Java – Why does Math.round(0.49999999999999994) return 1

The docs for java.io.Serializable are probably about as good an explanation as you'll get:

The serialization runtime associates with each serializable class a version number, called a serialVersionUID, which is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization. If the receiver has loaded a class for the object that has a different serialVersionUID than that of the corresponding sender's class, then deserialization will result in an InvalidClassException. A serializable class can declare its own serialVersionUID explicitly by declaring a field named serialVersionUID that must be static, final, and of type long:

ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L;

If a serializable class does not explicitly declare a serialVersionUID, then the serialization runtime will calculate a default serialVersionUID value for that class based on various aspects of the class, as described in the Java(TM) Object Serialization Specification. However, it is strongly recommended that all serializable classes explicitly declare serialVersionUID values, since the default serialVersionUID computation is highly sensitive to class details that may vary depending on compiler implementations, and can thus result in unexpected InvalidClassExceptions during deserialization. Therefore, to guarantee a consistent serialVersionUID value across different java compiler implementations, a serializable class must declare an explicit serialVersionUID value. It is also strongly advised that explicit serialVersionUID declarations use the private modifier where possible, since such declarations apply only to the immediately declaring class — serialVersionUID fields are not useful as inherited members.

The transient keyword in Java is used to indicate that a field should not be part of the serialization (which means saved, like to a file) process.

From the Java Language Specification, Java SE 7 Edition, Section 8.3.1.3. transient Fields:

Variables may be marked transient to indicate that they are not part of the persistent state of an object.

For example, you may have fields that are derived from other fields, and should only be done so programmatically, rather than having the state be persisted via serialization.

Here's a GalleryImage class which contains an image and a thumbnail derived from the image:

class GalleryImage implements Serializable
{
    private Image image;
    private transient Image thumbnailImage;

    private void generateThumbnail()
    {
        // Generate thumbnail.
    }

    private void readObject(ObjectInputStream inputStream)
            throws IOException, ClassNotFoundException
    {
        inputStream.defaultReadObject();
        generateThumbnail();
    }    
}

In this example, the thumbnailImage is a thumbnail image that is generated by invoking the generateThumbnail method.

The thumbnailImage field is marked as transient, so only the original image is serialized rather than persisting both the original image and the thumbnail image. This means that less storage would be needed to save the serialized object. (Of course, this may or may not be desirable depending on the requirements of the system -- this is just an example.)

At the time of deserialization, the readObject method is called to perform any operations necessary to restore the state of the object back to the state at which the serialization occurred. Here, the thumbnail needs to be generated, so the readObject method is overridden so that the thumbnail will be generated by calling the generateThumbnail method.

For additional information, the article Discover the secrets of the Java Serialization API (which was originally available on the Sun Developer Network) has a section which discusses the use of and presents a scenario where the transient keyword is used to prevent serialization of certain fields.