Using HashMap Problem

[Mohit G Gupta]

KB book

    

import java.util.*;
class Dog {
  public Dog(String n) { name = n; }
  public String name;
  public boolean equals(Object o) {
    if((o instanceof Dog) && 
       (((Dog)o).name == name)) {
      return true;
    } else {
      return false;
    }
  }
  public int hashCode() {return name.length(); }
}
class Cat { }
enum Pets {DOG, CAT, HORSE }
class MapTest {
  public static void main(String[] args) {
    Map<Object, Object> m = new HashMap<Object, Object>();
    m.put("k1", new Dog("aiko"));   // add some key/value pairs
    m.put("k2", Pets.DOG);
    m.put(Pets.CAT, "CAT key");
    Dog d1 = new Dog("clover");     // let's keep this reference
    m.put(d1, "Dog key");
    m.put(new Cat(), "Cat key");
d1.name = "magnolia";
    System.out.println(m.get(d1));                  // #1 it gives complie-time error 
    d1.name = "clover";
    System.out.println(m.get(new Dog("clover")));   // #2  works fine
    d1.name = "arthur";
    System.out.println(m.get(new Dog("clover")));   // #3 it gives complie time error
}}

1.what does d1.name does ?

2.can anyone explain why
#1 gives complie-time error
#2, works fine
#3 gives complie time error

3.In the last line of code,

The complier is comparing arthur length with clover length.
my question is why is it not comparing the length of name clover passed in the object (new Dog("clover") with that kept in the hashmap.

the hashmap holds d1 with Dog key
where d1 is the key and Dog Key is the value.
here d1 is

  
 Dog d1 = new Dog("clover");    

[Christophe Verré]

Please QuoteYourSources.

[Abimaran Kugathasan]

Change the name length of d1 and check....Like this....

import java.util.*;
class Dog {
  public Dog(String n) { name = n; }
  public String name;
  public boolean equals(Object o) {
    if((o instanceof Dog) &&
       (((Dog)o).name == name)) {
      return true;
    } else {
      return false;
    }
  }
  public int hashCode() {return name.length(); }
}
class Cat { }

enum Pets {DOG, CAT, HORSE }

public class MapTest {
  public static void main(String[] args) {
    Map<Object, Object> m = new HashMap<Object, Object>();
    m.put("k1", new Dog("aiko"));   // add some key/value pairs
    m.put("k2", Pets.DOG);
    m.put(Pets.CAT, "CAT key");
    Dog d1 = new Dog("clover");     // let's keep this reference
    m.put(d1, "Dog key");
    m.put(new Cat(), "Cat key");
    System.out.println(m.get(d1));
d1.name = "magnol";				// chenged the length of name as 6. now try this code
    System.out.println(m.get(d1));                  // #1
    d1.name = "clover";
    System.out.println(m.get(new Dog("clover")));   // #2
    d1.name = "arthur";
    System.out.println(m.get(new Dog("clover")));   // #3
}}

[Mohit G Gupta]

see the changes