Chapter 3:Decision Structures

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Chapter 3Decision Structures
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Chapter 3Decision Structures

• 3.1 The if Statement
• 3.2 The if-else Statement
• 3.3 The if-else-if Statement 
• 3.4 Nested if Statements 
• 3.5 Logical Operators
• 3.6 Comparing String Objects
• 3.7 More About Variable Declaration and Scope
• 3.9 The switch Statement
• 3.10 Creating Objects with the DecimalFormat
  Class
• 3.12 Common Errors to Avoid

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Nested if Statements

• If an if statement appears inside of another if
  statement (single or block) it is called a nested
  if statement.
• The nested if is only executed if the if
  statement it is in results in a true condition.
• Nested if statements can get very complex, very
  quickly.

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LoanQualifier.java

• double salary, yearsOnJob
• String input
• input JOptionPane.showInputDialog("Enter your "
  "annual salary.")
• salary Double.parseDouble(input)
• input JOptionPane.showInputDialog("Enter the
  number of " "years at your current job.")
• yearsOnJob Double.parseDouble(input)

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LoanQualifier.java

• if (salary gt 30000)
• if (yearsOnJob gt 2)
• JOptionPane.showMessageDialog(null, "You
  qualify " "for the loan.")
• else
• JOptionPane.showMessageDialog(null, "You
  must have " "been on your current job for
  at least "
• "two years to qualify.")
• else
• JOptionPane.showMessageDialog(null, "You must
  earn at least 30,000 per year to qualify.")
  

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Nested if Statement Flowcharts
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if-else Matching

• Curly brace use is not required if there is only
  one statement to be conditionally executed.
• However, sometimes curly braces can help make the
  program more readable.
• Additionally, proper indentation makes it much
  easier to match up else statements with their
  corresponding if statement.

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if-else Matching

• if (employed 'y')
• if (recentGrad 'y')
• System.out.println("You qualify for the special
  interest rate.")
• else
• System.out.println("You must be a recent
  college graduate to qualify.")
• else
• System.out.println("You must be employed to
  qualify.")

This else matches with this if.
This else matches with this if.
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Nested if Statements

• If an if statement appears inside of another if
  statement (single or block) it is called a nested
  if statement.
• The nested if is only executed if the if
  statement it is in results in a true condition.
• Nested if statements can get very complex, very
  quickly.

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LoanQualifier.java

• double salary, yearsOnJob
• String input
• input JOptionPane.showInputDialog("Enter your "
  "annual salary.")
• salary Double.parseDouble(input)
• input JOptionPane.showInputDialog("Enter the
  number of " "years at your current job.")
• yearsOnJob Double.parseDouble(input)

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LoanQualifier.java

• if (salary gt 30000)
• if (yearsOnJob gt 2)
• JOptionPane.showMessageDialog(null, "You
  qualify " "for the loan.")
• else
• JOptionPane.showMessageDialog(null, "You
  must have " "been on your current job for
  at least "
• "two years to qualify.")
• else
• JOptionPane.showMessageDialog(null, "You must
  earn at least 30,000 per year to qualify.")
  

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Nested if Statement Flowcharts
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if-else Matching

• Curly brace use is not required if there is only
  one statement to be conditionally executed.
• However, sometimes curly braces can help make the
  program more readable.
• Additionally, proper indentation makes it much
  easier to match up else statements with their
  corresponding if statement.

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if-else Matching

• if (employed 'y')
• if (recentGrad 'y')
• System.out.println("You qualify for the special
  interest rate.")
• else
• System.out.println("You must be a recent
  college graduate to qualify.")
• else
• System.out.println("You must be employed to
  qualify.")

This else matches with this if.
This else matches with this if.
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Logical Operators

• Java provides two binary logical operators (
  and ) that are used to combine boolean
  expressions.
• Java also provides one unary (!) logical operator
  to reverse the truth of a boolean expression.

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Logical Operators
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The Operator

• The logical AND operator () takes two operands
  that must both be boolean expressions.
• The resulting combined expression is true iff (if
  and only if) both operands are true.

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LogicalAND

• double salary // Annual salary
• double yearsOnJob // Years at current
  job
• String input // To hold string
  input
• input JOptionPane.showInputDialog("Enter
  your " "annual salary.")
• salary Double.parseDouble(input)
• input JOptionPane.showInputDialog(
  "Enter the number of " "years at your
  current job.")
• yearsOnJob Double.parseDouble(input)

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LogicalAND

• // Determine whether the user qualifies for the
  loan.
• if (salary gt 30000 yearsOnJob gt 2)
• JOptionPane.showMessageDialog(null,
  "You qualify " "for the loan.")
• else
• JOptionPane.showMessageDialog(null,
• "You do not " "qualify for the loan.")

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The Operator

• The logical OR operator () takes two operands
  that must both be boolean expressions.
• The resulting combined expression is false iff
  (if and only if) both operands are false.

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LogicalOR

• double salary // Annual salary
• double yearsOnJob // Years at current
  job
• String input // To hold string
  input
• input JOptionPane.showInputDialog("Enter
  your " "annual salary.")
• salary Double.parseDouble(input)
• input JOptionPane.showInputDialog(
  "Enter the number of " "years at your
  current job.")
• yearsOnJob Double.parseDouble(input)

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LogicalOR

• // Determine whether the user qualifies for the
  loan.
• if (salary gt 30000 yearsOnJob gt 2)
• JOptionPane.showMessageDialog(null,
  
• "You qualify " "for the
  loan.")
• else
• JOptionPane.showMessageDialog(null,
  "You do not " "qualify for the
  loan.")

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The ! Operator

• The ! operator performs a logical NOT operation.
• If an expression is true, !expression will be
  false.
• if (!(temperature gt 100))
• System.out.println(Below the maximum
  temperature.")
• If temperature gt 100 evaluates to false, then the
  output statement will be run.

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Short Circuiting

• Logical AND and logical OR operations perform
  short-circuit evaluation of expressions.
• Logical AND will evaluate to false as soon as it
  sees that one of its operands is a false
  expression.
• Logical OR will evaluate to true as soon as it
  sees that one of its operands is a true
  expression.

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Order of Precedence

• The ! operator has a higher order of precedence
  than the and operators.
• The and operators have a lower precedence
  than relational operators like lt and gt.
• Parenthesis can be used to force the precedence
  to be changed.

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Order of Precedence
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Comparing String Objects

• In most cases, you cannot use the relational
  operators to compare two String objects.
• Reference variables contain the address of the
  object they represent.
• Unless the references point to the same object,
  the relational operators will not return true.
• StringCompareTo.java

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StringCompare.java

• String name1 "Mark",
• name2 "Mark",
• name3 "Mary"
• // Compare "Mark" and "Mark"
• if (name1.equals(name2))
• System.out.println(name1 " and "
  name2
• " are the same.")
• else
• System.out.println(name1 " and "
  name2
• " are the NOT the
  same.")

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StringCompare.java

• // Compare "Mark" and "Mary"
• if (name1.equals(name3))
• System.out.println(name1 " and "
  name3
• " are the same.")
• else
• System.out.println(name1 " and "
  name3
• " are the NOT the
  same.")

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StringCompareTo.java

• String name1 "Mary",
• name2 "Mark"
• // Compare "Mary" and "Mark"
• if (name1.compareTo(name2) lt 0)
• System.out.println(name1 " is less
  than " name2)
• else if (name1.compareTo(name2) 0)
• System.out.println(name1 " is equal to
  " name2)
• else if (name1.compareTo(name2) gt 0)
• System.out.println(name1 " is greater
  than " name2)

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Ignoring Case in String Comparisons

• In the String class the equals and compareTo
  methods are case sensitive.
• In order to compare two String objects that might
  have different case, use
• equalsIgnoreCase, or
• compareToIgnoreCase

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SecretWord.java

• if (input.equalsIgnoreCase("PROSPERO"))
• System.out.println("Congratulations!
  You know the secret word!")
• else
• System.out.println("Sorry, that is NOT
  the " "secret word!")