarity) {
    double x = b*b - 4*a*c;
    // When x < 0, Math.sqrt(x) returns NaN
    if (x < 0) {
        throw new Exception("NaN")
    }
    return (-b + Math.sqrt(x) * polarity) / (2*a);
}

Here's your client code for an Exception.

a=1; b=-7; c=12;

// x = 4.0
try {
    System.out.println("x = " + quadratic(a, b, c, 1));
} catch (Exception iae) {
    System.out.println("Oopsie: " + iae.getMessage());
}

// x = 3.0
try {
    System.out.println("x = " + quadratic(a, b, c, -1));
} catch (Exception iae) {
    System.out.println("Oopsie: " + iae.getMessage());
}

// "invalid" coefficients.
a=4; b=4; c=16;

// Oopsie: NaN
try {
    System.out.println("x = " + quadratic(a, b, c, 1));
} catch (Exception iae) {
    System.out.println("Oopsie: " + iae.getMessage());
}

// Oopsie: NaN
try {
    System.out.println("x = " + quadratic(a, b, c, -1));
} catch (Exception iae) {
    System.out.println("Oopsie: " + iae.getMessage());
}

Union Types

To truly pass or return unrelated types to or from a method, you want Union types which Java does not really support. But Paguro provides Union Types which you can use in Java like this (using Or):

public static Or<Double,String> quadratic(double a, double b,
                                          double c, int polarity) {
    double x = b*b - 4*a*c;

    // When x < 0, Math.sqrt(x) retruns NaN
    if (x < 0) {
        return Or.bad("NaN");
    }
    return Or.good((-b + Math.sqrt(x) * polarity) / (2*a));
}

@Test public void testQuadradic() {
    double a, b, c;
    a=1; b=-7; c=12;

    // x = Good(4.0)
    System.out.println("x = " + quadratic(a, b, c, 1));

    // x = 3.0
    System.out.println(
            (String) quadratic(a, b, c, -1)
                    .match(good -> "x = " + good,
                           bad -> "Oopsie: " + bad));

    // "invalid" coefficients.
    a=4; b=4; c=16;

    // x = Bad("NaN")
    System.out.println("x = " + quadratic(a, b, c, 1));

    // Oopsie: NaN
    System.out.println(
            (String) quadratic(a, b, c, -1)
                    .match(good -> "x = " + good,
                           bad -> "Oopsie: " + bad));
}

Conclusion

For your specific example, just use Floating Point. For a more general solution, I find union types more useful than Exceptions. You can use union types as arguments to a method that might take two different inputs which have no common interface or ancestor. They are also more friendly to Functional Programming.

Answer 2

Absolutely! In your case, since the return values can be positive, 0, negative, or false, probably the best return value would be a JSONObject. In your code, you would add this import:

import org.json.JSONObject;

and in the function:

JSONObject retval = new JSONObject();
double x = b*b - 4*a*c;
if(x < 0){
    retval.put("result", false);
}
else{
    retval.put("result", x);
}

You potentially could also add another key:value pair to the retval to include the result type to help with checking it in the calling function.

Answer 3

I would say it is possible. But not directly.

I haven't gone through the program given in the question. I am answering this knowing this is an old post. Might help someone.

Add your values to a Map like below. And get it with its key!

public static HashMap returnAnyType(){
    String x = "This";
    int a = 9;

    HashMap<String, Object> myMap = new HashMap();
    myMap.put("stringVal", x);
    myMap.put("intVal", a);
    return myMap;
}

String theStringFromTheMap = (String) returnAnyType().get("stringVal");
int theIntFromTheMap = (Integer) returnAnyType().get("intVal");

System.out.println("String : " + theStringFromTheMap + " is great !");   
System.out.println("Integer: " + Math.addExact(theIntFromTheMap, 10));

Output (both are returned from same method):

String : This is great!
Integer: 19

Note:
This is not a good practice. But it helps sometimes when it is needed!

Answer 4

Write a function that returns an Object. Have it either return the Boolean or Integer wrapper objects. Then use instanceof to figure out which to use.

Answer 5

no. the best you can do is return on instance of a class that handles all the things you might want to return.

something like

public class ReturnObj {
   public bool yesno; // yes or no
   public int val; // for int values
   public String mode; // mode describing what you returned, which the caller will need to understand.
}

obviously, you need to play with the names....

Also, this seems like a code smell. You might be able to remove the need to do something like this by qualifying what path you want outside of your function, and then call a specific function to get a boolean or a specific function to get an int, depending on the qualification.