Bdtjtk

import java.util.*;



public class Tester

{

    public static void main(String args)

    {

        Set<Point> set = new HashSet<Point>();

        Point A = new Point(0, 0);

        set.add(A);

        Point B = new Point(0, 0);

        System.out.println(set.contains(B));

    }

}



class Point

{

    int x, y;

    public Point(int x, int y)

    {

        this.x = x;

        this.y = y;

    }



    public boolean equals(Object o)

    {

        // Not sure/

    }



    public int hashcode()

    {

        // Not sure?

    }

}

I'm using a Point object to represent every cell of a 2d matrix. For DFS, I need to mark nodes as visited. However, the code returns false although it has object with value (0, 0). I think this probably requires a hashcode and equals implementation since by default the equals() looks for object reference.

So my question is, what would be a good hashcode and equals implementation here?
1. No constraints on the x and y.
2. x and y at most would be 1000.

Your equals method should return true if and only if both objects are intances of Point and both have the same x and y values:

@Override

public boolean equals(Object o)

{

    if (o == this)

        return true;

    if (!(o instanceof Point))

        return false;

    Point other = (Point) o;

    return this.x == other.x && this.y == other.y;

}

For hashCode, you can use a helper method:

@Override

public int hashCode()

{

    return Objects.hash(x,y);

}

EDIT: using the @Override annotation is optional, but it will save you from typos such as hashcode instead of hashCode.

You used HashSet for Set implementation which uses hashing to store elements. hashcode() and equals() are used for hashing and comparing elements. There are some guidelines to override hashcode() and equals() method.

The general contract of hashCode is:

1. hashcode method should consistent. That is it should return same integer on mulitple call on same object.


2. If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.


3. It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

Guideline for equals method.

1. It is reflexive: for any non-null reference value x, x.equals(x) should return true.


2. It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.


3. It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.


4. It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.


5. For any non-null reference value x, x.equals(null) should return false.

Source:-Hashcode Javadoc, Equals javadoc