Java – why do fuzzy errors occur when using varargs overloads of primitive types and wrapper classes?

See English answers > ambiguous varargs methods4

Case 1

public class Test {

    public void display(int a) {
        System.out.println("1");
    }

    public void display(Integer a) {
        System.out.println("2");
    }

    public static void main(String[] args) {
        new test().display(0);
    }
}

Output: 1

Case #2

public class Test {

    public void display(int... a) {
        System.out.println("1");
    }

    public void display(Integer... a) {
        System.out.println("2");
    }

    public static void main(String[] args) {
        new test().display(0);
    }
}

Compilation error:

The method display(int[]) is ambiguous for the type Test

Solution

In the first example, the display (int) method is invoked in the strict call context, while display (Integer) is invoked in the loose call context (because automatic boxing is required). Therefore, the compiler selects the display (int) method according to JLS The call context is explained here in JLS 5.3 Invocation Contexts

In the second example, two methods are invoked in loose call context, so the compiler needs to find the most specific method, JLS 15.12.. 2.5 Choosing the Most Specific Method. Because int is not a subtype of integer, there is no specific method, and the compiler will throw a compilation error

You can find an explanation of similar compilation errors in method overload ambiguity with Java 8 internal conditions and unboxed primitives

Parts applicable to this case:

For the first example, only the display (int) method matches in the first stage, so select it For the second example, the two methods match in the third stage, so the most specific method algorithm is selected to play a role JLS 15.12 2.5 Choosing the Most Specific Method：

As mentioned earlier, due to int & lt;: Integer is not satisfied, so there is no most specific method