Java：如何使这个Serializable？

package helpers;

public class XY implements Comparable<XY>
{
    public int x;
    public int y;

    public XY (int x,int y)
    {
        this.x = x;
        this.y = y;
    }

    public int compareTo( XY other ) 
    {
        String compare1 = this.x + "-" + this.y;
        String compare2 = other.x + "-" + other.y;

        return compare1.compareTo( compare2 );
    }

    public String toString()
    {
        return this.x + "-" + this.y;
    }

}

截至目前,我不能将它作为一个对象与outputstream一起发送..我试图实现Serializable,但这不起作用.