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Chapter 14: Problem 5

Find the error in each block of code and show how to correct it. a) Assume that account, company and amount are declared. Object0utputStream outputStream; outputStream.writeInt( account ); outputStream.writeChars( company ); outputStream.writeDouble ( amount ); b) The following statements should read a record from the file "payables.txt". The Scanner variable inPayable should be used to refer to this file. Scanner inPayable = new Scanner( new File( "payables.txt" ) ); PayablesRecord record = ( PayablesRecord ) inPayable.readobject();

Short Answer

Expert verified
Initialize `ObjectOutputStream` correctly and use `ObjectInputStream` for reading objects.

Step by step solution

01

Identify the Error in Code Block (a)

In code block (a), the error is that the `ObjectOutputStream` object `outputStream` is not initialized before it is used. The stream must be associated with an underlying byte stream, such as a `FileOutputStream`.
02

Correct Code Block (a)

To correct the code, initialize the `ObjectOutputStream` with a `FileOutputStream`. For example: ```java outputStream = new ObjectOutputStream(new FileOutputStream("output.dat")); outputStream.writeInt(account); outputStream.writeChars(company); outputStream.writeDouble(amount); ``` This will attach the object output stream to a file named "output.dat".
03

Identify the Error in Code Block (b)

In code block (b), the error is that the `Scanner` class does not have a `readObject()` method. This method is used for object input streams, not scanners.
04

Correct Code Block (b)

To correct this, replace the `Scanner` with an `ObjectInputStream`, which does support `readObject()`. Here is the corrected code: ```java ObjectInputStream inPayable = new ObjectInputStream(new FileInputStream("payables.txt")); PayablesRecord record = (PayablesRecord) inPayable.readObject(); ``` Ensure that the required class imports are included, and handle exceptions such as `ClassNotFoundException` and `IOException`.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

ObjectOutputStream
In Java Input/Output operations, `ObjectOutputStream` plays a crucial role in serializing objects. Serialization makes it possible to write an object to a file, so it can be reconstructed later.
For the `ObjectOutputStream` to work, it needs to be initialized properly.
  • Always connect it with a `FileOutputStream` to specify the file destination.
  • Use the `writeObject()` method to save objects, but `writeInt()` or `writeChars()` can also be used for primitives and strings.
  • Remember to close your stream with `close()` method to release system resources.
This stream makes it easy to transfer objects into files, making it incredibly useful for preserving object states between program runs.
FileOutputStream
`FileOutputStream` is a byte stream class used in Java for writing streams of raw bytes to a file. This stream is particularly useful for writing binary data.
Connecting `FileOutputStream` with an `ObjectOutputStream` allows serialization of objects to a file. Here are some critical features:
  • It directs the bytes to a specified file, for example, "output.dat".
  • Using `write()` method, you can write data as bytes directly into the file.
  • Always handle exceptions like `IOException` to manage file access issues safely.
In essence, `FileOutputStream` is like the road that guides data bytes from your program to an external file.
ObjectInputStream
`ObjectInputStream` is the counterpart to `ObjectOutputStream`, enabling the reading and deserialization of objects that have been written to an output stream. When you want to access the previously stored data, this class comes into play.
  • You must link it with a `FileInputStream` to specify the input source file.
  • The `readObject()` method reads bytes and converts them back into the original object form.
  • Make sure to handle exceptions like `IOException` and `ClassNotFoundException` which occur during deserialization.
In Java, `ObjectInputStream` makes retrieving data stored as objects a seamless process, simplifying data restoration and usage.
Java I/O streams
Java I/O (Input/Output) streams serve as a powerful abstraction for reading from and writing to byte and character streams. These streams simplify data processing by handling I/O through mechanisms like files, network sockets, or byte arrays.
Some key points about Java I/O streams include:
  • Character Streams deal with reading/writing of character-based data, while Byte Streams handle binary data.
  • Using `BufferedReader` or `BufferedOutputStream` can increase efficiency by buffering data.
  • Java streams use various mechanisms like `try-with-resources` to enforce automatic resource management and exception handling.
Java's I/O streams systematically manage data flow, freeing developers to focus more on application logic and less on intricate details.

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Most popular questions from this chapter

Complete the following tasks, assuming that each applies to the same program: a) Write a statement that opens file "oldmast.ser" for input-use objectInputstream variable in01dMaster to wrap a FileInputStream object. b) Write a statement that opens file "trans.ser" for input-use ObjectInputStream variable inTransaction to wrap a FileInputStream object. c) Write a statement that opens file "newmast.ser" for output (and creation)-use \(0 \mathrm{b}-\) ject0utputStream variable outNewMaster to wrap a File0utputStream. d) Write a statement that reads a record from the file "oldmast.ser". The record is an object of class AccountRecordSerializable- use ObjectInputStream variable in01dMaster. Assume class AccountRecordSerializable is the same as the AccountRecordSerializable class in Fig. 14.17 e) Write a statement that reads a record from the file "trans.ser". The record is an object of class TransactionRecord - use ObjectInputStream variable inTransaction. f) Write a statement that outputs a record to the file "newmast.ser". The record is an object of type AccountRecordSerializable-use Object0utputStream variable outNewMaster.

Determine which of the following statements are true and which are false. If \(f a l\) se, explain why. a) The programmer must explicitly create the stream objects System. in, System. out and System .err. b) When reading data from a file using class Scanner, if the programmer wishes to read data in the file multiple times, the file must be closed and reopened to read from the beginning of the file. This moves the file-position pointer back to the beginning of the file. c) Method exists of class File returns true if the name specified as the argument to the File constructor is a file or directory in the specified path. d) Binary files are human readable. e) An absolute path contains all the directories, starting with the root directory, that lead to a specific file or directory. f) Class Formatter contains method printf, which enables formatted data to be output to the screen or to a file.

Determine which of the following statements are true and which are false. If false, explain why. a) The impressive functions performed by computers essentially involve the manipulation of zeros and ones. b) People specify programs and data items as characters. Computers then manipulate and process these characters as groups of zeros and ones. c) Data items represented in computers form a data hierarchy in which data items become larger and more complex as we progress from fields to characters to bits and so on. d) A record key identifies a record as belonging to a particular field. companies store all their information in a single file to facilitate computer processing of the information. When a program creates a file, the file is retained by the computer for future reference.

Fill in the blanks in each of the following statements: a) Computers store large amounts of data on secondary storage devices as __________ b) \(A(n)\) ____________ is composed of several fields. c) To facilitate the retrieval of specific records from a file, one field in each record is chosen as a \((n)\) ____________ d) Files that are created using byte-based streams are referred to as _________ files, while files created using character-based streams are referred to as ___________ files. e) The standard stream objects are __________, _________ and ____________

Fill in the blanks in each of the following statements: a) Ultimately, all data items processed by a computer are reduced to combinations of ________ and _________ b) The smallest data item a computer can process is called a(n) ____________ c) \(A(n)\) ___________ can sometimes be viewed as a group of related records. d) Digits, letters and special symbols are referred to as _________ e) A database is a group of related ___________ f) Object _____________ normally enables a program to output error messages to the screen.
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