type information - Checking Before a Cast

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} */

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references:

1. On Java 8 - Bruce Eckel

2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/Individual.java

3. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/Person.java

4. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/Pet.java

5. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/Dog.java

6. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/Mutt.java

7. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/Pug.java

8. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/Cat.java

9. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/EgyptianMau.java

10. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/Manx.java

11. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/Cymric.java

12. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/Rodent.java

13. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/Rat.java

14. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/Mouse.java

15. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/Hamster.java

16. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/PetCreator.java

17. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/ForNameCreator.java

18. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/PetCount.java