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Why does ObjectOutputStream.writeObject(Object o) not take a Serializable? Why is it taking an Object?
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Basically, the ObjectOutputStream encodes Java objects using the class name and object values. And, hence generates corresponding streams. This process is known as serialization. Those converted streams can be stored in files and can be transferred among networks.
This process is known as serialization. Those converted streams can be stored in files and can be transferred among networks. Note: The ObjectOutputStream class only writes those objects that implement the Serializable interface. This is because objects need to be serialized while writing to the stream
Retrieve the object used to buffer persistent fields to be written to the stream. This method will allow trusted subclasses of ObjectOutputStream to substitute one object for another during serialization. Reset will disregard the state of any objects already written to the stream.
The method writeObject is used to write an object to the stream. Any object, including Strings and arrays, is written with writeObject. Multiple objects or primitives can be written to the stream. The objects must be read back from the corresponding ObjectInputstream with the same types and in the same order as they were written.
This is because writeObject in ObjectOutputStream overrides the method in the ObjectOutput interface which does not require that the object be Serializable.
The ObjectOutput interface specifies methods that allow objects to be written to a stream or underlying storage, but this may be achieved by a process other than serialization. The ObjectOutputStream implements this functionality, but requires serializable objects. However, it cannot modify the signature of the interface that it implements.
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It should be ObjectOutputStream.writeObject(serializable) rather than ObjectOutputStream. writeObject(Object). It is a proper use case where a marker interface like Serializable should have been used but unfortunately not. This would have made it possible the very real benefit of compile-time type checking instead of failing at runtime if the object does not implement Serializable interface.
I would like to take this opportunity to mention what Joshua Bloch has mentioned in his book Effective java:
A marker interface is an interface that contains no method declarations, but merely designates (or “marks”) a class that implements the interface as having some property. For example, consider the Serializable interface. By implementing this interface, a class indicates that its instances can be written to an
ObjectOutputStream(or “serialized”).In the case of the
Serializablemarker interface, theObjectOutputStream.write(Object)method will fail if its argument does not implement the interface. Inexplicably, the authors of theObjectOutputStreamAPI did not take advantage of theSerializableinterface in declaring the write method. The method’s argument type should have beenSerializablerather thanObject. As it stands, an attempt to callObjectOutputStream.writeon an object that doesn’t implementSerializablewill fail only at runtime, but it didn’t have to be that way.
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