User-defined object problem,why can't I use?

Question

as follows:
UObject.java
package test1;
public class UObject
private String num;
public UObject()
public void setNum(String num)
this.num=num;     
public String getNum()
return num;     
Test.java
package test1;
public class Test
public static void main(String args[])
UObject U=new UObject();
U.setNum("Squall");
System.out.println(U.getNum());
UObject[] U=new UObject[5];
for(int i=0;i<U.length;i++)
U.setNum(String.valueOf(i));     
for(int i=0;i<U.length;i++)
System.out.println(U[i].getNum());
error:
java.lang.NullPointerException
why can't I use?please tell me why?thanks.
ps:in Test.java's comments can run./**/.

Answer 1

Please use code formatting tags in future! http://forum.java.sun.com/features.jsp#Formatting
It would be easy to find the problem yourself if you printed out a stack trace e.g.
public static void main(String args[])
try
UObject U=new UObject();
U.setNum("Squall");
System.out.println(U.getNum());
UObject[] U=new UObject[5];
for(int i=0;i<U.length;i++)
U.setNum(String.valueOf(i)); 
for(int i=0;i<U.length;i++)
System.out.println(U.getNum());
catch( Exception e )
e.printStackTrace()
}Tho code doesnt look lke it compiles to me!