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for Loop
Writing to a file
String conversions
Random class
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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
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The for Loop Flowchart
initialization
boolean
expression?
true
statement(s)
update
false
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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
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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.
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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
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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.
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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
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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).
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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.
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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);
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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.
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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.
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Specifying a File Location
Windows’ 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);
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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);
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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.
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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)
We say end-of-file occurs when we
{
read a line, but there are no more
System.out.println(str);
lines left to read. End-of-file is
str = inputFile.readLine();
detected by checking for a null line.
}
inputFile.close();// close the file when done.
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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.
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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
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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();
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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
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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.
2
DiceGame
die1
numberSides
die2
generator
main()
toss()
A dicegame is
played with
two dice
Java code: DiceGame.java
1
Die
Random
nextInt()
A die uses a
random number
generator
Die.java
Using BlueJ
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ASIDE: a UML sequence diagram showing how the objects interact
Die is a class from which we instantiate two objects.
We toss the pair of dice 1,000,000 times.
System.out
DiceGame
main()
printLn()
new()
new()
die1:Die
new()
: Random
die2:Die
new()
Loop [1,000,000 times]
toss()
:Random
nextInt()
toss()
nextInt()
printLn()
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