Download 08 OO PLs

Dr. Philip Cannata
1
Dr. Philip Cannata
2
Dr. Philip Cannata
3
Java Upcasting and Downcasting
Click here for example html or see the “Java Upcasting and Downcasting” link on the class
webpage
Click here for example code or see the Test.java link on the class webpage or see the next page.
Dr. Philip Cannata
4
Java Upcasting and Downcasting – Test.java
class Animal {
static int value = 100;
}
class Cat extends Animal { int value = Animal.value + 1; }
class Dog extends Animal { int value = Animal.value + 2; }
public class Test {
public static void main(String[] args) {
Cat c = new Cat();
System.out.println(c.value);
Dog d = new Dog();
System.out.println(d.value);
Animal a = c;
System.out.println(a.value);
Object o = c;
// System.out.println(o.value);
a = d;
type(c);
type(d);
type(a);
// type(a.value);
Dog d2 = (Dog) a;
a = c;
Dog d3 = (Dog) a;
}
public static void type(Animal a){
System.out.println("I am a " + a);
}
}
Dr. Philip Cannata
5
Java Upcasting and Downcasting – Test.java
class Animal {
static int value = 100;
Textbook Chapter 24
}
In languages like Java, programmers think they have the benefit of a type
system, but in fact many common programming patterns force programmers
class Dog extends Animal { int value = Animal.value + 2; } to employ casts instead. Casts intentionally subvert the type system and leave
checking for execution time. This indicates that Java’s evolution is far from
public class Test {
complete. In contrast, most of the type problems of Java are not manifest in a
public static void main(String[] args) {
language like ML, but its type systems still holds a few (subtler) lurking
Cat c = new Cat();
problems. In short, there is still much to do before we can consider type
System.out.println(c.value);
system design a solved problem. . . .
Dog d = new Dog();
System.out.println(d.value);
Types form a very valuable first line of defense against program errors. Of
Animal a = c;
course, a poorly-designed type system can be quite frustrating: Java
System.out.println(a.value);
programming sometimes has this flavor. A powerful type system such as that
Object o = c;
of ML, however, is a pleasure to use. ML programmers, for instance, claim
// System.out.println(o.value);
that programs that type correctly often work correctly within very few
a = d;
development iterations.
type(c);
type(d);
Types that have not been subverted (by, for instance, casts in Java) perform
type(a);
several valuable roles:
// type(a.value);
• When type systems detect legitimate program errors, they help reduce the
Dog d2 = (Dog) a;
time spent debugging.
a = c;
• Type systems catch errors in code that is not executed by the programmer.
Dog d3 = (Dog) a;
This matters because if a programmer constructs a weak test suite, many
}
parts of the system may receive no testing. The system may thus fail after
public static void type(Animal a){
deployment rather than during the testing stage. (Dually, however, passing a
System.out.println("I am a " + a);
type checker makes many programmers construct poorer test suites—a most
}
undesirable and unfortunate consequence!)
}
class Cat extends Animal { int value = Animal.value + 1; }
Dr. Philip Cannata
6