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 ; Where do we store a package’s class files? –In sub-folders of the parent package folder –Name the sub-folder: –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 sub- package 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”);str = in.readUTF(); out.writeInt(x);int var = in.readInt(); out.writeDouble(3.5);double val = in.readDouble(); out.flush();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");