#vulnerable code 
                                public CtField lookupField(ASTList className, Symbol fieldName)
        throws CompileError
    {
        return lookupField2(Declarator.astToClassName(className, '.'),
                            fieldName);
    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public CtField lookupField(ASTList className, Symbol fieldName)
        throws CompileError
    {
        return lookupJavaField(Declarator.astToClassName(className, '.'),
                            fieldName);
    }