Dec. 16/ Deciding Which Loop to Use

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1 Dec. 16/ Deciding Which Loop to Use When is the while loop ideal to use? What is the do while loop ideal for? What is the for loop ideal for? What classes are used to write data to a file? What classes are used to read data from a file? Why have the programs written so far, require you to re-enter data each time the program runs? What is an input file and why is it called this? What is an output file and why is it called this? What are the two types of files? What does a text file contain? What happens to numbers stored in a file? What s the deal with a binary file? Deciding Which Loop to Use Although most repetitive algorithms can be written with any of the three types of loops, each works best in different situations. Each of Java s three loops is ideal to use in different situations. The following is a short summary of when each loop should be used: The while loop. The while loop is a pretest loop. It is ideal in situations where you do not want the loop to iterate if the condition is false from the beginning. It is also ideal if you want to use a sentinel value to terminate the loop. The do while loop. The do-while loop is a posttest loop. It is ideal in situations where you always want the loop to iterate at least once. The for loop. The for loop is a pretest loop that has built-in expressions for initializing testing and updating. These expressions make it very convenient to use a loop control

2 variable as a counter. The for loop is ideal in situations where the exact number of iterations is known. Introduction to File Input and Output Concept: The Java API provides several classes that you can use for writing data to a file and reading data from a file. To write data to a file, you can use the printwriter class, and optionally, the Filewriter class. To read data from a file, you can use the Scanner class and the File class. The programs have written so far require you to re-enter data each time the program runs. This is because the data stored in variables and objects in RAM disappears once the program stops running. To retain data between the times it runs, a program must have a way of saving the data. Data may be saved in a file, which is usually stored on a computer's disk. Once the data is saved in a file, it will remain there after the program stops running. The data can then be retrieved and used at a later time. In general, there are three steps that are taken when a file is used by a program: 1. The file must be opened. When the file is opened, a connection is created between the file and the program. 2. Data is then written to the file or read from the file. 3. When the program is finished using the file, the file must be closed. In this section we will discuss how to write Java programs that write data to files and read data from files. The terms input file and output file are commonly used. An input file is a file that a program reads data from. It is called an input file because the data stored in it serves as input to the program. An output file is a file that a program writes data to. It is called an output file because the program stores output in the file. In general, there are two types of files: text and binary. A text file contains data that has been encoded as text, using a scheme such as Unicode. Even if the file contains numbers, those numbers are stored in the file as a series of characters. As a result, the file may be opened and viewed in a text editor such as Notepad. A binary file contains data that has not been converted to text. As a consequence, you cannot view the contents of a binary file with a text editor. In this chapter, we will discuss how to work with text files. Binary files are discussed in Chapter 12. What import statement must you use to work with files? How do you write data to a file?

3 What does the PrintWriter class do? What does the following statement do: PrintWriter outputfile = new printwriter("studentdata.txt );? What happens If the file that you are opening with the PrintWriter object already exists? What methods you can you write data to a file with? What is a buffer? How does it work with data? What happens when the buffer is filled? What does the The PrintWriter class's println method do? What does a newline character do? What is a delimiter? The Java API provides a number of classes that you will use to work with files. To use theses classes, you will place the following import statement near the top of your program. import java.io.*; Using the PrintWriter Class to Write Data to a File To write data to a file you will create an instance of a PrintWriter class. The PrintWriter class allows you to open a file for writing. It also allows you to write data to the the same print and println methods that you have been using to display data on the screen. You pass the name of the file that you wish to open, as a string, to the PrintWriter class's constructor. For example, the following statement creates a printwriter object and passes the file name StudentData.txt to the constructor. PrintWriter outputfile = new printwriter("studentdata.txt ); This statement will create an empty file named StudentData.txt and establish a connection between it and the PrintWriter object that is referenced by outputfile. The file will be created in the current directory or folder. You may also pass a reference to a String object as an argument to the PrintWriter constructor. For example, in the following code the user specifies the name of the file.

4 String filename; filename = JoptionPane.showlnputDialog( Enter the filename."); PrintWriter outputfile = new printwriter(filename); WA R N I N G! If the file that you are opening with the PrintWriter object already exists, it will be erased and an empty file by the same name will be created. Once you have created an instance of the PrintWriter class and opened a file, you can write data to the file using the print and println methods. You already know how to use print and println with System.out to display data on the screen. They are used the same way with a PrintWriter object to write data to a file. For example, assuming that outputfile references a PrintWriter object, the following statement writes the string " Jim" to the file: outputfile.println( Jim"); When the program is finished writing data to the file, it must close the file. To close the file use the PrintWriter class's close method. Here is an example of the method's use: outputfile.close( ); Your application should always close files when finished with them. This is because the system creates one or more buffers when a file is opened. A buffer is a small "holding section" of memory. When a program writes data to a file, that data is first written to the buffer. When the buffer is filled, all the information stored there is written to the file. This technique increases the system's performance because writing data to memory is faster than writing it to a disk. The close method writes any unsaved data remaining in the file buffer. Once a file is closed, the connection between it and the PrintWriter object is removed. In order to perform further operations on the file, it must be opened again. More About the Printwriter Class's println Method The PrintWriter class's println method writes a line of data to a file. For example, assume an application creates a file and writes three students' first names and their test scores to the file with the following code: PrintWriter outputfile = new PrintWriter("StudentData.txt"); outputfile.println( Jim ); outputfile.println(95); outputfile.println( Karen"); outputfile.println(98);

5 outputfile.println( Bob ); outputfile.println(82); outputfile.close(); The println method writes data to the file and then writes a newline character immediately after the data. You can visualize the data written to the file in the following manner: Jim<newline> 9 5<newline> Karen <newline> 98 <newline> Bob<newline> 82 <newline> The newline characters are represented here as <newline>. You do not actually see the newline characters, but when the file is opened in a text editor such as Notepad, its contents will appear as shown in Figure As you can see from the figure, each newline character causes the data that follows it to be displayed on a new line. In addition to separating the contents of a file into lines, the newline character also serves as a delimiter. A delimiter is an item that separates other items. When you write data to using the println method, newline characters will separate the individual items of data. Later you will see that the individual items of data in a file must be separated in order for them to be read from the file.