Java HashMap.get() returns null when I create a new instance of an object as a key

Question

I\'m making a spreadsheet application and I\'m using a HashMap to store the data in the cells. As the key I\'m using a Point class, which only has the number of rows and col

Answer 1

By default, Point's equals method uses == i.e. object identity. Because Hashmap uses the equals method, it ends up comparing the key using == which would return false unless it's the same object.

To fix this, Implement Point's equals and hashCode method so that point1.equals(point2) returns true if the coordinates are same.

Answer 2

Since, you are using the Point as the key to the hashmap you would need to override the default equals and hashCode method

Untested Code

@Override
public boolean equals(Object obj){

if (obj == null) 
    return false;
if (obj == this) 
    return true;
if (!(obj instanceof Point.class))
    return false;
Point point = (Point) obj;
return this.x == point.x && this.y == point.y;
}

Answer 3

To detail my comment above, your Point class should implement hashcode and equals like following: (many implementations exist, it's just one that works) Supposing that your instance's variables are x and y.

    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Point point = (Point) o;

        if (x != point.x) return false;
        if (y != point.y) return false;

        return true;
    }

    public int hashCode() {
        int result = x;
        result = 31 * result + y;
        return result;
    }

Otherwise, if you don't override those methods, Object's Javadoc explains well your issue:

As much as is reasonably practical, the hashCode method defined by
     * class {@code Object} does return distinct integers for distinct
     * objects. (This is typically implemented by converting the internal
     * address of the object into an integer, but this implementation
     * technique is not required by the
     * Java<font size="-2"><sup>TM</sup></font> programming language.)
     *
     * @return  a hash code value for this object.
     * @see     java.lang.Object#equals(java.lang.Object)
     * @see     java.lang.System#identityHashCode
     */
    public native int hashCode();

Thus, new Point(1,2) would not be considered as equal to new Point(1,2) and therefore, could never be retrieved from your Map.