Equals & Hashcode implementation

A sample equals method implementation :
package view;

import java.util.HashMap;
import java.util.Map;

public class PhoneNumber {
    private final short areaCode;
    private final short prefix;
    private final short lineNumber;

    public PhoneNumber(int areaCode, int prefix, int lineNumber) {
        this.areaCode = (short)areaCode;
        this.prefix = (short)prefix;
        this.lineNumber = (short)lineNumber;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == this)
            return true;
        if (!(obj instanceof PhoneNumber)) {
            return false;
        }
        PhoneNumber pn = (PhoneNumber)obj;
        return pn.lineNumber == lineNumber && pn.prefix == prefix &&
            pn.areaCode == areaCode;
    }
    public static void main(String arg[]) {
        Map<PhoneNumber,String> m = new HashMap<PhoneNumber,String>();
            m.put(new PhoneNumber(77,44,55), "John");
            System.out.println(m.get(new PhoneNumber(77,44,55)));
       
    }
}

The console output will be null as the class is not overridden the hashcode.

    @Override

    public int hashCode() {

        int result = 17;

        result = 31 * result + areaCode;

        result = 31 * result + prefix;

        result = 31 * result + lineNumber;

        return result;

    }

This will return the correct result.