Session 5 More on Java Strings and Files & Intro. to Inheritance.

transcript

Session 5

More on Java Strings and Files& Intro. to Inheritance

Java File I/O

• Allows us to write and read “permanent” information to and from disk

• How would file I/O help improve the capabilities of the MemoPadApp?

Java File I/O Example: Echo.java• echoes all the words in one file to an output

file, one per line.

$ java Echo hamlet.txt hamlet.out

$ less hamlet.out

tragedy

hamlet

prince

denmark

william

shakespeare ...

Study Echo.java’s File I/O

• have constructors that allow convenient and flexible processing

• send input message: readLine()

• send output messages: print() and println()

• use a stereotypical loop to process a file of lines

• use of the stereotypical StringTokenizer loop as inner loop

import java.io.*;import java.util.StringTokenizer;

public class Echo { public static void main( String[] args ) throws IOException { String delimiters = " .?!()[]{}|?/&\\,;:-\'\"\t\n\r";

BufferedReader inputFile = new BufferedReader(new FileReader(args[0]) ); PrintWriter outputFile = new PrintWriter( new FileWriter( args[1] ) );

String buffer = null;

while( true ) { buffer = inputFile.readLine();

if ( buffer == null ) break;

buffer = buffer.toLowerCase(); StringTokenizer tokens = new StringTokenizer( buffer, delimiters );

while( tokens.hasMoreElements() ) { String word = tokens.nextToken(); outputFile.println( word ); } // end while } // end while(true)... } // end main} // end class Echo

wc - UNIX/Linux utility

• wc prints the number of lines, words, and characters in a file to standard output.

• For example:

$ wc hamlet.txt

4792 31957 196505 hamlet.txt

Exercise

• Using Echo.java as your starting point, create a WordCount.java program that does the same thing as wc, i.e., prints the number of lines, words, and characters in a file to standard output. For example:

$ java WordCount hamlet.txt lines = 4792 words = 32889 chars = 130156

public class WordCount { public static void main( String[] args ) throws IOException { String delimiters = " .?!()[]{}|?/&\\,;:-\'\"\t\n\r";

BufferedReader inputFile = new BufferedReader( new FileReader( args[0] ) );

String buffer = null; int chars = 0; int words = 0; int lines = 0;

lines++;

while( tokens.hasMoreElements() ) { String word = tokens.nextToken(); words++; chars += word.length(); } // end while } // end while( true )...

System.out.println( "" + lines + " " + words + " " + chars ); } // end main} // end class WordCount

Why the difference in the number of words and number of characters?

$ wc hamlet.txt

4792 31957 196505 hamlet.txt

$ java WordCount hamlet.txt

lines=4792 words=32889 chars=130156

Java File I/O Example: Echo.java• echoes all the words in one file to an output

file, one per line.

$ java Echo hamlet.txt hamlet.out

$ less hamlet.out

tragedy

hamlet

prince

denmark

william

shakespeare ...

public class Echo { public static void main( String[] args ) throws IOException { String delimiters = " .?!()[]{}|?/&\\,;:-\'\"\t\n\r";

BufferedReader inputFile = new BufferedReader(new FileReader(args[0]) ); PrintWriter outputFile = new PrintWriter( new FileWriter( args[1] ) );

while( tokens.hasMoreElements() ) { String word = tokens.nextToken(); outputFile.println( word ); } // end while } // end while(true)... } // end main} // end class Echo

Working with Standard Input and Output as Files

• Sometimes, we'd like to give the user an option of providing a file name or using standard I/O.

• We can call sort with its own file argument, or we can pipe the standard output of one program (cat hamlet.out) as the standard input to sort.

• How can we make our Java programs do the same thing?

Streams vs. Readers and Writers

• a stream is a device for transmitting or receiving a sequence of byte (8-bit) values– emphasis on reading/writing -- not on data itself– network and file systems are based on byte unit

• Readers and Writers use 16-bit Unicode – useful for I/O of textual values as opposed to

binary data such as images, colors, etc.– for example, BufferedRead has readLine method

Working with Standard Input and Output as Files

• Standard input is an instance of the InputStream class and does not respond to readLine(), which is how we would like to grab lines of text as Strings.

• Standard output does respond to println() messages, but it is a PrintStream, which cannot be stored in a PrintWriter variable.

What can we do?

• We could write duplicate code for the four different cases. (file-file, file-stdout, stdin-file, stdin-stdout)

• Every case would look the same except for one or two lines.

• That doesn't seem to be the correct solution.

• Maybe we can find a way to have them talk to objects that talk to standard input and output...

A Solution• Let's take advantage of an object-oriented idea: We ought

to be able to substitute an object with a common interface, even if somewhat different behavior, in place of one another, and let the new object fulfill the responsibilities of the replaced one.

• While BufferedReaders and PrintWriters don't know how to talk to standard input and output, respectively, we can use a translator to serve as a go-between.

• Java give us the classes we need: InputStreamReader and OutputStreamWriter.

public class EchoStandard { public static void main( String[] args ) throws IOException { String delimiters = " .?!()[]{}|?/&\\,;:-\'\"\t\n\r";

BufferedReader inputFile = new BufferedReader( new InputStreamReader( System.in ) ); PrintWriter outputFile = new PrintWriter( new OutputStreamWriter( System.out ) );

buffer = buffer.toLowerCase(); StringTokenizer tokens = new StringTokenizer(buffer,delimiters);

while( tokens.hasMoreElements() ) { String word = tokens.nextToken(); outputFile.println( word ); } // end while } // end while( true )... } // end main} // end class EchoStandard

BufferedReader inputFile = new BufferedReader(

new FileReader( args[0]) );

PrintWriter outputFile = new PrintWriter(

new FileWriter( args[1]) );

vs. EchoStandard

BufferedReader inputFile = new BufferedReader(

new InputStreamReader( System.in ) );

PrintWriter outputFile = new PrintWriter(

new OutputStreamWriter( System.out ) );

Exercise• Turn Echo.java into EchoV2.java, which behaves just like Echo,

except that it takes two optional command-line arguments: the names of the input file and output file, respectively.

• If the user omits the second argument, the program writes to standard output.

• If the user omits both arguments, the program reads from standard output and writes to standard output. For example:

$ java EchoV2 hamlet.txt hamlet.out

$ less hamlet.out

tragedy

Exercise - More Examples$ java EchoV2 EchoV2.java

$ java EchoV2 hamlet.txt | less (interesting that the pipe “|” is not args[1])

tragedy

$ java EchoV2

$ cat hamlet.txt | java EchoV2 | less

tragedy

Introduction to Inheritance

Accumulator Example• a simple calculator app• classes needed:

– AdderApp - contains main– AddingFrame - GUI

– CloseableFrame - allows X button

– Accumulator - internal representation and implementation of the accumulator

AdderApp

• contains the main() method that serves as the "Big Bang" for this part of the world

public class AdderApp {

public static void main( String[] args ) {

AddingFrame f = new AddingFrame();

f.show();

} // end main

} // end class AdderApp

AddingFrame

• Provides the graphical interaction between the user and the actual calculator methods

• AddingFrame extends CloseableFrame extends JFrame.

• AddingFrame depends on the Accumulator class do the mathematics for the program.

Accumulator Class

• Recall from CS I that a class contains three things. – Data / Instance Variables– Method(s)– Constructor(s)

Accumulator Class• What Data / Instance Variables are needed?

Accumulator Class• Data / Instance Variables needed:

– currentSum – the current value “accumulated” by the accumulator.

– currentNumber – the number that has been entered by the user. The value that will be added or subtracted.

– displayValue – the value visible on the graphical calculator. (Needed because sometimes we display the number the user is entering (currentNumber) and sometimes it is the current accumulated value (currentSum), so we will maintain a new value which holds whatever is on display.)

Accumulator Class

• What methods would the accumulator class need (hint, there are five of them)?

Accumulator Class

• Needed methods:– plus – adds the last number entered to the currentSum.– minus – subtracts the last number entered from the

currentSum.– clear – sets everything back to zero– addDigit – adjusts the currentNumber upon input of an

additional integer– getDisplay – returns the current displayValue. (This is

necessary because our graphical class will want to know what value to display any time some action occurs.)

Accumulator Class

• What would the constructor do?

AccumulatorV1public class AccumulatorV1 {

private int currentSum;

private int currentNumber;

private int displayNumber;

public Accumulator() {

currentSum=0;

currentNumber=0;

displayNumber=0;

public void clear() {

currentSum=0;

currentNumber=0;

displayNumber=0;

public void addDigit( int digit ) {

currentNumber=currentNumber*10+digit;

displayNumber=currentNumber;

public void plus() {

currentSum+=currentNumber;

currentNumber=0;

displayNumber=currentSum;

public void minus() {

currentSum-=currentNumber;

currentNumber=0;

public int getDisplay() {

return displayNumber;

} // end class AccumulatorV1

Refactoring Accumulator

• What is refactoring? – Changing a program in a way that does not

change its functionality.

• Why do it? – To improve the structure of your code based on

what you have learned since writing it.

• What common code can we refactor?

Refactoring Accumulator

• Using the clear() method in the constructor

• Refactoring the plus() and minus() methods to call a private helper method.

Refactoring Accumulator public void plus() {

currentSum+=currentNumber;

prepareForNextNumber();

currentSum-=currentNumber;

prepareForNextNumber();

private void prepareForNextNumber() {

currentNumber=0;

public int getDisplay() {

return displayNumber;

} // end class AccumulatorV2

public class AccumulatorV2 { private int currentSum; private int currentNumber; private int displayNumber; public Accumulator() { clear(); } public void clear() { currentSum=0; currentNumber=0; displayNumber=0; } public void addDigit( int digit ) { currentNumber=currentNumber*10+digit; displayNumber=currentNumber; }

Reinforcing the refactoring

• There is an old programmers adage that states

"There are only two numbers: 1 and many"

• Once you start to repeat code, it is time to start to think about refactoring and adding in a helper method.

Alternative structure of the program

• The complete “calculator” consists of four classes.– AdderApp– AddingFrame– CloseableFrame– Accumulator

Alternative structure of the program• We can think of the relationships between

these four classes as being “narrow and deep”– AdderApp creates an instance of AddingFrame

which creates an instance of Accumulator.– This is a good example of data hiding since

AdderApp doesn’t know/care that there is an instance of the Accumulator class.

public class AdderApp { public static void main( String[] args ) { AddingFrame f = new AddingFrame(); f.show(); } // end main} // end class AdderApp

public class AddingFrame extends CloseableFrame { private Accumulator myAccumulator; ... public AddingFrame( ) {

// create frame and accumulatormyAccumulator = new Accumulator();

Alternative structure of the program• But another way to structure this program would be to

create a relationship which is “wide and shallow”– AdderApp creates an an instance of Accumulator which it

passes to an instance of AddingFrame. public class AdderApp { public static void main( String[] args ) {

Accumulator a = new Accumulator();

AddingFrame f = new AddingFrame(a);

f.show();

} // end main

} // end class AdderApp

– This is a good example of composition. • We emphasize that AddingFrame is composed of an Accumulator

– This is a good example of writing code that is modular.• Now that we know the composition relation, we can compose

solutions using variations of Accumulator.

CountedAccumulator Extension

• Suppose we need a new kind of object, an Accumulator that counts how many operations it executes. Let’s call this class CountedAccumulator.

• It responds to all the same messages as a regular Accumulator and also responds to an operationsExecuted() message, by returning its count.

• What changes would you need to make to Accumulator?

Adding Behavior to a Class• Any time that we need to add behavior to a

class we have at least three options:– Add code to the class itself, keeping the

original class. – Copy all the old code into a new class and add

code to this new class.– Create a subclass that extends the original

class' behavior.

Pros and Cons

“Add code to the class itself, keeping the original class. “– Pros: Quick. Convenient. Simple.– Cons: May change the behavior of the class.

Thus, it isn’t always an option.

Pros and Cons

“Copy all the old code into a new class and add code to this new class. “– Pros: Quick. Convenient. Simple.– Cons: Duplicated code. Error trap! Error trap!

Pros and Cons

“Create a subclass that extends the original class' behavior.“– Pros: Doesn’t break existing code. Virtually

eliminates duplicate code. Provides the most flexibility.

– Cons: Slightly more time consuming.

Pros and Cons

Developing an Extended Class

• There are typically four steps in developing an extended class.– declare the class– declare the new data – create the constructors – adjust the methods

• declare the class

public class CountedAccumulator extends Accumulator {

• declare the new data

private int numberOfOperations;

• create the constructor

public CountedAccumulator () {

super();

numberOfOperations=0;

• Leave inherited methods alone – clear() and prepareForNextNumber() are both

inherited from Accumulator and there is no need to change them.

• Modify/Override inherited methods– plus() and minus() are inherited, but they don't do what

we want them to. – We can make them do more without completely

replacing the code however.

public void plus() {super.plus();numberOfOperations++;

• Add completely new methods – We need an accessor method for numberOfOperations

public void operationsExecuted() {

return numberOfOperations;

CountedAccumulator Solutionpublic class CountedAccumulator extends Accumulator {

private int numberOfOperations;

public CountedAccumulator() {

super(); // calls the superclass’ constructor

numberOfOperations=0;

public void plus() {

super.plus();

numberOfOperations++;

super.minus();

numberOfOperations++;

public int getOperations() {

return numberOfOperations;

} // end class CountedAccumulator

CountedAccumulator Solution• Now, before we can really work with this

we need to modify other files in our application.

• We need to set up the AddingFrame so that it works with a CountedAccumulator rather than a regular Accumulator. We do this in the AdderApp class for simplicity.

Accumulator a = new CountedAccumulator();

AddingFrame f = new AddingFrame(a);

A solution• Why do we do this in the AdderApp rather than

leave it alone and modify the AddingFrame? – Because in the end this makes our AddingFrame

slightly more versatile.

– Think about it...AddingFrame works with an Accumulator (or CountedAccumulator). If one is provided, it uses it. If one is not provided, it creates it.

– THAT, is more versatile than telling an AddingFrame to now always create a CountedAccumulator.

A solution

• Now we can run this... – Notice that we have basically returned to

having a Accumulator. Why? – Notice that even though I have private data and

methods in Accumulator, I didn't have to change this here. Why?

A solution that USES the counting functionality

• If we want to actually use the functionality of this new class, then something needs to call the new method in CountedAccumulator.

• Without discussing the details of exception handling, we could do this by writing:

try {Thread.sleep(10000);

} catch(Exception e) {}System.out.println("Performed“

+a.getOperations()+"operations");

Another Exercise

Create a class named EvenOddAccumulator that subclasses Accumulator to implement this behavior.

EvenOddAccumulators respond to all the same messages as regular Accumulators. But, in response to plus() and minus() messages, an EvenOddAccumulator both computes the new sum and writes a congratulatory message if the sum is even.

Toward a Solution

Here is the critical new piece of the EvenOddAccumulator class:

if ( currentSum % 2 == 0 ) {System.out.println( "Hurray! You made an even number." );

The big question is, what else is a part of the class?

Toward a Solution

• Let’s look at one version of this…

A Problem Accessing Inherited Data$ javac EvenOddAccumulator.javaEvenOddAccumulator.java:17: currentSumhas private access in Accumulatorif ( currentSum % 2 == 0 )^EvenOddAccumulator.java:24: currentSumhas private access in Accumulatorif ( currentSum % 2 == 0 )^2 errors

Oops!currentSum is declared as a private instance variable in class Accumulator. private means private: no code outside the Accumulator class can access

that variable.

A Possible Solution for Accessing Inherited Data

• Change currentSum to be public or protected.

public class Accumulator {

protected int currentSum;

A Better Solutionfor Accessing Inherited Data

(2) Add a protected “accessor” method to theAccumulator class. Use that method to access thecurrentSum instance variable in the subclass.

public class Accumulator {...protected int currentSum() {

return currentSum;}

Then use currentSum() in EvenOddAccumulator.

Programming with Inheritance

Inheritance is an object-oriented programming construct that enables us to add behavior to an existing system without modifying the existing classes.

Programming with InheritanceOur new EvenOddAccumulator class adds behavior to a program

that uses Accumulators without modifying:• the behavior of the existing Accumulator class or• the existing AddingFrame class!

That means...• No chance of introducing an unnecessary, unexpected errors

into the working Accumulator class.• No need to modify programs that use instances of

Accumulator but which don’t need instances of EvenOddAccumulator.

• The ability to use EvenOddAccumulators in programs that expect to use Accumulators.

Programming with Inheritance

We could have achieved some of these results without using inheritance by creating a new class named EvenOddAccumulator that simply duplicated the behavior of existing Accumulator class.

Using inheritance means that...• No need to reimplement existing methods.• No need to duplicate code.

One of the most important features of object-oriented programming is that it encourages us to create new classes that reuse existing code as much as possible. Without inheritance, you have only one tool for doing that, composition. With inheritance, you have two tools.

Session 5 More on Java Strings and Files & Intro. to Inheritance.

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