13.4.22 Method and Constructor Overloading

Adding new methods that overload existing method names does not break compatibility with pre-existing binaries. The method signature to be used for each method invocation was determined when these existing binaries were compiled; therefore newly added methods will not be used, even if their signatures are both applicable and more specific than the method signature originally chosen.

While adding a new overloaded method or constructor may cause a compile-time error the next time a class or interface is compiled because there is no method or constructor that is most specific (§15.11.2.2), no such error occurs when a Java program is executed, because no overload resolution is done at execution time.

If the example program:

class Super {
    static void out(float f) { System.out.println("float"); }
}

class Test {
    public static void main(String[] args) {
        Super.out(2);
    }
}

is compiled and executed, it produces the output:

float

Suppose that a new version of class Super is produced:

class Super {
    static void out(float f) { System.out.println("float"); }
    static void out(int i) { System.out.println("int"); }
}

If Super is recompiled but not Test, then running the new binary with the existing binary of Test still produces the output:

float

However, if Test is then recompiled, using this new Super, the output is then:

int

as might have been naively expected in the previous case.