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Trail: Essential Java Classes
Lesson: I/O: 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 straightforward 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(new Date());
ObjectOutputStream 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.

Thus, the writeObject method serializes the specified object, traverses its references to other objects recursively, and writes them all. In this way, relationships between objects are maintained.

ObjectOutputStream 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 ObjectInputStream

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 straightforward. Here's code that reads in the String and the Date objects that were written to the file named theTime in the previous 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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