为什么我不能正确地从哈希表中检索？

Question

Why can't I properly retrieve from my hashtable? I create a hashtable which consists of key and values as both type Coordinate (a class I created). And then I can't retrieve the x value from my coordinates object.

public coordinates translateOffsetToPixel(int x, int y){
    //Translate given coordinates to pixel coordinates for the cell
    coordinates input = new coordinates(x,y);
    coordinates outputPixelCoord;

    Hashtable <coordinates, coordinates> table = new Hashtable<coordinates, coordinates>();

    for (int r = 0 ; r<row; r++){
        for(int c = 0; c< col; c++){
            System.out.println("hit translation");
            table.put(new coordinates(r,c), new coordinates(r*2,c*2));
        }
    }

    outputPixelCoord = table.get(input);
    System.out.println("outputX:" + outputPixelCoord.getX()); //ERROR
    return outputPixelCoord;

}

Coordinates Class:

public class coordinates {
    private int x,y;
    public coordinates(int x, int y){
        this.x = x;
        this.y = y;
    }

    public int getX() {return x;}

    public int getY() {return y;}

}

LOGTABLE:

03-17 13:55:53.690    1961-1961/com.example.sam.matrix D/ViewRootImpl﹕ ViewPostImeInputStage ACTION_DOWN
03-17 13:55:53.780    1961-1961/com.example.sam.matrix I/System.out﹕ hit board
03-17 13:55:53.780    1961-1961/com.example.sam.matrix I/System.out﹕ 5
03-17 13:55:53.780    1961-1961/com.example.sam.matrix I/System.out﹕ hit translation
03-17 13:55:53.780    1961-1961/com.example.sam.matrix E/InputEventReceiver﹕ Exception dispatching input event.
03-17 13:55:53.780    1961-1961/com.example.sam.matrix E/MessageQueue-JNI﹕ Exception in MessageQueue callback: handleReceiveCallback
03-17 13:55:53.800    1961-1961/com.example.sam.matrix E/MessageQueue-JNI﹕ java.lang.NullPointerException

Answer 1

For a Hashtable (and HashMap ) to store and retrieve keys properly, the key type must override hashCode and equals properly.

To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the hashCode method and the equals method.

You haven't overridden those methods, so the Hashtable can't find your keys. Override those methods.

Answer 2

You need to override the hashCode and equals method like so, so that it may be stored and retrieved from the HashTable/HashMap, etc..

public class Coordinates {
    private int x,y;
    public coordinates(int x, int y){
        this.x = x;
        this.y = y;
    }

    public int getX() {return x;}

    public int getY() {return y;}

    public int hashCode() {
        // This is just a random way to generate hash
        // see other ways to generate hash before you implement this
        return x + (37 * y)
    }

    public boolean equals(Object obj) {
        if (obj instance of Coordinates) {
            Coordinates c = (Coordinates)obj;
            return c.x == this.x && c.y == this.y;
        }
        return false;
    }
}