The Scanner class has very nice look-ahead capabilities. You can query a scanner to determine whether more tokens are available and whether the next token is of a given type. If scanner is of type Scanner :

•scanner.hasNext() — returns a boolean value that is true if there is at least one more token in the input source.

•scanner.hasNextInt(), scanner.hasNextDouble(), and so on — returns a boolean value that is true if there is at least one more token in the input source and that token represents a value of the requested type.

•scanner.hasNextLine() — returns a boolean value that is true if there is at least one more line in the input source.

Although the insistence on defining tokens only in terms of delimiters limits the usability of scanners to some extent, they are easy to use and are suitable for many applications.

11.1.6Serialized Object I/O

The classes PrintWriter, TextReader, Scanner, DataInputStream, and DataOutputStream allow you to easily input and output all of Java’s primitive data types. But what happens when you want to read and write objects? Traditionally, you would have to come up with some way of encoding your object as a sequence of data values belonging to the primitive types, which can then be output as bytes or characters. This is called serializing the object. On input, you have to read the serialized data and somehow reconstitute a copy of the original object. For complex objects, this can all be a major chore. However, you can get Java to do all the work for you by using the classes ObjectInputStream and ObjectOutputStream. These are subclasses of InputStream and Outputstream that can be used for writing and reading serialized objects.

ObjectInputStream and ObjectOutputStream are wrapper classes that can be wrapped around arbitrary InputStreams and OutputStreams. This makes it possible to do object input and output on any byte stream. The methods for object I/O are readObject(), in ObjectInputStream, and writeObject(Object obj), in ObjectOutputStream. Both of these methods can throw IOExceptions. Note that readObject() returns a value of type Object, which generally has to be type-cast to a more useful type.

ObjectOutputStream also has methods writeInt(), writeDouble(), and so on, for outputting primitive type values to the stream, and ObjectInputStream has corresponding methods for reading primitive type values.

Object streams are byte streams. The objects are represented in binary, machine-readable form. This is good for e ciency, but it does su er from the fragility that is often seen in binary data. They su er from the additional problem that the binary format of Java objects is very specific to Java, so the data in object streams is not easily available to programs written in other programming languages. For these reasons, object streams are appropriate mostly for short-term storage of objects and for transmitting objects over a network connection from one Java program to another. For long-term storage and for communication with non-Java programs, other approaches to object serialization are usually better. (See Subsection 11.6.2 for a character-based approach.)

ObjectInputStream and ObjectOutputStream only work with objects that implement an interface named Serializable. Furthermore, all of the instance variables in the object must be serializable. However, there is little work involved in making an object serializable, since the Serializable interface does not declare any methods. It exists only as a marker for the compiler,