1Fall 2007 ACS-1903

for Loop

Writing to a file

String conversions

Random class

2Fall 2007 ACS-1903

for Loop

• a specialized form of the while loop … a pre-test loop.

• The for loop allows the programmer to initialize a control variable, test a condition, and modify the control variable all in one line of code.

• The for loop takes the form:for(initialization; test; update){

loop statements;}

• Example: Squares.java

3Fall 2007 ACS-1903

The for Loop Flowchart

statement(s)trueboolean

expression?

false

update

initialization

4Fall 2007 ACS-1903

for Loop

• The initialization section … initialize its control variable.

• The test section of the for statement is similar to the test in a while loop.

• The update section of the for loop is the last thing to execute in the loop.

• Example: UserSquares.java

5Fall 2007 ACS-1903

The for Loop Initialization

• Optional - however, usually provided.

• Typically, for loops initialize a counting variable

• Can initialize multiple variables.

• Variables declared in this section have scope only for the for loop.

6Fall 2007 ACS-1903

The Update Expression

• usually used to increment or decrement the counting variable(s)

• last section to execute in the loop.

• may update multiple variables.

• Each variable updated is executed as if it were on a line by itself.

7Fall 2007 ACS-1903

Modifying The Control Variable

• bad programming style to update the control variable of a for loop within the body of the loop - leads to hard to maintain code and difficult debugging.

• update section should be used to update the control variable.

8Fall 2007 ACS-1903

Multiple Initializations and Updates

• The for loop may initialize and update multiple variables.for(int i = 5, int j = 0; i < 10 || j < 20; i++, j+=2){

loop statements;}

• only the semicolons are mandatory.for(;;){

loop statements;}//infinite loop.

• If left out, the test section defaults to true.

9Fall 2007 ACS-1903

Nested Loops

• Like if statements, loops can be nested.

• If a loop is nested, the inner loop will execute all of its iterations for each time the outer loop executes once.for(int i = 0; i < 10; i++)

for(int j = 0; j < 10; j++)

loop statements;

• The loop statements in this example execute 100 times.

• Example: Clock.java

10Fall 2007 ACS-1903

The break And continue Statements

• a break statement can be used to abnormally terminate a loop.

• use of the break statement in loops bypasses the normal mechanisms and makes the code hard to read and maintain.

• considered bad form to use the break statement in this manner.

11Fall 2007 ACS-1903

The continue Statement

• a continue statement will cause the currently executing iteration of a loop to terminate and the next iteration will begin.

• a continue statement will cause the evaluation of the condition in while and for loops.

• as with the break statement, the continue statement should be avoided because it makes the code hard to read and debug.

12Fall 2007 ACS-1903

Deciding Which Loops to Use

• The while loop:

• Pretest loop• Use it where you do not want the statements to execute if the

condition is false in the beginning.

• The do-while loop:

• Post-test loop• Use it where you want the statements to execute at least one time.

• The for loop:

• Pretest loop• Use it where there is some type of counting variable that can be

evaluated.

13Fall 2007 ACS-1903

Files

• Input … reading

• Output … writing

• Errors can occur

• Attempting to read a file that does not exist

• Writing to a file that is protected

• These types of errors need to be provided for

14Fall 2007 ACS-1903

Exceptions

• When something unexpected happens in a Java program, an exception is thrown.

• The method currently executing when the exception is thrown must either handle the exception or pass it up the line.

• Handling the exception is discussed later.

• To pass it up the line, the method needs a throws clause in the method header.

15Fall 2007 ACS-1903

Exceptions

• To insert a throws clause in a method header, simply add the word throws and the name of the expected exception.

• The class Exception can be used to catch all exceptions.public static void main(String[] args) throws IOException{…}

• File I/O is a checked exception (meaning the exception must be handled or passed up).

• A program with file I/O will generate a compile-time error if the exception is not handled or passed up.

• Example: FileWriteDemo.java

16Fall 2007 ACS-1903

Writing Data To a File

• When writing to a file we need objects from the following classes are used to write data to files:

• FileWriter – This class allows basic file writing functionality.

• PrintWriter – This class allows the programmer to write files using the same style that is used to write to the screen (i.e. print and println).

17Fall 2007 ACS-1903

The FileWriter Class

• The FileWriter clas provides other classes with the basic file writing functionality.

System.out.println(“Enter the filename.”);

filename = Keyboard.readString();

FileWriter fwriter = new FileWriter(filename);

• This will create an object that can access the file filename.

• Warning: if the file above already exists, it will be erased and replaced with the new file.

18Fall 2007 ACS-1903

The PrintWriter Class

• The PrintWriter class adds to the functionality of the FileWriter class.

• The PrintWriter cannot directly access the file but must work through the FileWriter class.

• The PrintWriter needs a FileWriter object in order to work:FileWriter fwriter = new FileWriter("StudentData.txt");

PrintWriter outputFile = new PrintWriter(fwriter);

19Fall 2007 ACS-1903

The PrintWriter Class

• Once linked to the fwriter object, the outputFile object can talk to the file. outputFile.println(“Jim”);outputFile.close();

• Just as with the System.out object, the println method of the PrintWriter class will place a newline character after the written data.

• The print method can be used to avoid writing the newline character.

20Fall 2007 ACS-1903

Appending Text to a File

• To avoid erasing a file that already exists:

• Create a FileWriter object using an optional boolean argument that tells the object to append data to the file.

FileWriter fwriter = new FileWriter(“filename”, true);

• Data written to a file created in such a manner will be appended to the end of the current file.

21Fall 2007 ACS-1903

Specifying a File LocationWindows’ Crazy Backslash

• Windows evolved from DOS.

• Since DOS was simply a hacked version of CP/M, it maintained the backslash (\) as a directory separator.

• Remember, if the backslash is used in a String literal, it is the escape character so there must be two of them.FileWriter fwriter = new FileWriter("A:\\PriceList.txt");

PrintWriter outputFile = new PrintWriter(fwriter);

22Fall 2007 ACS-1903

Specifying a File Location

• This is only necessary if the backslash is in a String literal.

• If the backslash is in a String object then it will be handled properly.

• Fortunately, Java allows Unix style filenames using the forward slash (/) to separate directories.FileWriter fwriter = new FileWriter("/home/rharrison/names.txt");PrintWriter outputFile = new PrintWriter(fwriter);

23Fall 2007 ACS-1903

Reading Data From a File

• Java provides several classes to read data from a file.

• FileReader

• Open an existing file for reading and establish a connection with it.

• BufferedReader

• Uses a buffer to allow the reading of full lines of text at a time rather than one byte at a time.

24Fall 2007 ACS-1903

Detecting The End of a File

• The readLine() method of the BufferedReader class will return null if the end of the file has been reached.FileReader freader = new FileReader(filename);BufferedReader inputFile = new BufferedReader(freader);// Read the first item.String str = inputFile.readLine();// If an item was read, display it // and read the remaining items.while (str != null){

System.out.println(str);str = inputFile.readLine();

}inputFile.close();// close the file when done.

We say end-of-file occurs when we read a line, but there are no more lines left to read. End-of-file is detected by checking for a null line.

25Fall 2007 ACS-1903

String Value Conversion

• The readLine() method of the BufferedReader class only reads in text as a String object.

• Strings that represent numbers can be converted and stored into primitive variables.

• Java provides wrapper classes that make conversion easy.

26Fall 2007 ACS-1903

String Value Conversion

Method Description

Integer.parseInt(str)This method accepts a string that contains a number and returns the number as an int.

Short.parseShort(str)This method accepts a string that contains a number and returns the number as a short.

Byte.parseByte(str)This method accepts a string that contains a number and returns the number as a byte.

Long.parseLong(str)This method accepts a string that contains a number and returns the number as a long.

Float.parseFloat(str)This method accepts a string that contains a number and returns the number as a float.

Double.parseDouble(str)This method accepts a string that contains a number and returns the number as a double.

Example: FileSum.java

27Fall 2007 ACS-1903

The Random Class

• Some applications, such as games and simulations, require the use of randomly generated numbers.

• The Java API has a class, Random, for this purpose. To use the Random class, use the import statement and create an instance of the class.

import java.util.Random;Random randomNumbers = new Random();

28Fall 2007 ACS-1903

Some Methods of the Random Class

Method Description

nextDouble() Returns the next random number as a double. The number will be within the range of 0.0 and 1.0.

nextFloat() Returns the next random number as a float. The number will be within the range of 0.0 and 1.0.

nextInt() Returns the next random number as an int. The number will be within the range of an int, which is –2,147,483,648 to +2,147,483,648.

nextInt(int n) This method accepts an integer argument, n. It returns a random number as an int. The number will be within the range of 0 to n.

Example: MathTutor.java

29Fall 2007 ACS-1903

Example

Suppose we want to program a game that involves two dice. Die will be a class from which we can instantiate two objects (one for each die).

We can toss the pair of dice 1,000,000 times and see what the average throw is.

ASIDE: a UML class diagram showing the classes the program has, each of their attributes and methods, and the relationships between classes.

Random 2 1

DieDiceGame

numberSides

generator

toss()

die1

die2

main()

nextInt()

A dicegame is played with two dice

A die uses a random number generator

DiceGame.java Die.javaJava code:

Using BlueJ

30Fall 2007 ACS-1903

ASIDE: a UML sequence diagram showing how the objects interact

DiceGamemain()

die1:Dienew()

die2:Dienew()

: Randomnew()

toss()

Loop [1,000,000 times]

System.out

printLn()

toss()

Die is a class from which we instantiate two objects.

We toss the pair of dice 1,000,000 times.

new() :Random

printLn()

nextInt()

nextInt()