I have a base class (A) that implements serializable. This class has a generated SerailVersionUID but its child class (B) does not have serialVersionUID.
I have the following in place (the input stream contains objects of type B)
public Object getObject() throws...
{
Object o = someObjectInputStream.readObject();
return o;
}
I know that it is best practice to cast the object returned by readObject() but the casting is done on the returned object by the method that calls getObject();
Querstion:
Is it true that the returned object could be of either type A or B?
Is there any guarantee that the returned type is always B considering that it does not have a UID?
Thanks
The fact that it doesn't have a serialVersionUID has no bearing on what class you receive. That is determined solely by what class you sent. Your question doesn't really make sense.
Related
What is the meaning of the following code (2nd line) in which inside class uvm_resource_pool definition, instance (object) rp is created?
class uvm_resource_pool;
static local uvm_resource_pool rp = get();
// Function: get
//
// Returns the singleton handle to the resource pool
static function uvm_resource_pool get();
if(rp == null)
rp = new();
return rp;
endfunction
This is how the singleton pattern gets coded in SystemVerilog. The singleton pattern is an OOP technique that makes sure only one instance of a class type is ever constructed. The constructor as well as the object rp are declared local. The only way to retrieve an instance of a object of the class type uvm_resource_pool is to call the static method get(), which constructs it the first time called, but then the next time it will just return rp. This is also how to solve the static class initialization order fiasco. You never reference a static variable directly, you always use a get() method that constructs it on the first reference.
If we dont have virtual constructors then why we have virtual destructors? Can constructors also be virtual?
There is no point in virtual constructor - you declare exactly what
type is created, and it is well known in compile time. The compiler
do not need [and actually cannot, since the dynamic dispatch is based
on information which is created only after the object was created].
So there are no virtual constructors.
Virtual destructors are important to prevent memory leaks, and
monitor the system. Assume you have A* a = new B; [B inherits
from A], and you later delete a; - the compiler has no way of
knowing a is a B [in the general case], and will invoke A's
destructor - if it wasn't virtual, and you might get a memory leak,
or other faults.
Using virtual destructor - you ensure that B's destructor is
invoked, since a B object is being destroyed.
Virtual destructors are needed because at destruction time, you don't always know what type you're dealing with:
Base *make_me_an_object()
{
if (the_moon_is_full())
return new Derived();
else
return new Base();
}
int main()
{
Base *p = make_me_an_object();
delete p;
}
The delete in the above program's main doesn't know whether its p points to a Base or a Derived object, but if the Base destructor is virtual (as it should be), then delete can use *p's vtable to find the right destructor.
By contrast, at construction time, you always know what kind of object you're creating. (And in case you don't, then you can create a factory or "virtual constructor" that does know.)
#include<iostream>
using namespace std;
class base {
protected:
int a;
};
class derived : public base {
};
int main() {
base * pointer_of_base = new derived;
delete pointer_of_base; // this will delete the base calss not the derived
}
The constructors are called on one time when we create the object of the class so when we inherit the base class constructors calls only one time so no need to be virtual.
But when we accessing the derived class from the pointer of the base class, if we want to delete the object of derived class we delete it by the pointer of base class but delete(pointer_of_base) will call the destructor of the base class but the actual motto is to delete the derived class . thus we need the destructor be virtual in nature.
I have an EggSac object which contains references to >100 000 Egg objects. Some variables in the Eggs have to be maintained to be consistent with EggSac, so I want to make these only changeable by EggSac. However EggSac passes references to its Eggs all over the application, so if I use public methods then any other code could modify the secure parts of the Eggs by accident.
What's a proper OO way to make sure only the EggSac object can call the "secure" methods of the Eggs, but still make the "safe" methods available to everyone?
My idea is to split Egg's class into a base class containing only safe methods and a derived class containing the secure methods that only EggSac should have access to. Then EggSac has members of the type of the derived class, but it casts them to their base class whenever something else wants one.
Have EggSack hold references to EggImpl, which implements all the necessary methods. Then pass around wrappers over the impl (the Egg class) which only call the "safe" methods on the impl.
When you say security, do you mean avoiding accidental code modification?
A structured way can be something like below.
If you want to make it really 'secure', then you can modify the code to store a string*HashCode* inside the calling class and only if it's matched (inside called ) in Egg, modification is allowed.
Interface ISecureModifier
{
String GetSecureModifierKEY();
String GetSecureModifierVALUE();
}
class Egg
{
Dictionary Secure_ata;
public secureDataModifier( ISecureModifier modifyingObject)//note the interface being used
{
//Here, try a cast (if your compiler still allowed other type objects not implementing ISecureModifier ) and throw exception stating not authorized to modify.
modifyingObject.GetSecureModifierKEY
modifyingObject.GetSecureModifierValue
/*Now write the code to modify Dictionary*/
}
}
class EggSac:ISecureModifier//implements interface
{
private string SecureModifierKEY;
private string SecureModifierVALUE
String GetSecureModifierKEY()//inteface impl
{
return SecureModifierKEY;
}
String GetSecureModifierVALUE();//interface impl
{
return SecureModifierVALUE;
}
ModifySecureData(Egg egg, string key, string value)
{
egg.secureDataModifier(this);//passing own reference
}
}
You may call like this
objEggSack.ModifySecureData(objEgg101, "firstKey","NewValue")
I have these classes:
[DataContract]
public class ErrorBase {}
[DataContract]
public class FileMissingError: ErrorBase {}
[DataContract]
public class ResponseFileInquiry
{
[DataMember]
public List<ErrorBase> errors {get;set;};
}
An instance of the class ResponseFileInquiry is what my service method returns to the client. Now, if I fill ResponseFileInquiry.errors with instances of ErrorBase, everything works fine, but if I add an instance of inherited type FileMissingError, I get a service side exception during serialization:
Type 'MyNamespace.FileMissingError' with data contract name 'FileMissingError'
is not expected. Add any types not known statically to the list of known types -
for example, by using the KnownTypeAttribute attribute or by adding them to the
list of known types passed to DataContractSerializer.'
So serializer is getting confused because it's expecting the List to contain the declared type objects (ErrorBase) but it's getting inherited type (FileMissingError) objects.
I have the whole bunch of error types and the List will contain combinations of them, so what can I do to make it work?
You should add KnownType attribute to your base class
[DataContract]
[KnownType(typeof(FileMissingError))]
public class ErrorBase {}
Read more about KnownType attribute in this blog
Try this:
[DataContract]
[KnownType(typeof(FileMissingError))]
public class ErrorBase {}
As the error message states, any information that cannot be know statically (like the polymorphic relationship you have expressed here) must be supplied via attributes. In this case you need to specify that your FileMissingError data contract is a known type of its base class, ErrorBase.
A tad bit late, but maybe for future generations. =)
If you don't want to add an attribute for every child class to your parent class, you could construct a list of known types in the parent classes static constructor using
IEnumerable<Assembly> assemblies = AppDomain.CurrentDomain
.GetAssemblies()
.Where(a => !a.GlobalAssemblyCache);
IEnumerable<Type> serializableTypes = assemblies.SelectMany(a => a.GetTypes())
.Where(t => IsSerializable(t));
// ...
private static bool IsSerializable(Type type)
{
return type.GetCustomAttributes(true).Any(a => a is DataContractAttribute);
}
and pass this list to the de/serializers constructor. I don't know how robust this solution is, but that's what I am doing and so far it works. It is a little slow, so make sure to cache the result.
Can a class return an object of itself.
In my example I have a class called "Change" which represents a change to the system, and I am wondering if it is in anyway against design principles to return an object of type Change or an ArrayList which is populated with all the recent Change objects.
Yes, a class can have a method that returns an instance of itself. This is quite a common scenario.
In C#, an example might be:
public class Change
{
public int ChangeID { get; set; }
private Change(int changeId)
{
ChangeID = changeId;
LoadFromDatabase();
}
private void LoadFromDatabase()
{
// TODO Perform Database load here.
}
public static Change GetChange(int changeId)
{
return new Change(changeId);
}
}
Yes it can. In fact, that's exactly what a singleton class does. The first time you call its class-level getInstance() method, it constructs an instance of itself and returns that. Then subsequent calls to getInstance() return the already-constructed instance.
Your particular case could use a similar method but you need some way of deciding the list of recent changes. As such it will need to maintain its own list of such changes. You could do this with a static array or list of the changes. Just be certain that the underlying information in the list doesn't disappear - this could happen in C++ (for example) if you maintained pointers to the objects and those objects were freed by your clients.
Less of an issue in an automatic garbage collection environment like Java since the object wouldn't disappear whilst there was still a reference to it.
However, you don't have to use this method. My preference with what you describe would be to have two clases, changelist and change. When you create an instance of the change class, pass a changelist object (null if you don't want it associated with a changelist) with the constructor and add the change to that list before returning it.
Alternatively, have a changelist method which creates a change itself and returns it, remembering the change for its own purposes.
Then you can query the changelist to get recent changes (however you define recent). That would be more flexible since it allows multiple lists.
You could even go overboard and allow a change to be associated with multiple changelists if so desired.
Another reason to return this is so that you can do function chaining:
class foo
{
private int x;
public foo()
{
this.x = 0;
}
public foo Add(int a)
{
this.x += a;
return this;
}
public foo Subtract(int a)
{
this.x -= a;
return this;
}
public int Value
{
get { return this.x; }
}
public static void Main()
{
foo f = new foo();
f.Add(10).Add(20).Subtract(1);
System.Console.WriteLine(f.Value);
}
}
$ ./foo.exe
29
There's a time and a place to do function chaining, and it's not "anytime and everywhere." But, LINQ is a good example of a place that hugely benefits from function chaining.
A class will often return an instance of itself from what is sometimes called a "factory" method. In Java or C++ (etc) this would usually be a public static method, e.g. you would call it directly on the class rather than on an instance of a class.
In your case, in Java, it might look something like this:
List<Change> changes = Change.getRecentChanges();
This assumes that the Change class itself knows how to track changes itself, rather than that job being the responsibility of some other object in the system.
A class can also return an instance of itself in the singleton pattern, where you want to ensure that only one instance of a class exists in the world:
Foo foo = Foo.getInstance();
The fluent interface methods work on the principal of returning an instance of itself, e.g.
StringBuilder sb = new StringBuilder("123");
sb.Append("456").Append("789");
You need to think about what you're trying to model. In your case, I would have a ChangeList class that contains one or more Change objects.
On the other hand, if you were modeling a hierarchical structure where a class can reference other instances of the class, then what you're doing makes sense. E.g. a tree node, which can contain other tree nodes.
Another common scenario is having the class implement a static method which returns an instance of it. That should be used when creating a new instance of the class.
I don't know of any design rule that says that's bad. So if in your model a single change can be composed of multiple changes go for it.