Bug in a kathy sierra prog...

[Sandip Sarkar]

Hi all,

the below given code is for map related manipulations taken fron kathy sierra book for scjp 1.5 (page 563 to be precise)...am pasting the code here...

   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");
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
}
} 

When i tryied compiling it ...itr gave me errors.......am bugged.
Please help.

Thanks.

[Kelvin Chenhao Lim]

Hi Sandip,

What version is your Java compiler? This program requires Java 1.5 or above, due to its use of generics. There's nothing wrong with the program, so you're probably using an older compiler.

[Jari Timonen]

It compiled run fine here. Can you paste some errors what compiler is complaining about?

[Sandip Sarkar]

Hi All,

I hace checked the version and its java 1.5
Also the installation folder says its 1.5
Additionally i have tried 2 compile it with the command:

javac -source 1.5 #filename.java

And its working fine in my friends compiler.
As requested here are the errors i got:

 MapTest.java:6: inconvertible types
found   : Object
required: Dog
if((o instanceof Dog) &&
    ^
MapTest.java:7: inconvertible types
found   : Object
required: Dog
(((Dog)o).name == name)) {
       ^
MapTest.java:20: put(Object,Object) in java.util.Map<Object,Object> cannot be applied to (java.lang.String,Dog)
m.put("k1", new Dog("aiko")); // add some key/value pairs
 ^
MapTest.java:21: put(Object,Object) in java.util.Map<Object,Object> cannot be applied to (java.lang.String,Pets)
m.put("k2", Pets.DOG);
 ^
MapTest.java:22: put(Object,Object) in java.util.Map<Object,Object> cannot be applied to (Pets,java.lang.String)
m.put(Pets.CAT, "CAT key");
 ^
MapTest.java:24: put(Object,Object) in java.util.Map<Object,Object> cannot be applied to (Dog,java.lang.String)
m.put(d1, "Dog key");
 ^
MapTest.java:25: put(Object,Object) in java.util.Map<Object,Object> cannot be applied to (Cat,java.lang.String)
m.put(new Cat(), "Cat key");
 ^
7 errors   

Please help.
Thanks.

[Kelvin Chenhao Lim]

Sandip,

Interesting. This looks like it might be a compiler bug. Perhaps you may want to try updating to the latest Sun JDK 5 distribution, or try a different version if you're already using the latest version.

[Vidhya Ramaswamy]

Hi,
I tried the same code, its working fine on my pc.

But I have another question regarding this.
With reference to line#5, it states in the book that(taken from pg 564 bottom para):

Why didn't we find the Cat key String? Why did it work to use an instance of Dog as a key, when using an instance of Cat as a key failed?
It's easy to see that Dog overrode equals() and hashCode() while Cat didn't.

1) I tried commenting the equals() and hashCode() used in Dog class, but the code runs and produces the same output.
2) If this code set is used:

   Cat c = new Cat();
   m.put(c, "Cat key");
   System.out.println(m.get(c));

instead of this one given in the book:

   
   m.put(new Cat(), "Cat key");
   System.out.println(m.get(new Cat()));

it produces the expected output (Cat key).

So, I feel with regards to 1) and 2), its really not a MUST for a class to override the equals() and hashCode() for using as apart of the key.
Please comment and explain why this behaviour?

[Jim Yingst]

Notice how whenever the error messages talk about String, they say java.lang.String. But when they talk about Object, they just say Object. The should give the fully qualified name, java.lang.Object. The fact that they don't suggests that you have done something extremely evil, and created your own class called Object (with no package). Delete the files Object.java and Object.class from your classpath, and never, ever do this again, as it only leads to confusion.

[Sandip Sarkar]

Hi Kelvin,

I tried installing java 5.0 again without any success at the code.
It still doesnt get compiled and am getting those 7 errors i mentioned earlier.
Kindly guide on how to proceed.

Thanks.

[prajal Mehta]

Originally posted by Vidhya Ramaswamy:
Hi,
I tried the same code, its working fine on my pc.

But I have another question regarding this.
With reference to line#5, it states in the book that(taken from pg 564 bottom para): .........



So, I feel with regards to 1) and 2), its really not a MUST for a class to override the equals() and hashCode() for using as apart of the key.
Please comment and explain why this behaviour?

Hi Vidhya,

I also tried yesterday similar code like yours ..!
I too have the same doubt.
I think you should start a new topic for this doubt, so that we will get some answers!

[Sandip Sarkar]

Hi Kelvin,

I tried installing java 5.0 again without any success at the code.
It still doesnt get compiled and am getting those 7 errors i mentioned earlier.
Kindly guide on how to proceed.

Thanks.

[Vidhya Ramaswamy]

Hi Sandip,
Have you checked the Environment variables: JAVA_HOME and PATH?
Are they pointing to 1.5 version ?

[Rizwan Qadri]

Hi,
Its working on myside

[Jim Yingst]

Sigh.

Here, run this:

        if (Object.class == java.lang.Object.class)
            System.out.println("Object class is fine, tell Jim to go away");
        else {
            ClassLoader cl = Object.class.getClassLoader();
            if (cl == null)
                System.out.println("Can't find Object's class loader");
            else {
                URL url = cl.getResource("Object.class");
                if (url == null)
                    System.out.println("Can't find Object.class file");
                else
                    System.out.println("False class file is located at " + url + " - DELETE IT NOW!!!");
                URL url2 = cl.getResource("Object.java");
                if (url2 != null)
                    System.out.println("False source file is located at " + url2 + " - DELETE IT NOW!!!");
            }
        }