K & B - Generics & Collections

[Venkat Sidh]

I was reading through this chapter and observed the following code on page 565,

Map<Object,Object> m = new HashMap<Object, Object>();
Dog d1 = new Dog("clover");
m.put(d1,"Dog key");
d1.name = "magnolia";
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

according to book, the answers to above code are

#1 fails
#2 succeeds
#3 fails

to my understanding #3 should also succeed because new Dog object is passed but not m.get(d1)

Correct me if I'm wrong, sorry you need to have K & B to know actual code of the above question.

Thanks.
[ October 18, 2006: Message edited by: Venkat Sidh ]

[Dinuka Arsakularatne]

#3 fails because you change the d1.name to arthur before getting the value from the map. so hash code is calculated for the string arthur. but then you try to get the value by passing in the key value as clover which the map does not have now because it has already calculates the hash code according to the previous string which was arthur.

hope this helps

[James Quinton]

  
 
Map<Object,Object> m = new HashMap<Object, Object>();
Dog d1 = new Dog("clover");
m.put(d1,"Dog key");
d1.name = "magnolia";
System.out.println(m.get(d1));  #1
d1.name = "clover";
System.out.println(m.get(new Dog("clover")));  #2
d1.name = "arthur"; //line
System.out.println(m.get(new Dog("clover")));  #3
  

d1.name was changed on "line", new Dog("clover") doesn't "equals" to d1 anymore, even though their hashCode() are same.
assuming you know the context of Dog object definition:

  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(); }
}
  

[Venkat Sidh]

James,

Are you saying #3 fails or succeeds ?

[James Quinton]

Originally posted by Venkat Sidh:
James,

Are you saying #3 fails or succeeds ?

fails, because the keys are not same anymore since equals() method returns false (clover not equals to arthur) even though both hashCode() methods return same value (clover length equals to arthur length)
[ October 18, 2006: Message edited by: James Quinton ]

[Bauke Scholtz]

Map just holds references to the objects, not the objects itself. So, when an object is changed, then those changes will be reflected in the map.

[Venkat Sidh]

I got what you guys are talking about.

I'm under impression that though d1.name changed to 'arthur' it will not affect m.get(new Dog("clover"))) as we're passing a new instance of Dog as a Key.

Have one more question though,

d1.name = "clover";
System.out.println(m.get(new Dog("clover")));  #2

here what is the Key passed in line #2..is it reference to Dog (d1) or new object of Dog

Thanks.