if选择结构
if（condition1）｛
statements;
.........
｝
多重if选择结构
if（condition1）｛
statements;
........
｝else if（condition2）｛
statements;
........
｝else｛
statements;
........
｝
嵌套if选择结构
if（condition1）｛
statements;
..........
if（condition2）｛
statements;
..........
｝else｛
statements;
..........
｝
｝else｛
｝
- 产生随机数：int random=（int）(Math.random（）＊10)；／／10～0中任意整数