Control Flow Statements

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Control Flow Statements

• The first step
• The first step in learning to use a new
  programming language is usually to learn the
  foundation concepts such as variables, types,
  expressions, flow-of-control, etc.

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Control Flow Statements

• Java programs accomplish their tasks by
  manipulating program data using operators and
  making decisions by testing the state of program
  data. When a program makes a decision, it is
  determining, based on the state of the program
  data, whether certain lines of code should be
  executed. For example, a program may examine a
  variable called hasChanged to determine if it
  should execute a block of code that saves data to
  disk. If hasChanged is true, the data saving code
  is executed. If hasChanged is false, the data
  saving code is not executed.

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Control Flow Statements

• At a simplistic level, a program executes
  statements sequentially in the order in which
  they appear. It starts with the first line of the
  "main()" method and ends with the last line,
  always moving from top to bottom.
• Most useful programs are vastly more complex than
  a simple linear sequence. Real programs contain
  numerous code blocks that must be executed only
  if certain conditions are present. Other code
  blocks must be executed repeatedly until certain
  conditions are met.
• A hypothetical example A program loops through
  an array of employee models and increases the
  value stored in the salary variable by 10 for
  each employee that has a hire date of one year
  ago. (I said it was hypothetical)

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Control flow statements

• Control flow statements are the tools programmers
  use to make decisions about which statements to
  execute and to otherwise change the flow of
  execution in a program. The four categories of
  control flow statements available in Java are
  selection, loop, exception, and branch.

5
Control Flow Statements
6
The first step

• The first step in learning to use a new
  programming language is usually to learn the
  foundation concepts such as variables, types,
  expressions, flow-of-control, etc

7
Flow of Control

• What is flow of control?
• Java support several different kinds of
  statements designed to alter or control the
  logical flow of the program, although in some
  cases, the behavior of those statements differs
  between Java and C.
• The ability to alter the logical flow of the
  program is often referred to as Flow of Control.

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Statements that support flow of control

• Statement Type
• if-else selection
• switch-case selectionfor
  loop
• while loop
• do-while loop
• try-catch-finally exception handling
• throw exception handling
• break miscellaneous
• continue miscellaneous
• label miscellaneous
• return miscellaneous
• goto reserved by Java but not
  supported

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Selection and Loop

• Selection and loop statements require a
  conditional expression that evaluates to true or
  false.
• If the conditional expression is true, the
  accompanying block of code is executed.
• If the conditional expression is false, the
  accompanying block of code is bypassed.

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Selection statements

• Selection statements will execute the code block
  only once. Loop statements repeatedly execute the
  code block, testing the conditional statement
  each time.
• Once the conditional statement evaluates to
  false, the loop exits and execution of the
  program continues to the next statement following
  the loop.

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Exception

• Exception statements are used to gracefully
  handle unusual events or errors that arise while
  a program is running.
• For instance, if a program attempts to open a
  file but the file doesn't exist, this will cause
  an I/O error. Exception handling in Java is a two
  step process.

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Exception

• First, an exceptional condition is detected
  during program execution by the JVM or
  application code.
• At this point a new exception object is created
  that describes the situation and is passed up up
  the call stack using the throw statement.
• . Each method in the call stack now has the
  option to handle it or let it continue to bubble
  up the call stack.

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Branch

• Branch statements explicitly redirect the flow of
  program execution. The most infamous branch
  statement in the history of computing is goto.
• In Java, goto is an unimplemented keyword, so it
  cannot be used in Java programs.
• In other languages, goto is used to redirect
  program execution to another line of code in the
  program by specifying a line number or other type
  of label.

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Branch

• In Java, break, continue, and label fill the
  role of goto, but without many of goto's
  negatives. return is a branching statement that
  redirects execution out of the current method and
  back to the calling method.
• It also returns a value to the calling method.
  This value is either a primitive, object, or
  void. void is used to return execution without
  returning a value.
• Also, whatever the return value is, its type must
  match the return type listed in the method
  declaration.

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Statements' general form

• statement_keyword
• (conditional_expression)
• statements_to_execute

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Three parts to the general form

• Notice from previous slide
• The statement_keyword is the Java language
  keyword that defines what type of control is
  being exerted on the statements_to_execute code
  block.
• The conditional_expression defines the
  conditions that must be present for
  statements_to_execute to be executed.

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statements' general form

• Notice statements_to_execute is wrapped in curly
  braces
• If statements_to_execute consists of a single
  statement only, the curly braces are optional.
• Most Java programmers consider it to be good
  coding style to always include the curly braces.
• Leaving out curly braces can cause subtle bugs
  because most Java programmers expect the curly
  braces out of habit.

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Examples

• Here are some examples of control statements. We
  will cover all of these and more in the future.
• //Example 1
• If (conditional_expression)
• statements_to_execute

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Examples

• //Example 2
• If (conditional_expression)
• statements_to_execute
• else
• statements_to_execute

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Example Cont

• //Example 3
• while(conditional_expression)
• statements_to_execute
• //Example 4
• for(int i 0 conditional_expression i)
• statements_to_execute

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Operators Revisited

• As we saw in Java Control Flow Statements ,
  making decisions in Java is accomplished by
  evaluating the state of program data using
  conditional operators such as , lt, and gt.
• if statements use the boolean value returned from
  these conditional expressions to determine
  whether the accompanying block of code should be
  executed.
• If the conditional expression returns true, the
  code block is executed. Otherwise, the code block
  is skipped.

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The if statement

• if(conditional_expression)
• statements_to_execute
• This general form shows a simple if structure.

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If statement Cont

• If the conditional_expression is true then
  statements_to_execute will be executed.
• The statements_to_execute are wrapped in curly
  braces .
• If statements_to_execute consists of a single
  statement only, the curly braces are optional.
• Java programmers consider it to be good coding
  style to always include the curly braces.

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Lets look at some examples of if

• //Example 1
• if(a b)
• c
• In example 1, the if tests the equality of a and
  b. If a and b are equal, c is incremented by 1.
• //Example 2
• if((x lt y) (y lt z))
• System.out.println("Of course\n")
  System.out.println("x is less than or equal to
  z")

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More Examples

• Example 2 evaluates a compound conditional
  expression.
• First it tests whether x is less than or equal
  to y. If this is true, then it tests if y is less
  than or equal to z. If that is also true, then it
  prints the two lines of output.
• //Example 3
• if(true)
• System.out.println("This line always
  executes")
• In example 3, instead of passing the if statement
  a conditional expression that returns a boolean
  value, we pass it the literal boolean value
  "true". The code block in this example will
  always be executed because true always evaluates
  to true.

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Example 4

• //Example 4
• if(!(employee.isManager()))
• System.out.println("Employee is not a
  manager.")
• Example 4 introduces the not operator !. !
  toggles a boolean value. In this example, if
  employee.isManager() returns true, the ! operator
  will toggle it to false and the if statement will
  not execute the code block. If employee.isManager(
  ) returns false, the ! operator will toggle it to
  true and the code block will execute

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if-else

• What if you want to execute one block of code
  when an expression evaluates to true but execute
  a different block of code when the same
  expression evaluates to false?
• The if statement alone is not enough because it
  only controls a single block of code.
• There are two ways to accomplish this.
• The first way is to use two if statements with
  mutually exclusive conditional expressions.
• The second way is to use an if-else structure.
  else is always used in conjunction with, and
  following, an if statement. else is the Java
  keyword designed for those situations where you
  must run exactly one of two code blocks.

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Lets look at some examples

• //Example 1
• if(a b)
• c
• If(a ! b)
• c--

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Example Cont

• //Example 2
• if(a b)
• c
• else
• c--

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Explanations

• Example 1 uses two mutually exclusive if
  statements to simulate an if-else construct. In
  this example only one of the two if statements
  will be executed. This is less efficient that
  using an if-else to do the same thing.
• Example 2 is the if-else version of the previous
  example . You can see the if-else requires less
  code than two ifs and requires one less operation
  be performed. (The a ! b is never explicitly
  tested.)

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Lets look at some examples of if-else-if

• //Example 1
• if(color BLUE))
• System.out.println("The color is blue.")
• else if (color GREEN)
• System.out.println("The color is green.")

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if-else-if

• //Example 2
• if(employee.isManager())
• System.out.println("Is a Manager")
• else if
• (employee.isVicePresident())
• System.out.println("Is a Vice-President")
• else
• System.out.println("Is a Worker")

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Explanations

• Example 1 shows a simple if-else-if.
• In this case if the color equals BLUE or GREEN,
  the respective code block is executed.
• If color equals any other value none of the code
  blocks will be executed.
• Example 2 uses the if-else-if structure with an
  ending else.
• In this example two different conditional
  expressions are tested and if neither is true,
  then the else statement's code block is executed