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Lesson 9 Working with text files
ObjectiveRead and write text files.

Working with Text Files

The FileReader and FileWriter classes are very useful for reading from and writing to text files.
Following are the two main constructors used to create FileReader and FileWriter objects: 
public FileReader(String fileName)
public FileWriter(String fileName)

Stream Objects

Since both of these objects represent streams, you can use them in conjunction with other stream objects. Additionally, they support the familiar read() and write() methods that are so prevalent in Java's stream classes.
The BufferedReader class is commonly used with FileReader since it is more efficient. It provides a readLine() method that is useful for reading entire lines of text at a time.
Following is an example of reading and printing lines of text from a file named Notes.txt: 
BufferedReader in = new BufferedReader(new FileReader("Notes.txt"));
String s;
while ((s = in.readLine()) != null)
System.out.println(s);

In this example, the while loop checks to see if the end of the file has been reached, as indicated by the readLine() method returning null. The readLine() method returns each line of text as a string, which is then printed to standard output.

Legacy Applet Files and Security

Java Applets are largely considered obsolete and are no longer widely supported. Here's a breakdown:
  • Deprecation:
    • Major web browsers have phased out support for the technology that Java Applets relied on.
    • Oracle deprecated Java Applets in Java SE 9, and they were removed from Java SE 11.
  • Browser Support:
    • Modern web browsers generally do not support Java Applets.
  • Alternatives:
    • Technologies like Java Web Start were used as alternatives, but even those have evolved.
    • modern web applications use different technology.
  • Oracle's Position:
    • Oracle's focus has shifted away from applets, with more emphasis on other Java technologies.
In essence, while there might be very niche legacy systems still relying on them, Java Applets are effectively no longer a viable or supported technology for general use.
It is important to understand that applets had security restrictions that prevented them from writing files under normal circumstances. More specifically, if an applet was not signed with a digital signature then you were not allowed to write files.
If you attempted to create a FileWriter object in an applet that was not signed, it wouldl trigger a security exception. Digitally signing an applet solved the problem before Oracle stopped their support for Java Applets.


Following is a similar example that reads a text file and writes its contents to another text file: 
BufferedReader in = new BufferedReader(new FileReader("Notes.txt"));
BufferedWriter out = new BufferedWriter(new FileWriter("Copy.txt"));
String s;
while ((s = in.readLine()) != null)
out.write(s, 0, s.length());

Provide 0 as offset

This example writes each line of text to the file named Copy.txt using the write() method. This version of the write() method expects a string, offset, and number of characters as its parameters. By providing 0 as the offset and s.length() as the number of characters, the entire string is written each time through the loop.
Note: It is worth noting that most of the Java I/O class constructors and methods are capable of throwing[1] exceptions, which means that you will typically place I/O code within a try-catch construct.

Data File Reader - Exercise

Click the Exercise link below to read from a file.
Data File Reader - Exercise

[1] Throwing (an exception): When an exceptional condition is encountered in a Java program, an exception is thrown, which means that a special exception object is created and passed out of the current context. The idea is that a higher context will be more capable of handling the exception.

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