Object equivalency in hashCode

Question

I'm working with hashCode for the first time and not sure how to check if two objects are equal. This is what I have so far.

/** Represents a City */
class City {
/** Decimal format to print leading zeros in zip code */
static DecimalFormat zipFormat = new DecimalFormat("00000");

    int zip;
    String name;
    String state;
    double longitude;
    double latitude;


    /** The full constructor */
    public City (int zip, String name, String state, 
            double longitude, double latitude) {
            this.zip   = zip;
            this.name  = name;
            this.state = state;
            this.longitude = longitude;
            this.latitude  = latitude;
}

/** to make sure the two cities have the same name, state, zip code,
    * and the same latitude and longitude */
    public boolean equals(Object obj){
        if (obj == null)
            return false;
            City temp = (City)obj;

            System.out.println(City.equals(obj));
    }

You have your answer in your javadoc there. To check if they are equal, check if their names, states, zip codes, lat, and long are equal. — gla3dr, Nov 15 '13 at 20:32
@turbo You're wrong there. If two objects are equal, it is required that they have the same hash code, but if two objects have the same hash code, they may be different. — Luiggi Mendoza, Nov 15 '13 at 20:37
@LuiggiMendoza You're right, deleted my comment so no one reads it and takes it as fact. — turbo, Nov 15 '13 at 20:39
Take a look to this: http://stackoverflow.com/questions/27581/overriding-equals-and-hashcode-in-java — jvazquez, Nov 15 '13 at 20:57

score 0 · Answer 1 · answered Nov 15 '13 at 20:36

0

Assuming equality in your case means all properties are same in your equals method add

City temp = (City)obj;
return this.zip == temp.zip &&
       this.name == temp.name &&
       ...
;

answered Nov 15 '13 at 20:36

Konstantin

3,254
15
20

Thanks! it works. Why use temp though? where did that come from/what does it mean? – Nov 15 '13 at 20:51
it's from your code... `equals` takes object, to be able to compare you need to cast it to `City` – Konstantin Nov 15 '13 at 20:52

score -1 · Answer 2 · edited Nov 15 '13 at 20:33

eclipse auto-generates for you:

    @Override
    public int hashCode()
    {
        final int prime = 31;
        int result = 1;
        long temp;
        temp = Double.doubleToLongBits(latitude);
        result = prime * result + (int) (temp ^ (temp >>> 32));
        temp = Double.doubleToLongBits(longitude);
        result = prime * result + (int) (temp ^ (temp >>> 32));
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        result = prime * result + ((state == null) ? 0 : state.hashCode());
        result = prime * result + zip;
        return result;
    }

    @Override
    public boolean equals(Object obj)
    {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        City other = (City) obj;
        if (Double.doubleToLongBits(latitude) != Double
                .doubleToLongBits(other.latitude))
            return false;
        if (Double.doubleToLongBits(longitude) != Double
                .doubleToLongBits(other.longitude))
            return false;
        if (name == null)
        {
            if (other.name != null)
                return false;
        }
        else if (!name.equals(other.name))
            return false;
        if (state == null)
        {
            if (other.state != null)
                return false;
        }
        else if (!state.equals(other.state))
            return false;
        if (zip != other.zip)
            return false;
        return true;
    }

If you're using Java 7, you could just call [`Objects.hashCode(this)`](http://docs.oracle.com/javase/7/docs/api/java/util/Objects.html#hashCode(java.lang.Object)). If not, use [`Arrays.hashCode(new Object[]{})`](http://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html#hashCode(java.lang.Object[])) — Luiggi Mendoza, Nov 15 '13 at 20:35
Is it intentional that a latitude of +0.0 will be considered different from -0.0? — supercat, Nov 16 '13 at 00:32

Object equivalency in hashCode

2 Answers2