14章类型信息-之类型转换前先做检查--之instanceof

instanceof用法：

对于instanceof有比较严格的限制，只可将其与命名类型进行比较，而不能与class对象做比较。

理解：这里所说的就是像if(pet instanceof Mutt)
        counter.count("Mutt");这里的Mutt,不能用对象比较，只能用类名，比如（Mutt mutt = new Mutt（）然后用mutt）或者 Mutt.class就不行了，

实例代码：

实体类父类：

//: typeinfo/pets/Individual.java
package typeinfo.pets;


public class Individual implements Comparable<Individual> {
  private static long counter = 0;
  private final long id = counter++;
  private String name;
  public Individual(String name) { this.name = name; }
  // 'name' is optional:
  public Individual() {}
  public String toString() {
    return getClass().getSimpleName() +
      (name == null ? "" : " " + name);
  }
  public long id() { return id; }
  public boolean equals(Object o) {
    return o instanceof Individual &&
      id == ((Individual)o).id;
  }
  public int hashCode() {
    int result = 17;
    if(name != null)
      result = 37 * result + name.hashCode();
    result = 37 * result + (int)id;
    return result;
  }
  public int compareTo(Individual arg) {
    // Compare by class name first:
    String first = getClass().getSimpleName();
    String argFirst = arg.getClass().getSimpleName();
    int firstCompare = first.compareTo(argFirst);
    if(firstCompare != 0)
    return firstCompare;
    if(name != null && arg.name != null) {
      int secondCompare = name.compareTo(arg.name);
      if(secondCompare != 0)
        return secondCompare;
    }
    return (arg.id < id ? -1 : (arg.id == id ? 0 : 1));
  }
} ///:~

继承类：

//: typeinfo/pets/Cat.java
package typeinfo.pets;


public class Cat extends Pet {
  public Cat(String name) { super(name); }
  public Cat() { super(); }
} ///:~

//: typeinfo/pets/Dog.java
package typeinfo.pets;


public class Dog extends Pet {
  public Dog(String name) { super(name); }
  public Dog() { super(); }
} ///:~

//: typeinfo/pets/Rodent.java
package typeinfo.pets;


public class Rodent extends Pet {
  public Rodent(String name) { super(name); }
  public Rodent() { super(); }
} ///:~

创建宠物的抽象类：

//: typeinfo/pets/PetCreator.java
// Creates random sequences of Pets.
package typeinfo.pets;
import java.util.*;


public abstract class PetCreator {
  private Random rand = new Random(47);
  // The List of the different types of Pet to create:
  public abstract List<Class<? extends Pet>> types();
  public Pet randomPet() { // Create one random Pet
    int n = rand.nextInt(types().size());
    try {
      return types().get(n).newInstance();
    } catch(InstantiationException e) {
      throw new RuntimeException(e);
    } catch(IllegalAccessException e) {
      throw new RuntimeException(e);
    }
  }
  public Pet[] createArray(int size) {
    Pet[] result = new Pet[size];
    for(int i = 0; i < size; i++)
      result[i] = randomPet();
    return result;
  }
  public ArrayList<Pet> arrayList(int size) {
    ArrayList<Pet> result = new ArrayList<Pet>();
    Collections.addAll(result, createArray(size));
    return result;
  }
} ///:~

实现（创建宠物抽象类）的实现类：

//: typeinfo/pets/ForNameCreator.java
package typeinfo.pets;
import java.util.*;


public class ForNameCreator extends PetCreator {

  private static List<Class<? extends Pet>> types =
    new ArrayList<Class<? extends Pet>>();
  // Types that you want to be randomly created:
  private static String[] typeNames = {
    "typeinfo.pets.Cat",
    "typeinfo.pets.Dog",
    "typeinfo.pets.Rodent"
  };
  @SuppressWarnings("unchecked")
  private static void loader() {
    try {
      for(String name : typeNames)
        types.add(
          (Class<? extends Pet>)Class.forName(name));
    } catch(ClassNotFoundException e) {
      throw new RuntimeException(e);
    }
  }
  static { loader(); }
  public List<Class<? extends Pet>> types() {return types;}
} ///:~

用于计算各个宠物个数的类：

package typeinfo.pets;


//: typeinfo/PetCount.java
// Using instanceof.
import typeinfo.pets.*;
import java.util.*;
import static net.mindview.util.Print.*;


public class PetCount {
  static class PetCounter extends HashMap<String,Integer> {
    public void count(String type) {
      Integer quantity = get(type);
      if(quantity == null)
        put(type, 1);
      else
        put(type, quantity + 1);
    }
  }
  public static void countPets(PetCreator creator) {
    PetCounter counter= new PetCounter();
    for(Pet pet : creator.createArray(20)) {
      // List each individual pet:
      printnb(pet.getClass().getSimpleName() + " ");
      if(pet instanceof Pet)
        counter.count("Pet");
      if(pet instanceof Dog)
        counter.count("Dog");
      if(pet instanceof Cat)
        counter.count("Cat");
      if(pet instanceof Rodent)
        counter.count("Rodent");
    }
    // Show the counts:
    print();
    print(counter);
  }
  public static void main(String[] args) {
    countPets(new ForNameCreator());
  }
} 

//测试打印：Rodent Rodent Dog Rodent Dog Rodent Dog Rodent Cat Dog Cat Cat Cat Dog Rodent Dog Dog Dog Rodent Dog 
//{Cat=4, Pet=20, Dog=9, Rodent=7}

工具类：

//: net/mindview/util/Print.java
// Print methods that can be used without
// qualifiers, using Java SE5 static imports:
package net.mindview.util;
import java.io.*;


public class Print {
  // Print with a newline:
  public static void print(Object obj) {
    System.out.println(obj);
  }
  // Print a newline by itself:
  public static void print() {
    System.out.println();
  }
  // Print with no line break:
  public static void printnb(Object obj) {
    System.out.print(obj);
  }
  // The new Java SE5 printf() (from C):
  public static PrintStream
  printf(String format, Object... args) {
    return System.out.printf(format, args);
  }
} ///:~

本文出自 “追随心的彼岸” 博客，谢绝转载！