问题
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 return Boolean FALSE
Is there a way to write a function that can return an int or a Boolean?
Ok, so this has received a lot of responses. I understand I'm simply approaching the problem incorrectly and I should throw some sort of Exception in the method. To get a better answer, I'm going to provide some example code. Please don't make fun :)
public class Quad {
public static void main (String[] args) {
double a, b, c;
a=1; b=-7; c=12;
System.out.println("x = " + quadratic(a, b, c, 1)); // x = 4.0
System.out.println("x = " + quadratic(a, b, c, -1)); // x = 3.0
// "invalid" coefficients. Let's throw an exception here. How do we handle the exception?
a=4; b=4; c=16;
System.out.println("x = " + quadratic(a, b, c, 1)); // x = NaN
System.out.println("x = " + quadratic(a, b, c, -1)); // x = 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) {
/*
throw exception!
I understand this code can be adjusted to accommodate
imaginary numbers, but for the sake of this example,
let's just have this function throw an exception and
say the coefficients are invalid
*/
}
return (-b + Math.sqrt(x) * polarity) / (2*a);
}
}
回答1:
No, you can't do that in Java.
You could return an Object though. And by returning an object you could technically return a derived class such as java.lang.Integer or java.lang.Boolean. However, I don't think it's the best idea.
回答2:
You could technically do this:
public <T> T doWork()
{
if(codition)
{
return (T) new Integer(1);
}
else
{
return (T) Boolean.FALSE;
}
}
Then this code would compile:
int x = doWork(); // the condition evaluates to true
boolean test = doWork();
But you could most certainly encounter runtime exceptions if the method returns the wrong type. You also must return objects instead of primitives because T is erased to java.lang.Object, which means the returned type must extend Object (i.e. be an object). The above example makes use of autoboxing to achieve a primitive return type.
I certainly wouldn't recommend this approach because IMO you need to evaluate your use of exception handling. You catch exceptions in exceptional cases if you can do something with that exception (i.e. recover, persist, retry, etc.). Exceptions are an exception to the expected workflow, not a part of it.
回答3:
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.
回答4:
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.
回答5:
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.
回答6:
No, one return reference to a customer.
You can write a response object that encapsulates a boolean and an int together and set the values according to your whim.
But if I was a user I'd think your design was confusing.
回答7:
my teacher had a cool idea for a tryParse C# like method in java hope it helps
import java.util.Scanner;
public class Program {
static Scanner scanner = new Scanner(System.in);
public static void main(String[] args) {
System.out.println("Enter number");
String input = scanner.next();
int[] num ={0};
if(tryParse(input,num))
System.out.println(num[0] * num[0]);
}
static boolean tryParse(String inputString,int[] outputInt){
try{
outputInt[0] = Integer.parseInt(inputString);
return true;
}catch(Exception e){
return false;
}
}
}
回答8:
This may not be the best solution depending on what you want to do, but an easy way to solve the problem might be to have the function return an int or a constant outside the possible range (such as RETURNED_FALSE/TRUE) and check for that.
回答9:
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!
回答10:
inout parameters only work for objects that can be mutated. For String inout parameters, pass in StringBuilders instead of Strings, then replace() their values.
来源：https://stackoverflow.com/questions/4952856/how-to-write-java-function-that-returns-values-of-multiple-data-types