Transcript Week 2 Power Point Slides

The Package Statement
• Group related interfaces and classes together
• Purpose: encapsulation and reduces name conflicts
– private package classes not visible outside the package
– Classes in different packages can have the same names
• Creating a package class file
– The first statement: package <package name>;
• Where do we store a package’s class files?
– In sub-folders of the parent package folder
– Name the sub-folder: <package name>
– Alternatives
• Create jar file and add to classpath environment variable
• Put into the lib/ext folder of JVM
• Package and sub packages only related by where they are stored
The Import Statement
• Example: import java.awt.event.*;
• Java automatically imports the package, java.lang
• The import statement allows references to package
classes that are not fully qualified. It doesn’t
actually load anything
• Sub packages are referenced using dot notation.
Example: java.awt.Button. Button is a subpackage of java.awt
• Fully qualified names are always needed if the
same class name appears in two or more packages.
Notes on the Import Statement
• Import can reference either a single package class or
all classes of a package.
• Wildcard references do not apply to sub-packages.
• Import java.awt.Button allows us to WRITE
b = new Button(“Clear”);
instead of
b = new java.awt.Button(“Clear”);
• Legal uses of import:
a. import java.awt.*;
okay
b. import java.awt.E* no good.
c. import java.awt.*; does not import java.awt.Event.*
Input and Output Streams
• Streams handle Java input and output.
• File transfer moves data to/from disk rather than
to/from keyboard or screen
File I/O Concepts
• Files allow programs to access persistent data.
• Key terms: database, file, record, and field
• File access operations: open, read, write, close.
• Handle exceptions when dealing with files.
– FileNotFoundException, IOException, EOFException
• File types are: Sequential and Random
• Data types are: Text, Binary, and Object
• File class: File file = new File(“pathname”)
if (!file.exists())  file doesn’t exist.
Sequential Text File Streams
•
•
Stream: Flow of source to sink
Operations: sequential reads and writes of Strings
BufferedReader in
= new BufferedReader( new FileReader(“path”));
strVar = in.readLine();
in.close();
PrintWriter out = new
PrintWriter(new BufferedWriter(new FileWriter(“path”)));
out.println(“data”);
out.close();
•
BufferedReader wraps FileReader
–
–
•
Reads entire line instead of a character at a time
The readLine() method returns null if at the end of file.
PrintWriter contains print and println methods.
Binary Files Streams
Operations: Reads and Writes of binary or text data
DataOutputStream out=new DataOutputStream(new
FileOutputStream(“path”));
DataInputStream in=new DataInputStream(new
FileInputStream(“path”));
out.writeUTF(“data”);
out.writeInt(x);
out.writeDouble(3.5);
out.flush();
str
= in.readUTF();
int var = in.readInt();
double val = in.readDouble();
in.close();
out.close();
1.UTF stands for Unicode Text Format for writing and reading strings
2.DataOutputStream wraps FileOutputStrea to provide more methods
for different data types.
Random File Read and Write
RandomAccessFile raf = new RandomAccessFile(“path”,”access”);
Read(byte[]), readBoolean(), readChar(), readDouble(), readFloat(),
readInt(), readLine(), readLong(), readShort(), readUTF(), length(),
setLength(), getFilePosition(), writeBoolean(), writeChar(),
writeDouble(), writeFloat(), writeInt(), writeLine(), writeLong(),
writeShort(), writeUTF(),seek(long), close().
• “access” notes
–
–
–
–
Can be “rw”, “r”, “w”.
If “r”, the file must pre-exist.
If “rw” or “w” and the file doesn’t exist, it will be created.
“rwd” writes with immediate updates to storage.
• Seek first and then use the various methods
Writing objects to files
• Make the object serializable
– Add Implements java.io.Serializable onto class signature line of
any object that can be written to disk.
– Place the keyword transient on any instance variables that are not
to be serialized (ex: private transient int x;);
• Which streams?
– ObjectOutputStream stream = new ObjectOutputStream(
new BufferedOutputStream(newFileOutputStream(name)));
– ObjectInputStream stream = new ObjectInputStream(
new BufferedInputStream(new FileInputStream(name)));
• Which methods?
stream.writeObject(objVariable);
objVariable = (ObjectClassName)stream.readObject();
stream.close();
Check if File is Readable
Boolean isReadable(String fileName)
{ File file = new File(fileName);
if (!file.exists())
throw new FileNotFoundException();
if (!file.canRead())
throw new IOException();
}
Dialog for Choosing a File
String str = System.getProperty ("user.dir");
JFileChooser chooser = new JFileChooser(str);
int result = chooser.showOpenDialog(null);
if (result == JFileChooser.APPROVE_OPTION)
{
File file = chooser.getSelectedFile();
String fileName = file.getName();
System.out.println("You selected " + fileName);
// Insert code here to open and access data from file
} else System.out.println("You cancelled the file dialog");