Java -- Runtime typing differences

Question

I have two situations drawn up, and the strange differences between them are causing me a bit of grief. I will attempt to detail them below in code.

Situation 1:

public void doSomething(Object obj) {
  //do something with obj
}

public void doSomething(String str) {
  //do something similar to str, but apply some custom
  //processing for String's
}

Object o = new String("s");
doSomething(o); // this will use the Object version...

Situation 2:

class Dog {
  void makeSound() {
    System.out.println("woof");
  }
}

class Chihuahua extends Dog {
  void makeSound() {
    System.out.println("yip");
  }
}

Dog dog = new Chihuahua();
dog.makeSound(); //will print 'yip', the Chihuahua version...

Why, in situation one, is the runtime type of the parameter not used, but in situation two it is? I understand that the examples are actually different things, but I am more curious about what is going on 'under the covers' here.

Answer 1

In the first example, a method signature is chosen at compile-time from among a number of overloaded signatures. The Java language specification says that this is done based on the static type of the variables.

In the second example, an implementation of a given method signature is chosen at runtime based on virtual method dispatch. This is done based on the runtime type of the class containing the method definition.

Answer 2

Situation #1 is method overloading, which is static at compile time. Situation #2 is polymorphism, which is dynamic at runtime.