Doubt in K&B SCJP 5: Using Maps

[Marcin Skrzek]

There is a misleading (in my opinion) example code on page 563:

    
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");
System.out.println(m.get("k1")); // #1
String k2 = "k2";
System.out.println(m.get(k2)); // #2
Pets p = Pets.CAT;
System.out.println(m.get(p)); // #3
System.out.println(m.get(d1)); // #4
System.out.println(m.get(new Cat())); // #5
System.out.println(m.size()); // #6
}
}

I claim that, because of the way in which equals() method is overriden for Dog class (Dog`s member variable - name - is String, so shouldn`t it be compared to another variable using equals() instead of ==, should it?)

The side effect is, that if we slightly change code on page 565 from:

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

which gives the output: "Dog key"

to:

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

than the output will be: null

and we have now String constant pool issue in chapter about Collections, what isn`t even mentioned, but we read later (page 566, line 6):

In the second call to get(), the hashcodes are both 6, so step 1 succeeds. Once in the correct bucket (the "length of name = 6" bucket), the equals() method is invoked, and since Dog's equals() method compares names, equals() succeeds, and the output is Dog key.

Bert, sorry, if my tone seems to be to impertinent. My intention was only to clearly explain my doubt.

Thanks

[Burkhard Hassel]

Hi all,

I think, Marcin is right, the comparison should be made with equals. Personally I like to override equals with what I call the this-and-that-method:

public boolean equals(Object o) {
if (o instanceof Dog) {
Dog that = (Dog) o;
return this.name.equals(that.name);
} else
return false;
}

Yours,
Bu.

[Marcin Skrzek]

Bu,

Maybe I`m wrong, but your method:

    
public boolean equals(Object o) {
if (o instanceof Dog) {
Dog that = (Dog) o;
return this.name.equals(that.name);
} else
return false;
}

seems to throw NullPointerException on runtime, when this.name is null.

Cheers,
Marcin

[Burkhard Hassel]

Hi Marcin,

yes, all members of reference type to be compared should be checked that they are not null before they are compared. Unless the rest of the class doesn't provide that they can never be null (which is not the case in the Dog class).

Yours
Bu.