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Chapter 7

Additional Control Structures

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Knowledge Goals

• Understand the role of the switch statement

• Understand the purpose of the break statement

• Understand the distinctions among the alternative looping statements

• Understand what is and what is not an exception

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Knowledge Goals

• Know when throwing an exception is appropriate

• Know how an exception should be handled

• Be aware of Java's additional operators and their place in the precedence hierarchy with respect to one another

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Skill Goals

• Write a switch statement for a multiway branching problem

• Write a do statement and contrast it with a while statement

• Write a for statement as an alternative to a while statement

• Choose the most appropriate looping statement for a given problem

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Skill Goals

• Use the Java exception-handling facilities try, catch, and throw

• Use a throw statement to throw a predefined exception

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Switch StatementSwitch statement

A selection control structure for multiway branching

Switch label

"case" + constant which labels a code segment

Switch expression

The integral expression whose value determines which switch label is selected

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Switch Statement

Switch expression

Statement, besideSwitch label

that matches Switchexpression, is

executed

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Switch Statement

switch (digit)

{

case 1 : Statement1;

break;

case 2 :

case 3 : Statement2;

break;

case 4 : Statement3;

break;

default: Statement4;

}

Statement5;

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Switch Statement

The value of IntegralExpression (Switch expression) that determines which branch is executed must be of types byte, char, short, or int

Case labels are constant (possibly named) integral expressions; several case labels can be associated with a statement

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Switch Statement

Summary of flow of control Control branches to the statement associated with the case

label that matches the value of the integral switch expression

Control proceeds through all remaining statements, including the default, unless redirected with break

If no case label matches the value of the switch expression, control branches to the default label, if present; otherwise control passes to the statement following the entire switch structure

Forgetting to use break can cause logical errors because after a branch is taken, control proceeds sequentially until either break or the end of the switch structure occurs

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double weightInPounds = 165.8;char weightUnit;System.out.print(“Weight in “);switch (weightUnit) { case ‘P’ : case ‘p’ : System.out.println(“pounds is “ + weightInPounds); break; case ‘O’ : case ‘o’ : System.out.println(“ounces is “ + 16.0*weightInPounds); break; case ‘K’ : case ‘k’ : System.out.println(“kilos is “ + weightInPounds/2.2); break; case ‘G’ : case ‘g’ : System.out.println(“grams is “ + 454.0*weightInPounds); break; default : System.out.println(“That unit is not handled!”); break;}

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Do Statement

do

{

Statement1;

Statement2;

…

StatementN;

} while (Expression);

Called post-test loop because expression is tested at end of loop

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// Count-controlled repetition sum = 0; // initialize counter = 1;

do {

sum = sum + counter;

counter++; // increment

} while(counter <= 10); // condition

Do Statement

Write an event-controlled loop that reads and sums until a 0 is found

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Do Statement

Understand the difference?

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For Statement

for (count = 1; count <= limit; count++)

outFile.println("" + count);

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For Statement

init can be nothing local variable declaration an expression a series of local variable

declarations and expressions separated by commas

Expression must be Boolean

Update can be omitted an expression a series of

expressions separate by commas

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For Statement

int num;

for (num = 1; num <= 3; num++)

{

System.out.println(“Potato “ + num);

}

Trace:

num num<=3 printed:

1 1<=3 Potato 1

2 2<=3 Potato 2

3 3<=3 Potato 3

4 4<=3 nothing

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For Statement

What is output from this loop? int count;

for (count = 1; count <= 10; count++);

System.out.println(“*”);

Be careful!

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Did you say no output from the for loop?The semicolon right after the parentheses means that the body statement is a null statement

The body of the for loop is whatever statement immediately follows the ()

That statement can be a single statement, a compound statement, or a null statement

For Statement

Actually, the code outputs one *.

Can you see why?

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Guidelines for Looping Statements

1. For a simple count-controlled loop, use the for statement

2. For an event-controlled loop that must execute at least once, use the do statement

3. For an event-controlled loop about which nothing is known, use the while statement

4. When in doubt, use the while statement

The while is the work horse of looping statements

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Break Statement

Break statement

A control statement that causes an immediate exit from the statement in which it appears

If inside a nested structures, control exits only the innermost structure containing it

Can be used in a switch statement and any looping statements

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Continue Statement

Continue statement

A control statement that causes a loop to jump to the start of the next iteration

Can only be used in a looping statement

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Java Operators

We have examined arithmetic operators relational operators logical operators (both full and short-circuit

evaluation increment and decrement operators assignment operator see the back of the book for additional operators

that make some operations easier

Can you give the symbols for each of these?

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Java Operators

A further word about the assignment operator: When combined with its two operands, it forms an assignment expressionAssignment expressionA Java expression with (1) a value and (2) the side effect of storing the expression value into a memory locationExpression statement (assignment statement)A statement formed by appending a semicolon to an assignment expression, an increment expression, or a decrement expression

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Exceptions

• Recall that an exception is an unusual

situation that occurs when a program is

running

• So far we have "passed the buck" by

throwing the exceptions to the next level

• Now we show how to manage our own

exceptions

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Exceptions

Exception Management

The process of defining and taking care of exceptions, which includes

defining the exception (determining what is and is not an exception)

raising (throwing) the exception (recognizing when it occurs)

handling the exception (code that is executed when an exception is caught)

try-catch-finally statement allows all this

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Exceptions

Code with exceptions

Code to handle an exception

Code to handle another exception

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Execution of try-catch

No statements throw

an exception

Statement following entire try-catch

statement

A statement throws

an exception

Exception Handler

Statements to deal with exception are executed

Control moves directly to exception handler

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try-catch with Built-In Exception

Scanner in = new Scanner(System.in); System.out.println("Enter file name"); String filename = in.nextLine();

try

{

Scanner inFile = new Scanner(new FileReader(filename));

}

catch(IOException except)

{

System.out.println(“Unable to open file ” + filename);

}

Exceptions

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Exceptions

Same

Always executes

Yes, but how do you generate an exception?

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Exceptions

You generate an excepton by throwing it!

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Exceptions

try

{

if (someproblem)

throw new Exception("You have problems");

}

catch(Exception except)

{

System.out.println("Exception throw: " +

except.getMessage());

}

getMessage is method in class Exception ;it returns the constructor's argument

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Exceptions

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Exceptions

try

{

// block

}

An exception can bethrown in the block

or any method called from within the block or

any method calledfrom within any

method called ….

The system looks for a catch in the calling block. If notfound, the method is exited and the next higher level

block is searched. The process continues until acatch is found or the next level is the JVM.

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TestingTesting axioms:

An application or class is never finished until it has been thoroughly tested!

Testing activities begin at the design stage and run in parallel with design and implementation

Desk checkingTracing the execution of a design on paperWalkthroughA verification method in which a team performs a manual simulation of the code or design

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Testing

Inspection

A verification method in which one member of a team reads the code or design line by line and the other team members point out errors

Execution trace

Going through the code with actual values, recording the state of the variables

Hint: Use data from your test plan in your execution trace

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Testing

How manycases

must yourtest planinclude

?

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Extras

I became aprofessor ofmathematicsat age 22 and

have a lawnamed

after me

Who am I?

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Extras - GUI Track

Output from JOptionPane.showInputDialog

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Extras - GUI Track

Input read as a string; use Scanner to parse