关于地图滚屏有关问题，

关于地图滚屏问题，急
为什么我这个地图往左右能滚屏的，但是往下就不行了，屏幕也会花掉，谢谢
以下是我的代码：
这是Map类：
public class Map 
{
int mapRow = 30;//地图总行数
int mapCol = 30;//地图总列数

int w = 16;//每个格子宽
int h = 16;//每个格子高

int map_x = 0;//地图的x初始坐标
int map_y = 0;//地图的y初始坐标

int MapNum;//一行占的格子数 

int cx;
int cy;

int x;
int y;

int x0 = 0;//屏幕左上角的世界坐标
int y0 = 0;

int gamemap_map0[][] = 
{
{ 4, 18, 18, 18, 18, 22, 2, 3, 2, 3, 24, 18, 18, 18, 4, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 4 },
{ 9, 18, 18, 18, 18, 22, 7, 8, 7, 8, 24, 18, 18, 18, 9, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 9 },
{ 14, 18, 18, 18, 18, 22, 12, 13, 12, 13, 24, 18, 18, 18, 14, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 14 },
{ 18, 18, 18, 18, 18, 22, 23, 23, 23, 23, 24, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18 },
{ 18, 18, 18, 18, 18, 22, 23, 23, 23, 23, 24, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18 },
{ 18, 18, 18, 18, 18, 22, 23, 23, 23, 23, 24, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18 },
{ 18, 18, 18, 18, 18, 18, 18, 19, 17, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18 },
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