java 为何写入序列化数字时发生异常

一下程序, 我输入1和3的时候, 发生异常，输出结果为：
input number: 1
input number: 3
write exception
原因也查到了，就是因为我在class data加了一个happy pp = new happy(aa, bb);如果去掉就正常了。但我想知道在保留这个语句的前提下怎样可以不发生异常呢？请大侠们指教！   import java.util.*; import java.io.*;
    public class TestDat2{   
      int aa;
      double bb;
      data[] cards;
     
       public TestDat2(){
         try{    
            FileInputStream fi = new FileInputStream("cards.dat");
            ObjectInputStream input = new ObjectInputStream(fi);
            cards = (data[])(input.readObject());
         }
             catch(IOException ex){
               cards = new data[1];
            }
             catch(ClassNotFoundException a){}
      }
      
       public static void main(String [] args){
         TestDat2 td = new TestDat2();
         while (true){
            Scanner kb = new Scanner(System.in);
            System.out.print("input number: ");
            int num = kb.nextInt();
            if (num == 1)
               td.add(); 
            if (num == 2)
               td.display();
            if (num == 3)
               td.save(); 
            if (num == 4)
               break; 
         }
      }
   
       public void add(){  //1
         aa = 1;
         bb = 2.2;
         data da = new data(aa, bb);
         cards[0] = da;
      }
      
       public void display(){ //2 
         System.out.println(cards[0]);
      }
      
       public void save(){//3
         try{
            FileOutputStream fo = new FileOutputStream("cards.dat");
            ObjectOutputStream output = new ObjectOutputStream(fo);
            output.writeObject(cards);
            System.out.println("write succeed");//test
         }
             catch(IOException ex){
               System.out.println("write exception");//test
            }
      }
   }
      import java.util.*; import java.io.*;
    public class data implements Serializable {
      int aa;
      double bb;
      happy pp = new happy(aa, bb);
      
       public data(int aa, double bb){
         this.aa = aa;
         this.bb = bb;
      }
      
       public String toString(){
         String info = "aa: " + aa + "\tbb" + bb;
         return info;
      }
   }
    public class happy{
      int nn;
      double mm;
   
       public happy(int nn, double mm){
         this.nn = nn;
         this.mm = mm;
      }
   
   }