holder3.java

Thinking In Java 第四版练习题答案

// generics/Holder3.java
// TIJ4 Chapter Generics, Exercise 1, page 621
// Use Holder3 with the typeinfo.pets library to show that a Holder3 that is
// specified to hold a base type can also hold a derived type.
import typeinfo.pets.*;

public class Holder3<T> {
	private T a;
	public Holder3(T a) { this.a = a; }
	public void set(T a) { this.a = a; }
	public T get() { return a; }
	public static void main(String[] args) {
		Holder3<Pet> h3 = 
			new Holder3<Pet>(new Pet());
		Pet a = h3.get(); // No cast needed
		System.out.println(h3.get());
		// can also hold a subclass of Pet:
		h3.set(new Dog());
		System.out.println(h3.get());
		// can also hold a sububclass of Pet:
		h3.set(new Gerbil());
		System.out.println(h3.get());
	}
}