Maps and custom objects in java

Question

I am trying the below piece of code.

class dog{
    private String name;
    public dog(String n){
        name = n;
    }
    public String getname(){ return name; }
    public void setname(String n){ name =n;}
    public boolean equals(Object o){
        //if (( o instanceof dog )&& (((dog)o).name == name)) return true;
        if (( o instanceof dog )&& (((dog)o).name.equals(name))) return true;
        else return false;
    }

    public int  hashcode(){
        return name.length();
    }
    public String toString(){
        return "Name:"+name;
    }
}

This is my Dog class . Now in Main method , I am trying to do the following

Map<Object,Object> m = new HashMap <Object, Object>();

dog p = new dog("GM");
dog q = new dog ("GM");
System.out.println(p.equals(q));
m.put ( new dog("GM"),"K2"); 

System.out.println(m.get(new dog("GM")));

I am getting a true and a null value. i Was expecting a K2 instead of null . Can somebody help me with this . I have overridden hashcode and equals methods . What is the thing i am missing ??

EDIT : - Changed equals function. Same results .

The String value comparison is incorrect in dog class in equal method. — Mengjun
– Mengjun, Commented Oct 29, 2013 at 3:09
You used (((dog)o).name == name)). Actually, it should use (((dog) o).name.equals(name). Please change it first and then check something further. — Mengjun
– Mengjun, Commented Oct 29, 2013 at 3:10
Change your hashCode to return name.hashCode() instead of name.length(), you are using one of the worsts hash functions you can. It won't return negatives, and it rarely uses large numbers. — Amir Pashazadeh
– Amir Pashazadeh, Commented Oct 29, 2013 at 4:13
And it is preferred to apply Java Coding Conventions: 1- Capitalize class name to Dog not dog 2- rename the getter and setter methods to getName and setName so you (and most of libraries) can access them using interpspection. — Amir Pashazadeh
– Amir Pashazadeh, Commented Oct 29, 2013 at 4:15

Kevin · Accepted Answer · 2013-10-29 03:37:05Z

3

The immediate problem is that hashCode needs a capital C, you are implementing hashcode with a lowercase c.

((dog)o).name == name compares the identities of the strings. That means if you have two instances of the string "GM", they will .equals() each other, but not ==.

edited Oct 29, 2013 at 3:37

answered Oct 29, 2013 at 3:05

Kevin

56.6k15 gold badges107 silver badges139 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

Bibin Mathew Over a year ago

thanks for the input. But p.equals(q) is returning true. That means equals method is working as expected. Isn't ? Also in case of strings i think this is fine. Please correct me if am wrong. In fact tried that too. same results even if i use equals instead of ==

Kevin Over a year ago

You are wrong. This is not fine in case of String; in fact it's in the case of strings where it most often confuses people. Sometimes it works (look up "string interning"), and sometimes it doesn't. Apparently for you it's working in the println but if it were working in the map we wouldn't be having this discussion.

Kevin Over a year ago

Got it, that took way too long

Bibin Mathew Over a year ago

You meant you got the mistake i made ? If yes please do help .

Bibin Mathew Over a year ago

Ah Thanks a lot.. Stupid mistake :)

|

Collectives™ on Stack Overflow

Maps and custom objects in java

1 Answer 1

6 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

6 Comments

Your Answer

Sign up or log in

Post as a guest

Related