和罕见神曲差不多的歌:java里面问题！求助

public class a {

public static final int CM_PER_M = 100;
public static final int MM_PER_CM = 10;
public static final int MM_PER_M = MM_PER_CM*CM_PER_M;//毫米转换为米为100*10

private int meters = 0;
private int centimeters = 0;
private int millimeters = 0;
public a(int mm)
{
meters = mm/MM_PER_M;
centimeters = (mm - meters*MM_PER_M)/MM_PER_CM;
millimeters = mm - meters*MM_PER_M - centimeters*MM_PER_CM;
}

public String toString()
{
return meters + "m " + centimeters + "cm " + millimeters + "mm";
}

public a a2(a q)
{
return q;
}
public static void main(String[] args) {
a i=new a(3);

System.out.println(i.a2(i));
}
}
public a a2(a q)
{
return q;
}
看这个
这是我自己定义的
在public static void main(String[] args) {
a i=new a(3);
System.out.println(i.a2(i));
}
时输出的竟然是方法toString的结果
这是怎么回事啊？