winter 2006cisc121 - prof. mcleod1 last time misc. useful classes in java: –string...

Winter 2006 CISC121 - Prof. McLeod 1

Last Time

• Misc. useful classes in Java:– String– StringTokenizer– Math– System


Stuff…

• Assignment 1 is posted (finally!).


Today

• Wrapper classes

• JFileChooser

• Text File I/O

• The File class


Wrapper Classes

• Sometimes it is necessary for a primitive type value to be an Object, rather than just a primitive type.– Some data structures only store Objects.– Some Java methods only work on Objects.

• Wrapper classes also contain some useful constants and a few handy methods.


Wrapper Classes - Cont.

• Each primitive type has an associated wrapper class:

• Each wrapper class Object can hold the value that would normally be contained in the primitive type variable, but now has a number of useful static methods.

char Character

int Integer

long Long

float Float

double Double


Integer Wrapper Class - Example

Integer number = new Integer(46);//”Wrapping”

Integer num = new Integer(“908”);

Integer.MAX_VALUE // gives maximum integer

Integer.MIN_VALUE // gives minimum integer

Integer.parseInt(“453”) // returns 453

Integer.toString(653) // returns “653”

number.equals(num) // returns false

int aNumber = number.intValue(); // aNumber is 46


Aside - Why an “equals” Method for Objects?

• The String class also has “equals” and “equalsIgnoreCase”.

• These wrapper classes also have an equals method.

• Why not use the simple boolean comparators (==, !=, etc.) with Objects?

• These comparators just compare memory addresses.

• How are you going to sort Objects?


Aside - Why an “equals” Method for Objects?, Cont.

• == can only compare memory addresses when Objects are compared.

• Most Data Container Objects will have both an equals method and a compareTo method.

• The equals method tests for equality using whatever you define as “equal”.

• The compareTo method returns a postive or negative int value (or zero to indicate “equal”), again depending on how you define one Object to be greater or less than another.


Wrapper Classes – Cont.

• The Double wrapper class has equivalent methods:

Double.MAX_VALUE // gives maximum double value

Double.MIN_VALUE // gives minimum double value

Double.parseDouble(“0.45E-3”) // returns 0.45E-3

• parseDouble is only available in Java 2 and newer versions.

• See the Java documentation for more on Wrapper classes.


Character Wrapper Class

• Many useful methods to work on characters:• “character” is a char

• getNumericValue(character)• isDigit(character)• isLetter(character)• isLowerCase(character)• isUpperCase(character)• toLowerCase(character)• toUpperCase(character)


Built - In GUI Windows

• We will learn to build our own GUI windows, but you should be aware of the GUI Windows already built into Java:

– JOptionPane– JColorChooser– JFileChooser

• These are all built to perform common tasks and are very easy to use.

• Imported from the javax.swing package.• See “BuiltInDemo.java”.


JFileChooser

• A built in file browser/selector dialog box.• The demo only used the chooser in the most

simple way.• For example, you can specify a starting folder and

add as many file extension filters as you like.• The chooser returns a File object, from which

you can obtain much information about the file.


JFileChooser Window


JFileChooser Example Code

• (At the top:

– import java.swing.JFileChooser;)

JFileChooser chooser = new JFileChooser();

int result = chooser.showOpenDialog(null);

if (result == JFileChooser.APPROVE_OPTION)

System.out.println(chooser.getSelectedFile());


JFileChooser, Cont.

• Upon completion of the dialog, the getSelectedFile() method returns a File object.

• This object can easily be used with file I/O code…


Simple Alternative

• Prompt the user for a filename as a String, using the console window.


File I/O

• Files provide a convenient way to store and re-store to memory larger amounts of data.

• We will use arrays to store the data in memory, and we’ll talk about these things later.

• Three kinds of file I/O to discuss:– Text– Binary– Random access

• For now, we’ll stick with text I/O.


Text File Output in Java 5.0

• Use the PrintWriter class. (As usual), you must import the class:

import java.io.PrintWriter;

• In your program:

PrintWriter fileOut = new PrintWriter(outFilename);

• (outFilename is a String filename we obtained somewhere else…)


Text File Output in Java 5.0, Cont.

• Unfortunately the instantiation of the PrintWriter object can cause a FileNotFoundException to be thrown and you must be ready to catch it:

try {

writeFile = new PrintWriter(outputFile);

} catch (FileNotFoundException e) {

System.out.println(e.getMessage());

System.exit(0);

} // end try catch


Aside - File Paths in Strings

• Sometimes you might have to include a path in the filename, such as “C:\Alan\CISC212\Demo.txt”

• Don’t forget that if you have to include a “\” in a String, use “\\”, as in:

“C:\\Alan\\CISC212\\Demo.txt”



• The PrintWriter constructor can also accept a File object (such as provided from JFileChooser!)



• The Object fileOut, owns a couple of familiar methods: print() and println().

• When you are done writing, don’t forget to close the file with:

fileOut.close();

• Way easy!!


Text File Input in Java 5.0

• Use the FileReader and Scanner classes. Our usual import statements:

import java.util.Scanner;

import java.io.FileReader;

import java.io.FileNotFoundException;

• We’ll get to that last one in a minute.


Text File Input in Java 5.0, Cont.• In my program:

fileIn = new FileReader("Test.txt");

Scanner fileInput = new Scanner(fileIn);

• Scanner class’ constructor can also accept a File object directly.

• Unfortunately the FileReader constructor (what’s a “constructor” anyways?) throws a kind of exception that I cannot ignore - so the code above cannot be used exactly in this way.


Text File Input in Java 5.0, Cont.

• This works:

FileReader fileIn = null;

try{

fileIn = new FileReader("Test.txt");

}

catch (FileNotFoundException e) {

// Do something clever here!

}

Scanner fileInput = new Scanner(fileIn);


Without using FileReader

• You can also send a File object to the Scanner class when you instantiate it instead of a FileReader object.

• You will still need to do this in a try catch block as shown in the previous slide.

• See the demo program “TextFileReaderDemo.java”


Text File Input in Java 5.0, Cont.

• We are going to have to talk about try/catch blocks soon! But for now, let’s get back to file input.

• To get the file contents, and print them to the console, for example:

while (fileInput.hasNextLine()) {

System.out.println(fileInput.nextLine());

}


Aside - Scanner Class’ Tokenizer

• The Scanner class has a built in String Tokenizer.

• Set the delimiters using the useDelimiter(delimiter_String) method.

• Obtain the tokens by calling the next() method.• The hasNext() method will return false when

there are no more tokens.


• File is a class in the java.io.* package.• It contains useful utility methods that will help prevent

programs crashing from file errors.• For example:

File myFile = new File(“test.dat”);

myFile.exists(); // returns true if file exists

myFile.canRead(); // returns true if can read from file

myFile.canWrite(); // returns true if can write to file

The File Class


The File Class, Cont.

myFile.delete(); // deletes file and returns true if successful

myFile.length(); // returns length of file in bytes

myFile.getName(); // returns the name of file (like “test.dat”)

myFile.getPath(); // returns path of file (like “C:\AlanStuff\JavaSource”)


Binary and Random Access

• Binary files contain data exactly as it is stored in memory – you can’t read these files in Notepad!

• Text file is sequential access only.• What does that mean?

• Random access can access any byte in the file at any time, in any order.

• More about Binary and Random File Access later!

winter 2006cisc121 - prof. mcleod1 last time misc. useful classes in java: –string...

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