Java Language Generics Creating a Bounded Generic Class
Example
You can restrict the valid types used in a generic class by bounding that type in the class definition. Given the following simple type hierarchy:
public abstract class Animal {
public abstract String getSound();
}
public class Cat extends Animal {
public String getSound() {
return "Meow";
}
}
public class Dog extends Animal {
public String getSound() {
return "Woof";
}
}
Without bounded generics, we cannot make a container class that is both generic and knows that each element is an animal:
public class AnimalContainer<T> {
private Collection<T> col;
public AnimalContainer() {
col = new ArrayList<T>();
}
public void add(T t) {
col.add(t);
}
public void printAllSounds() {
for (T t : col) {
// Illegal, type T doesn't have makeSound()
// it is used as an java.lang.Object here
System.out.println(t.makeSound());
}
}
}
With generic bound in class definition, this is now possible.
public class BoundedAnimalContainer<T extends Animal> { // Note bound here.
private Collection<T> col;
public BoundedAnimalContainer() {
col = new ArrayList<T>();
}
public void add(T t) {
col.add(t);
}
public void printAllSounds() {
for (T t : col) {
// Now works because T is extending Animal
System.out.println(t.makeSound());
}
}
}
This also restricts the valid instantiations of the generic type:
// Legal
AnimalContainer<Cat> a = new AnimalContainer<Cat>();
// Legal
AnimalContainer<String> a = new AnimalContainer<String>();
// Legal because Cat extends Animal
BoundedAnimalContainer<Cat> b = new BoundedAnimalContainer<Cat>();
// Illegal because String doesn't extends Animal
BoundedAnimalContainer<String> b = new BoundedAnimalContainer<String>();
