java调集的缺陷:典范未知[Java编程]

//: CatsAndDogs.java
// Simple collection example (Vector)
import java.util.*;

class Cat {
  private int catNumber;
  Cat(int i) {
    catNumber = i;
  }
  void print() {
    System.out.println("Cat #" + catNumber);
  }
}

class Dog {
  private int dogNumber;
  Dog(int i) {
    dogNumber = i;
  }
  void print() {
    System.out.println("Dog #" + dogNumber);
  }
}

public class CatsAndDogs {
  public static void main(String[] args) {
    Vector cats = new Vector();
    for(int i = 0; i < 7; i++)
      cats.addElement(new Cat(i));
    // Not a problem to add a dog to cats:
    cats.addElement(new Dog(7));
    for(int i = 0; i < cats.size(); i++)
      ((Cat)cats.elementAt(i)).print();
    // Dog is detected only at run-time
  }
} ///:~

//: WorksAnyway.java
// In special cases, things just seem
// to work correctly.
import java.util.*;

class Mouse {
  private int mouseNumber;
  Mouse(int i) {
    mouseNumber = i;
  }
  // Magic method:
  public String toString() {
    return "This is Mouse #" + mouseNumber;
  }
  void print(String msg) {
    if(msg != null) System.out.println(msg);
    System.out.println(
      "Mouse number " + mouseNumber);
  }
}

class MouseTrap {
  static void caughtYa(Object m) {
    Mouse mouse = (Mouse)m; // Cast from Object
    mouse.print("Caught one!");
  }
}

public class WorksAnyway {
  public static void main(String[] args) {
    Vector mice = new Vector();
    for(int i = 0; i < 3; i++)
      mice.addElement(new Mouse(i));
    for(int i = 0; i < mice.size(); i++) {
      // No cast necessary, automatic call
      // to Object.toString():
      System.out.println(
        "Free mouse: " + mice.elementAt(i));
      MouseTrap.caughtYa(mice.elementAt(i));
    }
  }
} ///:~

//: GopherVector.java
// A type-conscious Vector
import java.util.*;

class Gopher {
  private int gopherNumber;
  Gopher(int i) {
    gopherNumber = i;
  }
  void print(String msg) {
    if(msg != null) System.out.println(msg);
    System.out.println(
      "Gopher number " + gopherNumber);
  }
}

class GopherTrap {
  static void caughtYa(Gopher g) {
    g.print("Caught one!");
  }
}

class GopherVector {
  private Vector v = new Vector();
  public void addElement(Gopher m) {
    v.addElement(m);
  }
  public Gopher elementAt(int index) {
    return (Gopher)v.elementAt(index);
  }
  public int size() { return v.size(); }
  public static void main(String[] args) {
    GopherVector gophers = new GopherVector();
    for(int i = 0; i < 3; i++)
      gophers.addElement(new Gopher(i));
    for(int i = 0; i < gophers.size(); i++)
      GopherTrap.caughtYa(gophers.elementAt(i));
  }
} ///:~