java空指向异常``求助高手

import java.util.*;
import java.io.*;
interface Shuiguo
{
public void zhang();
public void shu();
};
class Banan implements Shuiguo
{
public void zhang()
{
System.out.println("香蕉。。");
}
public void shu()
{
System.out.println("摘了");
}
};
class Orange implements Shuiguo
{
public void zhang()
{
System.out.println("橘子");
}
public void shu()
{
System.out.println("熟了");
}
};
class Shu
{
BufferedReader b = null;
public void shu()
{
b = new BufferedReader(new InputStreamReader(System.in));
}
public String string()
{
String str = null;
try
{
str = b.readLine();
}
catch (Exception e)
{
}
return str;
}
};
class Factory
{