More on if Statements

1
More on if Statements

• Comparison operator

2
Equality Testing vs. Assignment

• The operator tests for equalityif (x 0)
  . . // if x equals zero
• The operator assigns a value to a variablex
  0 // assign 0 to x
• Don't confuse them.

3
Comparing Floating-Point Numbers

• Consider this code
• double r Math.sqrt(2) double d (r r) -
  2
• if (d 0)
• System.out.println("sqrt(2)squared
  minus 2 is 0")
• else
• System.out.println( "sqrt(2)squared
  minus 2 is not 0 but " d)
• It printssqrt(2)squared minus 2 is not 0 but
  4.440892098500626E-16
• Don't use to compare floating-point numbers
• Use gt or lt instead

4
String Comparison

• Don't use for strings!if (input "Y") //
  WRONG!!!
• Use equals methodif (input.equals("Y"))
• tests identity, equals tests equal contents
• Case insensitive test ("Y" or "y")if
  (input.equalsIgnoreCase("Y"))

5

• public class StringExample
• // instance variables - replace the example
  below with your own
• private String name
• private String nickname
• /
• Constructor for objects of class
  StringExample
• /
• public StringExample()
• // initialize instance variables
• name "Robert"
• nickname name.substring(0,3)
• public void displayNames()
• if (nickname "Rob")

6
Object Comparison

• tests for identity, equals for identical
  content
• Rectangle cerealBox new Rectangle(5, 10, 20,
  30)Rectangle oatmealBox new Rectangle(5, 10,
  20, 30)Rectangle r cerealBox
• cerealBox ! oatmealBox, but cerealBox.equals(oat
  mealBox)
• cerealBox r
• Note equals must be defined for the class

7
Object Comparison
8
Multiple Alternatives

• if (condition1)
• statement1
• else if (condition2)
• statement2
• else if (condition3)
• statement3
• else
• statement4
• The first matching condition is executed.
• Order matters.

9

• public class Earthquake
• private double richter
• public Earthquake(double magnitude)
• richter magnitude
• public String getDescription()
• String description
• if (richter gt 8.0)
• description "Most structures fall"
• else if (richter gt 7.0)

10
Multiple Alternatives

• Order matters
• if (richter gt 0) // always passes
• description "Generally not felt by
  people"
• else if (richter gt 3.5) // not tested
• description "Felt by many people, no
  destruction"
• . . .
• Be careful about omitting the else
• if (richter gt 8.0)
• description "Most structures fall"
• if (richter gt 7.0) // omitted else--ERROR
• description "Many buildings destroyed

11
Multiple Alternatives

• The dangling else problem
• if (richter gt 0)
• if (richter lt 4)
• System.out.println(The earthquake is
  harmless)
• else
• System.out.println(Negative value not
  allowed)
• An else always belongs to the closest if.
• Use braces to avoid this problem.
• if (richter gt 0)
• if (richter lt 4)
• System.out.println(The earthquake is
  harmless)
• else