"Because Groovy dispatches by the runtime type, the specialized implementation of oracle(String) is used in the second case." "Wherever there’s a Java API that uses the static type Object, this code effectively loses the strength of static typing. ... This is why the Java type concept is called weak static typing" Source: Groovy in Action Second…

Dispatcher.groovy

def oracle(Object o) { return 'object' }
def oracle(String o) { return 'string' }
Object x = 1
Object y = 'foo'
assert 'object' == oracle(x)
assert 'string' == oracle(y)

Dispatcher.java

public class Dispatcher {

    static String oracle(Object o) { return "object"; }
    static String oracle(String o) { return "string"; }

    public static void main(String... args) {
        Object x = 1;
        Object y = "foo";
        assert "object".equals(oracle(x));
        assert "object".equals(oracle(y));
    }

}