Groovy and Java static variable behaviour

Question

I have been doing some Java and Grails3 programming recently. I found some behavior that I don't understand.

There are two groovy classes:

class Super {
    static String desc = "Super"
}

and

class Sub extends Super {
    static String desc = "Sub"
}

Now, I'm running following code in Java and Groovy:

Super aSuper = new Super();
Sub sub = new Sub();
Super superSub = new Sub();

System.out.println("Super object: [" + aSuper.getDesc() + "]"); //1
System.out.println("Sub object: [" + sub.getDesc() + "]");//2
System.out.println("Sub object, super reference: [" + superSub.getDesc()+ "]");//3
System.out.println("Super reference: [" + Super.getDesc()+ "]");//4
System.out.println("Sub reference: [" + Sub.getDesc()+ "]");//5

Results of 1,2,4,5 are the same in both cases and easy to predict ([Super], [Sub], [Super], [Sub])

But in case number 3 when running above code form Java class output will be: Sub object, super reference: [Super] And from Groovy it will result in: Sub object, super reference: [Sub]

Why is Groovy interpretation of static variable different?

Answer 1

As @dmahapatro states, it's based on Multi Methods, but it's a subtle example. In the documentation, the example is based on method selection given arguments where the variable is of a parent type of the instance held by the argument. Java chooses the method signature at compile time and, since at that time it only has the class of the argument variable declaration (Object), rather than the class of the instance (String). Groovy delays its decision, so it can determine the class of the instance being referred to by the argument variable, and uses that to determine which method signature matches most closely.

In your example above, as you're referring to a static member, which is Class-based, not instance-based, Java's inheritance idea (virtual methods) do not come into play. Java again chooses the static from the Class of the reference variable (Super.desc). Groovy again takes the delayed, instance-based path, and asks the instance what it's static member is (Sub.desc).

Note that most would consider case 3 bad style (asking an instance what the value of a class-static member is), so it shouldn't come up, normally.