Files & Streams cont..

Input File Exceptions

A FileNotFoundException is thrown if the file is not found when an attempt is made to open a file

Each read method throws IOException we still have to write a catch block for it

If a read goes beyond the end of the file an EOFException is thrown

Avoiding Common ObjectInputStream File Errors

There is no error message (or exception)

if you read the wrong data type!

Input files can contain a mix of data types it is up to the programmer to know their order and use the

correct read method

ObjectInputStream works with binary, not text files

As with an output file, close the input file when you are done with it

Common Methodsto Test for the End of an Input File

A common programming situation is to read data from an input file but not know how much data the file contains

In these situations you need to check for the end of the file

There are three common ways to test for the end of a file:1. Put a sentinel value at the end of the file and test for it.

2. Throw and catch an end-of-file exception.

3. Test for a special character that signals the end of the file (text files often have such a character).

The EOFException Class

Many (but not all) methods that read from a file throw an end-of-file exception (EOFException) when they try to read beyond the file

all the ObjectInputStream methods in Display 9.3 do throw it

The end-of-file exception can be used in an "infinite" (while(true)) loop that reads and processes data from the file

the loop terminates when an EOFException is thrown

The program is written to continue normally after the EOFException has been caught

Using EOFException

try { ObjectInputStream inputStream = new ObjectInputStream(new FileInputStream("numbers.dat")); int n; System.out.println("Reading ALL the integers"); System.out.println("in the file numbers.dat."); try { while (true) { n = inputStream.readInt(); System.out.println(n); } } catch(EOFException e) { System.out.println("End of reading from file."); } inputStream.close(); } catch(FileNotFoundException e) { System.out.println("Cannot find file numbers.dat."); } catch(IOException e) { System.out.println("Problem with input from file numbers.dat."); }

Chapter 9 6

main method fromEOFExceptionDemo

Intentional "infinite" loop to process data from input file

Note order of catch blocks:the most specific is first

and the most general last

Loop exits when end-of-file exception is thrown

Processing continues after EOFException: the input file is closed

Binary I/O of Class Objects

read and write class objects in binary file

class must be serializable

import java.io.*

implement Serializable interface

add implements Serializable to heading of class definition

methods used:

to write object to file:writeObject method in ObjectOutputStream

to read object from file: readObject method in ObjectInputStream

public class Species implements Serializable

outputStream = new ObjectOutputStream(

new FileOutputStream("species.records"));

...

Species oneRecord =

new Species("Calif. Condor, 27, 0.02);

...

outputStream.writeObject(oneRecord);

inputStream = new ObjectInputStream( new FileInputStream("species.records"));...Species readOne = null;...readOne = (Species)inputStream.readObject(oneRecord);

readObject returns a reference to type Object so it must be cast to Species before assigning to readOne

ClassIODemo Excerpts

The Serializable Interface

Java assigns a serial number to each object written out.

If the same object is written out more than once, after the first write only the serial number will be written.

When an object is read in more than once, then there will be more than one reference to the same object.

If a serializable class has class instance variables then they should also be serializable.

Why aren't all classes made serializable?

security issues: serial number system can make it easier for programmers to get access to object data

doesn't make sense in all cases, e.g., system-dependent data

SummaryPart 1

Text files contain strings of printable characters; they look intelligible to humans when opened in a text editor.

Binary files contain numbers or data in non-printable codes; they look unintelligible to humans when opened in a text editor.

Java can process both binary and text files, but binary files are more common when doing file I/O.

The class ObjectOutputStream is used to write output to a binary file.

SummaryPart 2

The class ObjectInputStream is used to read input from a binary file.

Always check for the end of the file when reading from a file. The way you check for end-of-file depends on the method you use to read from the file.

A file name can be read from the keyboard into a String variable and the variable used in place of a file name.

The class File has methods to test if a file exists and if it is read- and/or write-enabled.

Serializable class objects can be written to a binary file.

Reading Objects From FileWhy use files

Quickly load objects from file.Populate your system with data at runtime

Possible to incorporate into your projectLoad room types in at runtime.Load stored data in at runtime

public List<Vehicle> readVehicles(String file)

{

Scanner reader = new Scanner(file);

List<Vehicle> vehicles = new ArrayList<Vehicle>();

try

{

while (reader.hasNextLine())

{

vehicles.add(Vehicle.readFromScanner(reader));

}

}

finally

{

reader.close();

}

return vehicles;

}

Reading Objects from File// In vehicle Class

public Vehicle readFromScanner(Scanner scanner)

{

String make = reader.next();

String model = reader.next();

int year = reader.nextInt();

String vin = reader.next();

// Don't use double for currency values

BigDecimal value = reader.nextBigDecimal();

int milesDriven = reader.nextInt();

// Shouldn't this be some sort of date type?

Date lastOilChange = ??? //what do we do here???????

//Assuming the following constructor exists

return new Vehicle(make, model, year, vin, value, milesDriven,lastOilChange);

}

File would look as follows

Ford Fiesta 1990 ABCDE1234 5555.99 84000 2015/01/10

Toyota Avensis1999 DCADE1234 15555.99 14000 2015/11/10

What do we do about the date?

Read it as a String if you like and keep it as a string

OR have a Date field in your object, then we need to parse the date to string when saving or reading

Write date to filetry {

DateFormat df = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss");

Date today = Calendar.getInstance().getTime();

String reportDate = df.format(today);

String dateToPrintToFile = reportDate;

File file = new File("testDate.txt");

if (!file.exists()) {

file.createNewFile();

}

FileWriter fw = new FileWriter(file.getAbsoluteFile());

BufferedWriter bw = new BufferedWriter(fw);

bw.write(dateToPrintToFile);

bw.close();

} catch (Exception e) {

e.printStackTrace();

}

Read Date from File 2015/11/10 Date has been saved in previous example as String

We can read the date using reader.next() to get the next word

Read in the string and parse the string to a date using SimpleDateFormat

SimpleDateFormat sdf=new SimpleDateFormat("YYYY/MM/DD"); Date date=sdf.parse(reader.next()); MyObject.setDate(date);

Further ExampleDEBBIE STARR F 3 W 1000.00

JOAN JACOBUS F 9 W 925.00

DAVID RENN M 3 H 4.75

ALBERT CAHANA M 3 H 18.75

DOUGLAS SHEER M 5 W 250.00

SHARI BUCHMAN F 9 W 325.00

SARA JONES F 1 H 7.50

RICKY MOFSEN M 6 H 12.50

JEAN BRENNAN F 6 H 5.40

JAMIE MICHAELS F 8 W 150.00

ObjectList.txt

public class Payroll { private LineWriter lw; private PersonList list; private Person;

public Payroll(LineWriter lw) { this.lw = lw; } public void readfile() { try{ Scanner scanner = new Scanner(System.in); while(scanner.hasNextLine()) { String line = scanner.nextLine(); Scanner lineScanner = new Scanner(line); lineScanner.useDelimiter(" ");

person = new Person(); while(lineScanner.hasNext()) { person.setFirstName(lineScanner.next()); person.setLastName(lineScanner.next()); person.setGender(lineScanner.next()); person.setTenure(lineScanner.next()); person.setRate(Integer.parseInt(lineScanner.next())); person.setSalary(Double.parseDouble(lineScanner.next()));

list.add(person); } } }catch(FileNotFoundException e) {e.printStackTrace();} }

public class Person {

private String firstName, lastName, tenure; private char gender; private int rate; private double salary; public Person(){

} //settors . . . . . //gettors . . . . .

}

public class PersonList {

private ArrayList<Person> personList; public PersonList(){

personList = new ArrayList<Person>(); }

//list methods..//add//remove//search

}

Initialise the list

public class Driver{

public static void main(String args[]) { LineWriter lw = new LineWriter("ObjectList.txt"); Payroll payroll = new Payroll(lw);

payroll.readfile(); lw.close(); }

}

DEBUGGING

Overview

Testing and debugging are important activities in software development.

Techniques and tools are introduced.

Part of the material here comes from chapter 13 of Cohoon and Davidson’s book and chapter 6 of Barnes and Kolling’s book “Objects first with Java”.

Programming Errors Compilation errors

Syntax error (example: missing a semi-colon). Semantic error. (For example, applying modulus % on floating-

point value for certain programming languages. In Java ,is it fine? Yes!)

Easiest type of errors to fix. Runtime errors

Occur at runtime. Java’s exception mechanism can catch such errors.

Logic errors Program runs but produces incorrect result. Hard to characterize, hence hardest to fix.

Programming errors are also known as bugs Origin: a moth in the Mark I computer.

Testing and Debugging (1/7) Testing

To determine if a code contains errors. Debugging

To locate the error and fix it. Documentation

To improve maintainability of the code. Include sensible comments, good coding style and clear logic.

Testing

Yes

Error?

Debug

Testing and Debugging (2/7) Unit testing

Test of individual parts of an application – a single method, a single class, a group of classes, etc.

Positive versus negative testing Positive testing – testing of functionality that we expect to work. Negative testing – testing cases we expect to fail, and handle

these cases in some controlled way (example: catch handler for exception).

Test automation Regression testing – re-running tests that have previously been

passed whenever a change is made to the code. Write a test rig or a test harness.

This and the next slides involve materials not covered yet or to be covered in other modules (such as Software Engineering). You will need to gain relevant experience before you can appreciate them. Hence I will skip them for the moment.

Testing and Debugging (3/7) Modularization and interfaces

Problem is broken into sub-problems and each sub-problem is tackled separately – divide-and-conquer.

Such a process is called modularization. The modules are possibly implemented by different programmers,

hence the need for well-defined interfaces. The signature of a method (its return type, name and parameter

list) constitutes the interface. The body of the method (implementation) is hidden – abstraction.

Good documentation (example: comment to describe what the method does) aids in understanding.

static double

max(double a, double b)           Returns the greater of two double values.

Testing and Debugging (4/7) Manual walkthroughs

Pencil-and-paper.Tracing the flow of control between

classes and objects.Verbal walkthroughs

Testing and Debugging (5/7) Print statements

Easy to add Provide information:

Which methods have been called The value of parameters The order in which methods have been called The values of local variables and fields at strategic points

Disadvantages Not practical to add print statements in every method Too many print statements lead to information overload Removal of print statements tedious

Testing and Debugging (6/7) Debugger

Provides Stepping (step and step-into) Breakpoint Tracking of every object’s state

Program testing can be used to show the presence of bugs, but never to show their absence. – Edgar Dijkstra

Testing and Debugging (7/7) Tips and techniques

Start off with a working algorithm Incremental coding/test early Simplify the problem Explain the bug to someone else Fix bugs as you find them Recognize common bugs (such as using ‘=’ instead of ‘==’, using

‘==’ instead of equals( ), dereferencing null, etc.) Recompile everything Test boundaries Test exceptional conditions Take a break

Turning Debugging Info On/Offpublic static int sum(int a, int b) {int left = 0;int right = data.length - 1;if (debugging) { System.out.println("sum called with a = " + a

+ " b = " + b);}int total = a + b;if (debugging) { System.out.println("total = " + total);}return total;

}

debugging = true;int ans = sum(3,5);

Debugging on.debugging = false;int ans = sum(3,5);

Debugging off.

Add an extra boolean debugging field to the class.

Testing Thoroughly (1/2) Recall our discussion last week. Richard couldn’t spot the

error in this code of his. // To find the maximum among 3 integer// values in variables num1, num2, num3.int max = 0;if (num1 > num2 && num1 > num3) max = num1;if (num2 > num1 && num2 > num3) max = num2;if (num3 > num1 && num3 > num2) max = num3;

He tested it on many sets of data: <3,5,9>, <12,1,6>, <2,7,4>, etc. and the program works for all these data.

But he didn’t test it with duplicate values! Eg: <3,3,3>, <7,2,7>, etc.

Testing Thoroughly (2/2) Richard wrote another program.

// To find the maximum among 3 integer// values in variables num1, num2, num3.int max = 0;if (num1 > max) max = num1;if (num2 > max) max = num2;if (num3 > max) max = num3;

He was told that the program doesn’t work but again he couldn’t figure out why. He has tested it on many data sets, including duplicate values!

Can you tell him what he missed out in his testing? Don’t forget the special cases!

Testing Boundaries It is important to test the boundary conditions.

Input Year Input Month

1582 2

1583 0

1583 13

1583 1

1583 12

final int CALENDAR_START = 1583;// validate input if ((year < CALENDAR_START) || (month < 1) || (month > 12)) {

System.output.println("Bad request: " + year + " " + month);}

Path Testing Paths: different routes that your program can take

Design test data to check all paths Example

if (x != 3) { y = 5;}else { z = z - x;}

if (z > 1) { z = z / x;}else { z = 0;}

if (x != 3)

y = 5 z = z - x

if (z > 1)

z = z / x z = 0

A

B

C

D

E

F

G

H

<x=0, z=1> Paths A, B, G, H.

<x=3, z=3> Paths E, F, C, D.

Integration and System Testing Integration testing is done as modules or components are

assembled. Attempts to ensure that pieces work together correctly Test interfaces between modules

System testing occurs when the whole system is put together

This comes in when you start to write bigger programs.

Debugger Using the debugger

Stepping Breakpoint Inspecting variables

I will illustrate in Lab