当我创建对象的新实例作为键时，Java HashMap.get() 返回 null [英] Java HashMap.get() returns null when I create a new instance of an object as a key

问题描述

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 colums. The problem I'm having is that if I use the HashMap.get() with a new Point, it returns a null value.

    HashMap<Point, String> cache = new HashMap<Point, String>();
    Point p1 = new Point(1,1);
    Point p2 = new Point(2,2);
    cache.put(p1, "Test: 1,1");
    cache.put(p2, "Test: 2,2");

    int maxRow = 2;
    int maxCol = 2;
    for (int i = 1; i <= maxRow; i++) {
        for (int j = 1; j <= maxCol; j++) {
            System.out.print(cache.get(new Point(i,j)));
            if (j < maxCol) 
                System.out.print("	");
            if (j == maxRow)
                System.out.println("");
        }
    }

This returns:

null     null
null     null

I'm probably missing something obvious, but I can't find it myself. Also, if you happen to know if there is a better data structure for storing data from cells I'd love to hear it. Thanks in advance!

解决方案

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.

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