Order of parameters - overriding

Question

According to javadoc

An instance method in a subclass with the same signature (name, plus the number and the type of its parameters) and return type as an instance method in the superclass overrides the superclass's method.

My question is - is the order of parameters immaterial? In case that thay have same name and they are the same type as well?

Answer 1

The order matters, so two methods with the same arguments in different order are not considered to have the same signature.
For example, this example does not compile:

interface Foo {
    void doIt(String what, int times);
}

class Bar implements Foo {
    public void doIt(int times, String what) {}
}

The names of the parameters however, are irrelevant. This is perfectly fine:

interface Foo {
    void doIt(String what, int times);
}

class Bar implements Foo {
    public void doIt(String andNowForSomeThingCompetelyDifferent, int theLarch) {}
}

Answer 2

The Java Language Specification says the order is material, but it requires some teasing out to explain why:

8.4.1. Formal Parameters

The formal parameters of a method or constructor, if any, are specified by a list of comma-separated parameter specifiers.

...

8.4.2. Method Signature

Two method or constructor declarations M and N have the same argument types if all of the following conditions hold:

1. They have the same number of formal parameters (possibly zero)
2. They have the same number of type parameters (possibly zero)
3. Let A₁, ..., A_n be the type parameters of M and let B₁, ..., B_n be the type parameters of N. After renaming each occurrence of a B_i in N's type to A_i, the bounds of corresponding type variables are the same, and the formal parameter types of M and N are the same.

...

Two method signatures m1 and m2 are override-equivalent iff either m1 is a subsignature of m2 or m2 is a subsignature of m1.

The formal parameters are specified as a list, so for "the formal parameter types of M and N" to be the same, they must be the same list and lists are order-dependant.

Since the correspondance in 3 is order dependent, the order of type parameters also matters.

This becomes more obvious when you deal with the byte-code/JNI convention for Method Descriptors.

MethodDescriptor:
    ( ParameterDescriptor* ) ReturnDescriptor

Answer 3

Order must also be same else overriding will not happen