Chapter Goals

• To be able to implement decisions using if statements
• To effectively group statements into blocks
• To learn how to compare integers, floating-point numbers, strings, and objects
• To correctly order decisions in multiple branches and nested branches
• To program conditions using Boolean operators and variables
• To be able to design tests that cover all parts of a program

The if Statement

• The if statement lets a program carry out different actions depending on a condition

  if (amount <= balance)
     balance = balance - amount;

  

The if/else Statement

if (amount <= balance)
   balance = balance - amount;
else
   balance = balance - OVERDRAFT_PENALTY;

Statement Types

• Simple statement:

  balance = balance - amount;

• Compound statement:

  if (balance >= amount) balance = balance - amount;

  Also loop statements – Chapter 6
• Block statement:

  {
     double newBalance = balance - amount;
     balance = newBalance;
  }

Syntax 5.1 The if Statement

Self Check 5.1

Why did we use the condition amount <= balance and not amount < balance in the example for the if/else statement?

• Answer: If the withdrawal amount equals the balance, the result should be a zero balance and no penalty.

Self Check 5.2

What is logically wrong with the statement

if (amount <= balance)
   newBalance = balance - amount; balance = newBalance;

and how do you fix it?

• Answer: Only the first assignment statement is part of the if statement. Use braces to group both assignment statements into a block statement.

Comparing Values: Relational Operators

• Relational operators compare values
  

  Java	Math Notation	Description
  >	>	Greater than
  >=	≥	Greater than or equal
  <	<	Less than
  <=	≤	Less than or equal
  ==	=	Equal
  !=	≠	Not equal

• The == denotes equality testing:

  a = 5; // Assign 5 to a
  if (a == 5) . . . // Test whether a equals 5

• Relational operators have lower precedence than arithmetic operators:

  amount + fee <= balance

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 prints:
   sqrt(2)squared minus 2 is not 0 but 4.440892098500626E-16

Comparing Floating-Point Numbers

• To avoid roundoff errors, don't use == to compare floating-point numbers
• To compare floating-point numbers test whether they are close enough: |x - y| ≤ ε

  final double EPSILON = 1E-14;
  if (Math.abs(x - y) <= EPSILON)
     // x is approximately equal to y

• ε is a small number such as 10^-14

Comparing Strings

• To test whether two strings are equal to each other, use equals method:

  if (string1.equals(string2)) . . .

• Don't use == for strings!

  if (string1 == string2) // Not useful

• == tests identity, equals tests equal contents
• Case insensitive test:

  if (string1.equalsIgnoreCase(string2))

Comparing Strings

• string1.compareTo(string2) < 0 means:
  string1 comes before string2 in the dictionary
• string1.compareTo(string2) > 0 means:
  string1 comes after string2 in the dictionary
• string1.compareTo(string2) == 0 means:
  string1 and string2 are equal
• "car" comes before "cargo"
• All uppercase letters come before lowercase:
  "Hello" comes before "car"

Lexicographic Comparison

Syntax 5.2 Comparisons

Comparing Objects

• == tests for identity, equals for identical content

• Rectangle box1 = new Rectangle(5, 10, 20, 30);
  Rectangle box2 = box1;
  Rectangle box3 = new Rectangle(5, 10, 20, 30);

• box1 != box3,
  but box1.equals(box3)
• box1 == box2
• Caveat: equals must be defined for the class

Object Comparison

Testing for null

• null reference refers to no object:

  String middleInitial = null; // Not set
  if ( . . . )
     middleInitial = middleName.substring(0, 1);

• Can be used in tests:
  

  if (middleInitial == null)
     System.out.println(firstName + " " + lastName);
  else
     System.out.println(firstName + " " + middleInitial + ". " + lastName);

• Use ==, not equals, to test for null
• null is not the same as the empty string ""

Relational Operator Examples

Self Check 5.3

What is the value of s.length() if s is

1. the empty string ""?
2. the string " " containing a space?
3. null?

• Answer: (a) 0; (b) 1; (c) an exception occurs.

Self Check 5.4

Which of the following comparisons are syntactically incorrect? Which of them are syntactically correct, but logically questionable?

String a = "1";
String b = "one";
double x = 1;
double y = 3 * (1.0 / 3);

1. a == "1"
2. a == null
3. a.equals("")
4. a == b
5. a == x
6. x == y
7. x - y == null
8. x.equals(y)

• Answer: Syntactically incorrect: e, g, h. Logically questionable: a, d, f.

Multiple Alternatives: Sequences of Comparisons

• if (condition1)
     statement1;
  else if (condition2)
     statement2;
     . . .
  else
     statement4;

• The first matching condition is executed
• Order matters:

  if (richter >= 0) // always passes
     r = "Generally not felt by people";
  else if (richter >= 3.5) // not tested
     r = "Felt by many people, no destruction";
  . . .

• Don't omit else:

  if (richter >= 8.0)
     r = "Most structures fall";
  if (richter >= 7.0) // omitted else--ERROR
     r = "Many buildings destroyed"

ch05/quake/Earthquake.java

ch05/quake/EarthquakeRunner.java

Program Run:

• Enter a magnitude on the Richter scale: 7.1
  Many buildings destroyed

Multiple Alternatives: Nested Branches

• Branch inside another branch:

  if (condition1)
  {
     if (condition1a)
        statement1a;
     else
        statement1b;
  }
  else
     statement2;

Tax Schedule

If your filing status is Single		If your filing status is Married
Tax Bracket	Percentage	Tax Bracket	Percentage
$0 . . . $32,000	10%	$0 . . . $64,000	10%
Amount over $32,000	25%	Amount over $64,000	25%

Nested Branches

• Compute taxes due, given filing status and income figure:
  1. Branch on the filing status
  2. For each filing status, branch on income level
• The two-level decision process is reflected in two levels of if statements
• We say that the income test is nested inside the test for filing status
  

ch05/tax/TaxReturn.java

ch05/tax/TaxCalculator.java

Program Run:

• Please enter your income: 50000
  Are you married? (Y/N) N
  Tax: 11211.5

Self Check 5.5

The if/else/else statement for the earthquake strength first tested for higher values, then descended to lower values. Can you reverse that order?

• Answer: Yes, if you also reverse the comparisons:

  if (richter < 3.5)
     r = "Generally not felt by people";
  else if (richter < 4.5)
     r = "Felt by many people, no destruction";
  else if (richter < 6.0)
     r = "Damage to poorly constructed buildings";
  .. .

Self Check 5.6

Some people object to higher tax rates for higher incomes, claiming that you might end up with less money after taxes when you get a raise for working hard. What is the flaw in this argument?

• Answer: The higher tax rate is only applied on the income in the higher bracket. Suppose you are single and make $31,900. Should you try to get a $200 raise? Absolutely: you get to keep 90 percent of the first $100 and 75 percent of the next $100.

Using Boolean Expressions: The boolean Type

• George Boole (1815-1864): pioneer in the study of logic
• value of expression amount < 1000 is true or false
  
• boolean type: one of true or false

Using Boolean Expressions: Predicate Method

• A predicate method returns a boolean value:

  public boolean isOverdrawn()
  {
     return balance < 0; // Returns true or false
  }

• Use in conditions:

  if (harrysChecking.isOverdrawn())

• Useful predicate methods in Character class:

  isDigit
  isLetter
  isUpperCase
  isLowerCase

• if (Character.isUpperCase(ch)) . . .

• Useful predicate methods in Scanner class: hasNextInt() and hasNextDouble():

  if (in.hasNextInt()) n = in.nextInt();

Using Boolean Expressions: The Boolean Operators

• &&  and
• ||  or
• !   not

• if (0 < amount && amount < 1000) . . . 

• if (input.equals("S") || input.equals("M")) . . .

• if (!input.equals("S")) . . .

&& and || Operators

Boolean Operators

Truth Tables

A	B	A && B
true	true	true
true	false	false
false	Any	false

A	B	A || B
true	Any	true
false	true	true
false	false	false

A	! A
true	false
false	true

Using Boolean Variables

• private boolean married;
• Set to truth value:

  married = input.equals("M");

• Use in conditions:

  if (married) . . . else . . .
  if (!married) . . .

• Also called flag
• It is considered gauche to write a test such as

  if (married == true) . . . // Don't

  Just use the simpler test

  if (married) . . .

Self Check 5.7

When does the statement

system.out.println (x > 0 || x < 0);

print false?

• Answer: When x is zero.

Self Check 5.8

Rewrite the following expression, avoiding the comparison with false:

if (Character.isDigit(ch) == false) . . .

• Answer: if (!Character.isDigit(ch)) . . .

Code Coverage

• Black-box testing: test functionality without consideration of internal structure of implementation
• White-box testing: take internal structure into account when designing tests
• Code coverage: measure of how many parts of a program have been tested
• Make sure that each part of your program is exercised at least once by one test case
  E.g., make sure to execute each branch in at least one test case
• Include boundary test cases: legal values that lie at the boundary of the set of acceptable inputs
• Tip: write first test cases before program is written completely → gives insight into what program should do

Self Check 5.9

How many test cases do you need to cover all branches of the getDescription method of the Earthquake class?

• Answer: 7.

Self Check 5.10

Give a boundary test case for the EarthquakeRunner program. What output do you expect?

• Answer: An input of 0 should yield an output of "Generally not felt by people". (If the output is "Negative numbers are not allowed", there is an error in the program.)