Varargs in method overloading in Java

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You can either Widen or Box but you cannot do both, unless you are boxing and widening to Object (An int to Integer(Boxing) and then Integer to Object(Widening) is legal, since every class is a subclass of Object, so it is possible for Integer to be passed to Object parameter)

Similarly an int to Number is also legal (int -> Integer -> Number)
Since Number is the super class of Integer it is possible.

Let’s see this in your example:

public static void test(Integer...i)

public static void test(Float...f)

There are some rules that are followed when selecting which overloaded method to select, when Boxing, Widening, and Var-args are combined:

  1. Primitive widening uses the smallest method argument possible
  2. Wrapper type cannot be widened to another Wrapper type
  3. You can Box from int to Integer and widen to Object but no to Long
  4. Widening beats Boxing, Boxing beats Var-args.
  5. You can Box and then Widen (An int can become Object via Integer)
  6. You cannot Widen and then Box (An int cannot become Long)
  7. You cannot combine var-args, with either widening or boxing

So, based on the above given rules:

When you pass two integers to above functions,

  • according to rule 3, it will have to be first Widened and then
    Boxed to fit into a Long, which is illegal according to rule 5 (You cannot Widen and then Box).
  • So, it is Boxed to store in Integer var-args.

But in first case, where you have methods with var-args of primitive types:

public static void test(int...i)
public static void test(float...f)

Then test(1, 2) can invoke both the methods (Since neither of them is more suitable for rule 1 to apply):

  • In first case it will be var-args
  • In second case, it will be Widening and then Var-args (which is allowed)

Now, when you have methods with exactly one int and one float:

public static void test(int i)
public static void test(float f)

Then on invoking using test(1), rule 1 is followed, and smallest possible widening (i.e. the int where no widening is needed at all) is chosen. So 1st method will be invoked.

For more information, you can refer to JLS - Method Invocation Conversion

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