rodent14.java

JAVA编程思想第四版英文原版习题答案. pdf原版的

// polymorphism/Rodent14.java
// TIJ4 Chapter Polymorphism, Exercise 14, page 301
/* Modify Exercise 12 so that one of the member objects is a shared object with 
* reference counting, and demonstrate that it works properly.
*/
/* Solution includes, in same package, the following two files: 
* import static org.greggordon.tools.Print.*;
*
* public class Shared {
*	private int refcount = 0;
*	private static long counter = 0;
*	private final long id = counter++; 
*	public Shared() { println("Creating " + this); }
*	public void addRef() { refcount++; }
*	public String toString() { return "Shared " + id; }
*	public void showRefcount() { println("refcount = " + refcount); }		
* }
* // and:
*import java.util.*;
*
* public class RandomRodentGenerator {
*	// same shared object to be passed to every Rodent:
*	protected Shared shared = new Shared();
*	private Random rand = new Random();
*	public Rodent next() {
*		switch(rand.nextInt(3)) {
*			default:
*			case 0: return new Mouse(shared);
*			case 1: return new Rat(shared);
*			case 2: return new Squirrel(shared);			
*		}
*	}
* }
*/
package polymorphism.rodent;
import static org.greggordon.tools.Print.*;

class Characteristic {
	private String s;
	Characteristic(String s) {
		this.s = s;
		println("Creating Characteristic " + s);
	}	
}

class Description {
	private String s;
	Description(String s) {
		this.s = s;
		println("Creating Description " + s);
	}
}

class Rodent {
	private String name = "Rodent";
	private Shared shared;
	private static long counter = 0;
	private final long id = counter++; 
	private Characteristic c = new Characteristic("has tail");
	private Description d = new Description("small mammal");
	Rodent(Shared shared) { 
		println("Rodent() " + id); 
		this.shared = shared;
		this.shared.addRef();
	}
	protected void eat() { println("Rodent.eat()"); }
	protected void run() { println("Rodent.run()"); }
	protected void sleep() { println("Rodent.sleep()"); }
	public String toString() { return name + " " + id; }
}

class Mouse extends Rodent {
	private String name = "Mouse";
	private Characteristic c = new Characteristic("likes cheese");
	private Description d = new Description("nocturnal");
	Mouse(Shared shared) {
		super(shared); 
		println("Mouse()");		
	}
	protected void eat() { println("Mouse.eat()"); }
	protected void run() { println("Mouse.run()"); }
	protected void sleep() { println("Mouse.sleep()"); }
	public String toString() { return name + ", " + super.toString(); }
}

class Rat extends Rodent {
	private String name = "Rat";
	private Characteristic c = new Characteristic("larger");
	private Description d = new Description("black");
	Rat(Shared shared) { 
		super(shared);
		println("Rat()"); 
	}
	protected void eat() { println("Rat.eat()"); }
	protected void run() { println("Rat.run()"); }
	protected void sleep() { println("Rat.sleep()"); }
	public String toString() { return name + ", " + super.toString(); }
}

class Squirrel extends Rodent {
	private String name = "Squirrel";
	private Characteristic c = new Characteristic("climbs trees");
	private Description d = new Description("likes nuts");
	Squirrel(Shared shared) { 
		super(shared);
		println("Squirrel()"); 
	}
	protected void eat() { println("Squirrel.eat()"); }
	protected void run() { println("Squirrel.run()"); }
	protected void sleep() { println("Squirrel.sleep()"); }
	public String toString() { return name + ", " + super.toString(); }
}

public class Rodent14 {
	private static RandomRodentGenerator gen = new RandomRodentGenerator();
	public static void main(String[] args) {
		Rodent[] rodents = new Rodent[5];
		for(Rodent r : rodents) {
			r = gen.next();
			println(r);
		}
		gen.shared.showRefcount();		
	}
}