Sep 17, 2018

In Groovy, the methods which will be invoked are chosen at runtime.

This is called runtime dispatch or multi-methods. It means that the method will be chosen based on the types of the arguments at runtime. In Java, this is the opposite: methods are chosen at compile time, based on the declared types.

The following code, written as Java code, can be compiled in both Java and Groovy, but it will behave differently:
int method(String arg) {
    return 1;
}
int method(Object arg) {
    return 2;
}
Object o = "Object";
int result = method(o);
In Java, you would have:
assertEquals(2, result);
Whereas in Groovy:
assertEquals(1, result);
That is because Java will use the static information type, which is that o is declared as an Object, whereas Groovy will choose at runtime, when the method is actually called. Since it is called with a String, then the String version is called.