Why an abstract class generated?

[changzhou]

I try to create a class BasicType
public class BasicType implements java.io.Serializable{
String _ISBN;

public java.lang.String getISBN();
public void setISBN(java.lang.String);
public BasicType();
}

But I always get an abstract class, and the interface Serializable is not correct.
C:\>javap BasicType
Compiled from "BasicType.java"
public abstract class BasicType extends java.lang.Object implements java.io.Serializable{
public java.lang.String getISBN();
public void setISBN(java.lang.String);
public BasicType();
}

Anybody know why?

Here are my codes:
public class ClassGenerateSample extends ClassLoader{

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
try {
ClassPool pool = ClassPool.getDefault();
pool.importPackage("java.io.Serializable");

CtClass cls = pool.makeClass("BasicType");
//CtClass cls = pool.makeClass("FrameType");
//cls.setName("BasicType");
cls.addInterface(pool.get("java.io.Serializable"));

CtField cfld = CtField.make("private java.lang.String _ISBN;", cls);
cls.addField(cfld);

CtMethod m = CtNewMethod.make("public java.lang.String getISBN();", cls);
cls.addMethod(m);
m.setBody("return _ISBN;");
CtMethod m1 = CtNewMethod.make("public void setISBN(java.lang.String isbn);", cls);
cls.addMethod(m1);
m1.setBody(" _ISBN = $1;");

cls.writeFile("c:\\");
}
catch (CannotCompileException e) {
System.err.println("Cannot Compile Exception error");
}
catch (NotFoundException e) {
System.err.println("Not Found Exception error");
}
catch (IOException e) {
System.err.println("IO Exception error");
}
}
}