Method ambiguous when overloading with variable argument

Question

I am a newcomer into Java. Today I saw a piece of code in "Thinking in Java", but I cannot figure out why it produce compile time error.

Code:

public class OverloadingVarargs2 {
    static void f(float i, Character... args) {
        System.out.println("first");
    }
    static void f(Character... args) {
        System.out.println("second");
    }
    public static void main(String[] args) {
        f(1, 'a');
        f('a', 'b');
    }
}

Compile complained:

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
    The method f(float, Character[]) is ambiguous for the type OverloadingVarargs2

Eran · Accepted Answer · 2015-11-19 14:09:23Z

12

The problem is with f('a', 'b');

Both methods have a vararg argument, which means they will both be considered on the third and final phase of overloading resolution.

'a' is a char, which can be automatically converted to float via Widening Primitive Conversion.

Both 'a' and 'b' can be converted from char to Character using Boxing Conversion.

Therefore both f(float i, Character... args) and f(Character... args) are applicable for the arguments 'a' and 'b', and none of them has a precedence over the other.

answered Nov 19, 2015 at 14:09

Eran

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Comments

Konstantin Yovkov · Accepted Answer · 2015-11-19 14:06:16Z

6

The statement f('a', 'b'); is ambiguous, because the compiler cannot infer which exact method to invoke.

The reason behind this is that he char primitive type is considered as numeric (for each character there's a corresponding non-negative integer value between 0 and 65535).

That is why the compiler cannot infer if the parameter 'a' stands for the integer value of 97, which should be then cast to float or 'a' stands for the character 'a' which should be then autoboxed to Character.

answered Nov 19, 2015 at 14:06

Konstantin Yovkov

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Comments

Ramanlfc · Accepted Answer · 2015-11-19 14:16:31Z

3

this causes trouble because

f('a', 'b');

'a' can be converted to int and hence passed to float in the first f().

edited Nov 19, 2015 at 14:16

answered Nov 19, 2015 at 14:05

Ramanlfc

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