Upcasting

Bruce Eckel's Thinking in Java Contents | Prev | Next

//: Wind.java
// Inheritance & upcasting
import java.util.*;
 
class Instrument {
  public void play() {}
  static void tune(Instrument i) {
    // ...
    i.play();
  }
}
 
// Wind objects are instruments
// because they have the same interface:
class Wind extends Instrument {
  public static void main(String[] args) {
    Wind flute = new Wind();
    Instrument.tune(flute); // Upcasting
  }
} ///:~ 

What’s interesting in this example is the tune( ) method, which accepts an Instrument handle. However, in Wind.main( ) the tune( ) method is called by giving it a Wind handle. Given that Java is particular about type checking, it seems strange that a method that accepts one type will readily accept another type, until you realize that a Wind object is also an Instrument object, and there’s no method that tune( ) could call for an Instrument that isn’t also in Wind. Inside tune( ), the code works for Instrument and anything derived from Instrument, and the act of converting a Wind handle into an Instrument handle is called upcasting.

Why “upcasting”?

Composition vs. inheritance revisited
In object-oriented programming, the most likely way that you’ll create and use code is by simply packaging data and methods together into a class, and using objects of that class. Occasionally, you’ll use existing classes to build new classes with composition. Even less frequently than that you’ll use inheritance. So although inheritance gets a lot of emphasis while learning OOP, it doesn’t mean that you should use it everywhere you possibly can. On the contrary, you should use it sparingly, only when it’s clear that inheritance is useful. One of the clearest ways to determine whether you should use composition or inheritance is to ask whether you’ll ever need to upcast from your new class to the base class. If you must upcast, then inheritance is necessary, but if you don’t need to upcast, then you should look closely at whether you need inheritance. The next chapter (polymorphism) provides one of the most compelling reasons for upcasting, but if you remember to ask “Do I need to upcast?”, you’ll have a good tool for deciding between composition and inheritance.



Comments

  • There are no comments yet. Be the first to comment!

Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • Live Event Date: August 19, 2014 @ 11:00 a.m. ET / 8:00 a.m. PT You deployed your app with the Bluemix PaaS and it's gaining some serious traction, so it's time to make some tweaks. Did you design your application in a way that it can scale in the cloud? Were you even thinking about the cloud when you built the app? If not, chances are your app is going to break. Check out this upcoming webcast to learn various techniques for designing applications that will scale successfully in Bluemix, for the …

  • Specialization and efficiency are always in need. Whether it's replacing an aging roof, getting a haircut, or tuning up a car, most seek the assistance of trusted experts. The same is true in the business world, where an increasing number of companies are seeking the help of others to administer their IT systems and services. This special edition of Unleashing IT highlights a new breed of IT caretaker -- Cisco Powered service providers -- and the business advantages and operational efficiencies they …

Most Popular Programming Stories

More for Developers

Latest Developer Headlines

RSS Feeds