The Java TM Tutorial

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Trail: Essential Java Classes
Lesson: Reading and Writing (but no 'rithmetic)

## Serializing Objects

> Reconstructing an object from a stream requires that the object first be written to a stream. So let's start there.

### How to Write to an ObjectOutputStream

> Writing objects to a stream is a straight-forward process. For example, the following gets the current time in milliseconds by constructing a `Date` object and then serializes that object:

> >     FileOutputStream out = new FileOutputStream("theTime");
> >     ObjectOutputStream s = new ObjectOutputStream(out);
> >     s.writeObject("Today");
> >     s.writeObject(new Date());
> >     s.flush();

> `ObjectOutputStream` is a processing stream, so it must be constructed on another stream. This code constructs an `ObjectOutputStream` on a `FileOutputStream`, thereby serializing the object to a file named `theTime`. Next, the string `Today` and a `Date` object are written to the stream with the `writeObject` method of `ObjectOutputStream`.

> If an object refers to other objects, then all of the objects that are reachable from the first must be written at the same time so as to maintain the relationships between them. Thus the `writeObject` method serializes the specified object, traverses its references to other objects recursively, and writes them all.

> `ObjectOutputStream` stream implements the `DataOutput` interface that defines many methods for writing primitive data types, such as `writeInt`, `writeFloat`, or `writeUTF`. You can use these methods to write primitive data types to an `ObjectOutputStream`.

> The `writeObject` method throws a `NotSerializableException` if it's given an object that is not serializable. An object is serializable only if its class implements the `Serializable` interface.

### How to Read from an ObjectOutputStream

> Once you've written objects and primitive data types to a stream, you'll likely want to read them out again and reconstruct the objects. This is also straight-forward. Here's code that reads in the `String` and the `Date` object that was written to the file named `theTime` in the last example:

> >     FileInputStream in = new FileInputStream("theTime");
> >     ObjectInputStream s = new ObjectInputStream(in);
> >     String today = (String)s.readObject();
> >     Date date = (Date)s.readObject();

> Like `ObjectOutputStream`, `ObjectInputStream` must be constructed on another stream. In this example, the objects were archived in a file, so the code constructs an `ObjectInputStream` on a `FileInputStream`. Next, the code uses `ObjectInputStream`'s `readObject` method to read the `String` and the `Date` objects from the file. The objects must be read from the stream in the same order in which they were written. Note that the return value from `readObject` is an object that is cast to and assigned to a specific type.

> The `readObject` method deserializes the next object in the stream and traverses its references to other objects recursively to deserialize all objects that are reachable from it. In this way, it maintains the relationships between the objects.

> `ObjectInputStream` stream implements the `DataInput` interface that defines methods for reading primitive data types. The methods in `DataInput` parallel those defined in `DataOutput` for writing primitive data types. They include methods such as `readInt`, `readFloat`, and `readUTF`. Use these methods to read primitive data types from an `ObjectInputStream`.

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