type information - Checking Before a Cast

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This link: https://blog.csdn.net/wangbingfengf98/article/details/101944061

In C++, the classic cast "(Shape)" does not perform RTTI (Runtime type information, discovers and uses type information while a program is running). It simply tells the compiler to treat the object as the new type.
In Java, which does perform the type check, this cast is often called a "type-safe downcast." The reason for the term "downcast" is the historical arragement of the class hierarchy diagram.
There's a third form of RTTI in Java(The first two are upcast and downcast). This is the keyword instanceof, which tells us if an object is an instance of a particular type. It returns a boolean so you use it in the form of a question, like this

if(x instanceof Dog)
    ((Dog)x).bark();

// typeinfo/pets/Individual.java

// typeinfo/pets/Individual.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

import java.util.*;

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() {}

  @Override
  public String toString() {
    return getClass().getSimpleName() + (name == null ? "" : " " + name);
  }

  public long id() {
    return id;
  }

  @Override
  public boolean equals(Object o) {
    return o instanceof Individual && Objects.equals(id, ((Individual) o).id);
  }

  @Override
  public int hashCode() {
    return Objects.hash(name, id);
  }

  @Override
  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));
  }
}

// typeinfo/pets/Person.java

// typeinfo/pets/Person.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

public class Person extends Individual {
  public Person(String name) {
    super(name);
  }
}

// typeinfo/pets/Pet.java

// typeinfo/pets/Pet.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

public class Pet extends Individual {
  public Pet(String name) {
    super(name);
  }

  public Pet() {
    super();
  }
}

// typeinfo/pets/Dog.java

// typeinfo/pets/Dog.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

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

  public Dog() {
    super();
  }
}

// typeinfo/pets/Mutt.java

// typeinfo/pets/Mutt.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

public class Mutt extends Dog {
  public Mutt(String name) {
    super(name);
  }

  public Mutt() {
    super();
  }
}

// typeinfo/pets/Pug.java

// typeinfo/pets/Pug.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

public class Pug extends Dog {
  public Pug(String name) {
    super(name);
  }

  public Pug() {
    super();
  }
}

// typeinfo/pets/Cat.java

// typeinfo/pets/Cat.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

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

  public Cat() {
    super();
  }
}

// typeinfo/pets/EgyptianMau.java

// typeinfo/pets/EgyptianMau.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

public class EgyptianMau extends Cat {
  public EgyptianMau(String name) {
    super(name);
  }

  public EgyptianMau() {
    super();
  }
}

// typeinfo/pets/Manx.java

// typeinfo/pets/Manx.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

public class Manx extends Cat {
  public Manx(String name) {
    super(name);
  }

  public Manx() {
    super();
  }
}

// typeinfo/pets/Cymric.java

// typeinfo/pets/Cymric.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

public class Cymric extends Manx {
  public Cymric(String name) {
    super(name);
  }

  public Cymric() {
    super();
  }
}

// typeinfo/pets/Rodent.java

// typeinfo/pets/Rodent.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

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

  public Rodent() {
    super();
  }
}

// typeinfo/pets/Rat.java

// typeinfo/pets/Rat.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

public class Rat extends Rodent {
  public Rat(String name) {
    super(name);
  }

  public Rat() {
    super();
  }
}

// typeinfo/pets/Mouse.java

// typeinfo/pets/Mouse.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

public class Mouse extends Rodent {
  public Mouse(String name) {
    super(name);
  }

  public Mouse() {
    super();
  }
}

// typeinfo/pets/Hamster.java

// typeinfo/pets/Hamster.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

public class Hamster extends Rodent {
  public Hamster(String name) {
    super(name);
  }

  public Hamster() {
    super();
  }
}

// typeinfo/pets/PetCreator.java

// typeinfo/pets/PetCreator.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Creates random sequences of Pets
package typeinfo.pets;

import java.util.*;
import java.util.function.*;

public abstract class PetCreator implements Supplier<Pet> {
  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 get() { // Create one random Pet
    int n = rand.nextInt(types().size());
    try {
      return types().get(n).newInstance();
    } catch (InstantiationException | IllegalAccessException e) {
      throw new RuntimeException(e);
    }
  }
}

// typeinfo/pets/ForNameCreator.java

// typeinfo/pets/ForNameCreator.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package typeinfo.pets;

import java.util.*;

public class ForNameCreator extends PetCreator {
  private static List<Class<? extends Pet>> types = new ArrayList<>();
  // Types you want randomly created:
  private static String[] typeNames = {
    "typeinfo.pets.Mutt",
    "typeinfo.pets.Pug",
    "typeinfo.pets.EgyptianMau",
    "typeinfo.pets.Manx",
    "typeinfo.pets.Cymric",
    "typeinfo.pets.Rat",
    "typeinfo.pets.Mouse",
    "typeinfo.pets.Hamster"
  };

  @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();
  }

  @Override
  public List<Class<? extends Pet>> types() {
    return types;
  }
}

// typeinfo/PetCount.java

// typeinfo/PetCount.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Using instanceof

import java.util.*;
import typeinfo.pets.*;

public class PetCount {
  static class Counter 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) {
    Counter counter = new Counter();
    for (Pet pet : Pets.array(20)) {
      // List each individual pet:
      System.out.print(pet.getClass().getSimpleName() + " ");
      if (pet instanceof Pet) {
        counter.count("Pet");
      }
      if (pet instanceof Dog) {
        counter.count("Dog");
      }
      if (pet instanceof Mutt) {
        counter.count("Mutt");
      }
      if (pet instanceof Pug) {
        counter.count("Pug");
      }
      if (pet instanceof Cat) {
        counter.count("Cat");
      }
      if (pet instanceof EgyptianMau) {
        counter.count("EgyptianMau");
      }
      if (pet instanceof Manx) {
        counter.count("Manx");
      }
      if (pet instanceof Cymric) {
        counter.count("Cymric");
      }
      if (pet instanceof Rodent) {
        counter.count("Rodent");
      }
      if (pet instanceof Rat) {
        counter.count("Rat");
      }
      if (pet instanceof Mouse) {
        counter.count("Mouse");
      }
      if (pet instanceof Hamster) {
        counter.count("Hamster");
      }
    }
    // Show the counts:
    System.out.println();
    System.out.println(counter);
  }

  public static void main(String[] args) {
    countPets(new ForNameCreator());
  }
}
/* Output:
Rat Manx Cymric Mutt Pug Cymric Pug Manx Cymric Rat EgyptianMau Hamster EgyptianMau Mutt Mutt Cymric Mouse Pug Mouse Cymric
{EgyptianMau=2, Pug=3, Rat=2, Cymric=5, Mouse=2, Cat=9, Manx=7, Rodent=5, Mutt=3, Dog=6, Pet=20, Hamster=1}
*/