Chapter 5: Conditionals and Loops

(5.1) Boolean expressions

• Relational operators
• Logical operators

Relational operators

    a == b
    a != b
    a < b
    a <= b
    a > b
    a >= b

• Their precedence is lower than arithmetic operators.

Logical operators

    !p
    p && q
    p || q

• Precedence: not, and, or
• "Short circuit" operations

(5.2) The if statement

1. Simple if
2. if-else
3. Blocking
4. Nested ifs

Simple if

• Syntax:

  if (CONDITION) 
    STATEMENT
  

• Indenting STATEMENT one level deeper to show structure.

• Listing 5.1: Age

if-else statement

• Syntax:

  if (CONDITION)
    STATEMENT1
  else
    STATEMENT2
  

• Listing 5.2 Wages
  • Pay calculation with overtime considerations
  • Two numerical examples (under and over time)
• Listing 5.3–5.4 CoinFlip, Coin
  • What is (int) (Math.random() * 2)?

• Page 221: misleading indentation.

Blocking.

• A block is a sequence of statements enclosed in {} braces:

      {
        STATEMENT1; 
        STATEMENT2; 
        ... 
        STATEMENTN;
      }
  

• Replaces indenting in Python

• Block statements { ... } stand syntactically for a single statement.

Listing 5.5 Guessing

• What is generator.nextInt(MAX) + 1 ?
• Demonstrate effect of removing the blocking braces after 'else'

Blocking styles

    // 1.  Textbook style    // 2.  Alternate style

    if (condition)           if (condition) {
    {                          statement; ...
      statement; ...         }
    }                        else {
    else                       statement; ...
    {                        }
      statement; ... 
    }

Blocking styles

3. Avoid this:

   if (condition) {
     statement; ...
   } else { // don't do this!!!
      statement; ...
   }
   

   The else is hidden.

Nested if statements

How to write:

    // Textbook        // Simpler?

    if (c1)            if (c1)
      a1;                a1;
    else               else if (c2)
      if (c2)            a2;
        a2;            else
      else               a3;
        a3;

Listing 5.6 MinOfThree

• Which if does each else match?
• Using braces to make this clearer

• Study with inputs:

  2 5 7
  5 7 2 
  7 2 5
  

(5.3) Comparing data

• Floating point
• Characters
• Strings

Floating Point Comparisons

== requires exact equality, but floating point operations are inexact, so we usually want to find whether |a − b| < tolerance.

if (Math.abs(a - b) < tol)
   ...
else
   ...

Comparing characters

The Unicode character set determines the ordering of characters.

Comparing Strings (1)

• Normally don't use ==,!=, <, etc.

• Instead of ==, use the equals method:

  s1.equals(s2)  // value is boolean
  

• Instead of !=, use

  ! (s1.equals(s2))
  

• Instead of <, >, etc., use compareTo:

  s1.compareTo(s2) < 0 // true if s1 less than s2
  s1.compareTo(s2) == 0 // true if s1 equals s2
  s1.compareTo(s2) > 0 // true if s1 greater than s2
  

Comparing Strings (2)

The comparison is made on the basis of alphabetical order, or, to speak more precisely, lexicographic order, the order determined by the character codes.

• For Java, this is Unicode.
• Special cases:
  • "able" > "Baker" because capital letters have lower Unicode numbers.
  • "horse" < "horsefly" because "horse" is a prefix of "horsefly".

Part 2

Graphics applications

• 5.7 Determining event sources
• 5.8 Check boxes, radio buttons

(5.7) Determining event sources

• A single listener can listen for events from more than one object.
• May need to determine which object emitted the event.
• Use the ActionEvent method getSource, which returns a reference to the emitting object.

Example

LeftRight, LeftRightPanel (Listings 5.12–5.13)

• Two buttons with a single action listener.
• Note the subpanel used for grouping
  • But why is the arrangement of label and subpanel vertical?
  • Because of the size of the outer panel—as you can see by expanding it.

(5.8) Check and Radio Buttons

• Check boxes
• Radio buttons

Check boxes

• Square buttons that can be independently toggled (checked or unchecked)
  • More than one can be checked
• When toggled, they generate an ItemEvent
• An ItemListener can receive ItemEvents
• The listener typically doesn't ask the event source, but queries the state of each of the checkboxes

Example

StyleOptions, StyleOptionsPanel (Listings 5.14–5.15)

• Uses two checkboxes to select italic and/or bold fonts
• Shows the Font class, representing a character font
• If both are checked, the font is set to Font.ITALIC + Font.BOLD, representing the combination of italic bold

Radio buttons

• Round buttons, used in groups called ButtonGroups
• Can be toggled on/off, but are mutually exclusive
  • Only one selected within a group
• Generates an ActionEvent when toggled on, but not off
• Listener usually should determine the event source

Example

QuoteOptions, QuoteOptionsPanel (Listings 5.16–5.17)

• Uses three radio buttons to choose the type of quotation displayed
• Note the use of a ButtonGroup and adding each radio button to the group

Part 3: Iteration

• 5.4 while
• 5.5 Iterators
• 5.6 ArayLists

(5.4) The while statement

• Provides controlled repetition of statements (iteration).
• Iteration means either (1) the process of repeating a statement; or (2) a single repetition of a statement. E.g.:
  • "The iteration continues until all the input was read."
  • "The fifth iteration adds the fifth input to the sum."

Syntax and Semantics of while

Syntax:

while (expr)
  statement;

Semantics:

• As long as expr is true, keep executing statement
• (Usually a block statement)

Listing 5.7 Average

• Uses a while loop and a sentinel value to indicate end of input
• Uses priming read before loop
• Reads next value just at end of loop before next iteration.
• Uses an accumulator (running sum).

Listing 5.8 WinPercentage

• Uses while loop to validate input.

Infinite loops

• Occur if the expr never becomes false.
• Be sure the statement makes expr eventually become false.
• In Unix, you can use C-c (control-C) to interrupt.

Nested Loops

• Loops within loops.

• Example:

  int line = 1;
  while (line < 5) {
    char letter = 'a';
    while (letter < 'f') {
      System.out.print(letter);
      letter++;
    }
    System.out.println(); // why?
    line++;
  }
  

Listing 5.9 PalindromeTester

• Outer loop reads each line of input
• Inner loop compares characters forwards from beginning of line and backwards from end of line to middle
• Note how the values of left and right are updated
• Not sophisticated enough to allow for differences of spacing, capitalization, or punctuation
• Why is this wrong for ""?

Remarks on Lab 4

• Adapting PalindromeTester

While, Break, and Continue

• Using the break or continue statement in a while statement is possible
• Avoid it, because it obscures program structure
• Only use break in switch statements
• Don't use continue at all
• You can always use if instead

Why While/Break Is Bad

Normal exit from a while loop implies the condition is false.

while (x > 0)
{
    something;
    if (x == 2)
      break;
    x--;
}
// assert x <= 0 ???

The break statement makes it harder to reason about code.

(5.5) Iterators

• An Iterator is an object that allows us to iterate through a sequence of data
  • Usually a collection, such as a list
  • Processes each element in turn.
• Iterators have these methods:
  • hasNext() → boolean, tells us if there are any more data.
  • next() → Object, the next value in the sequence

Using an Iterator (Example)

(Assume c is some type of collection)

Iterator citer = c.iterator();
while (citer.hasNext()) {
  Object thing = citer.next();
  // process thing ...
}

Scanners as Iterators

• Scanners are iterators
• But they also define some more specialized
  • hasNextDouble
  • nextDouble
  • Etc.

Listing 5.10 URLDissector.java

Uses nested loops, with a Scanner as iterator for each loop.

• Opens a file and creates a Scanner
• Outer loop reads lines from file.
• Creates a new Scanner for the current line, which uses "/" to separate tokens.
• Inner loop processes (prints) the tokens of a URL (domain, directory, and file names).
• main declares throws IOException because things can go wrong in opening the file

(5.6) Array Lists

• java.util.ArrayList
• Dynamically resizable list implemented with an array
  • Arrays: chapter 8
• Elements are indexed as characters in strings.
• But we can insert and remove elements, expanding or shrinking the list.

Methods of ArrayList

See Figure 5.8 for details.

ArrayList() // constructor
add(object)
add(index, object)
get(index)
set(index, object)

Constructor

What is E?

ArrayList<E>()

E is the type of object we want the list to contain.

new ArrayList<String>()
new ArrayList<Integer>()

More ArrayList Methods

clear()
remove(index)
indexOf(object)
contains(object)
isEmpty()
size()

Examples

Demo in DrJava

Example

• Listing 5.11 Beatles
• Uses an ArrayList to store names of the Beatles
• Demonstrates constructor, add, indexOf, remove, add, get, and toString

Chapter Summary

• Flow of control is affected by control statements
  • Selection: if; conditional operator
  • Iteration: while, iterators, array lists
• Control statements in GUIs:
  • Event listeners can use if to determine event sources
  • Dialog boxes can return values to be checked by if statements
  • Check boxes and radio buttons can be used with control statements