Chapter 6: More Conditionals and Loops

(6.1) The switch statement

Evaluates one expression and compares it to different values; executes statements corresponding to the first value that the expression equals.

• break statement
• default case

Syntax (simplified)

switch (expr) {  // int or char expr
  case value1:   // int or char values
    statement;
    ...
    break;
  case value2:   // int or char values
    statement;
    ...
    break;
  default:
    statement;
    ...
    [break;]   // optional in default
}

Example (1)

int x = ...;
... 
switch (x + 2) { 
  case 4: 
    System.out.println("x is 2"); 
    break; 
  case 5: 
    System.out.println("x is 3"); 
    break; 
  default: 
    System.out.println("x is neither 2 nor 3"); 
    break; // optional 
}

Example (2)

Paraphrased as an 'if' statement:

int x = ...;
... 
if (x + 2 == 4) 
  System.out.println("x is 2"); 
else if (x + 2 == 5) 
  System.out.println("x is 3"); 
else 
  System.out.println("x is neither 2 nor 3");

If and Switch Statements

• A switch statement can always be paraphrased using if ... else if ...; but it is sometimes more convenient.
• However, nested ifs are really not that difficult to comprehend, contra p. 273.
• The if statement is more general than switch, because it can use comparisons other than equality, and types other than char and int.

Listing 6.1 GradeReport

• What happens if we remove one or more break statements?
• If we remove the default case?
• If we add a break statement at the end of the default case?

(6.2) The Conditional Operator

• An if-like expression:

  condition ? valueIfTrue : valueIfFalse
  

• Unlike the if statement, it has a value.
  • Expressions are things that have value.

Example

// with if        // with conditional operator

if (y > 10)       x = y > 10 ? 1 : 2;
  x = 1;
else
  x = 2;

(6.3) The do statement

• Like the while statement, but with the test for expr occuring at the end rather than at the beginning.
• Always executes its statement at least once.

• Syntax:

  do
    statement
  while (expr);
  

Example of do Statement

double x = 12.0;
do {
  System.out.println(x);
  x /= 2.0;
}
while (x > 3.5)

What if x is initialized as 3.1?

Listing 5.12 ReverseNumber

• Uses a do statement to print out the decimal digits of an integer, in reverse order.
• Uses % and / to extract the digits.
• Trace the execution with at least one example.

(6.4) The for Statement

• Another iteration control statement
• Especially good when we can tell the number of iterations at the outset.

• Simple examples:

  for (int i = 0; i < 5; i++)
    System.out.println(i);
  
  for (int i = 10; i > 0; i--)
    System.out.println(i);
  

Syntax and Semantics of for

Syntax:

for (init; test; update)
  statement

Semantics: equivalent to

    init;
    while (test) {
      statement;
      update;
    }

Variables declared in for

• The init may contain a variable declaration.

• If it does, the scope of the variable is only within the (block of the) for loop.

  for (int i; ...; ...)
     ...;
  // i doesn't exist
  

Variables declared in a block

• Incidentally, variables can be declared in any block, and their scope is the block.

  {
     int i;
     ...
  }
  // i doesn't exist
  

Updates in for

• The update can perform any kind of update.

• Examples:

  for (double x = 2.5; x < 100.0; x *= 2.0)
    System.out.println(x);
  
  for (double sum = 0.0; 
       scan.hasNext(); 
       sum = sum + scan.nextDouble())
    System.out.println(sum);
  

Listing 6.3 Multiples

• Uses a for loop to print out the multiples of a number up to a user-specified limit.
• The update is mult += value;

Listing 6.4 Stars

• Uses nested for loops.
• Outer loop prints one line per iteration.
• Inner loop prints one * per iteration.
• Each line is one * longer than the previous.

Enhanced for statement (1)

• New syntax in Java 1.5 (5)

• Usage: given an iterator,

  for (type elt : iterator)
    statement;
  

  is equivalent to

  while (iterator.hasNext()) {
    elt = iterator.next();
    statement;
  }
  

Enhanced for statement (2)

• Also works for Iterables as well as Iterators.
• An Iterable object is one one that has an iterator
  • Get it by calling the object's iterator() method
• Lists and other collections are usually Iterables.

Enhanced for statement (3)

Usage: given a collection,

for (type elt : collection)
  statement;

is equivalent to

Iterator iterator = collection.iterator();
for (type elt: iterator)
  statement;

Example: foreach.java

Comparing loops (1)

Number of iterations:

• for and while iterate 0 or more times
• do iterates at least 1 time

Comparing loops (2)

When is the condition evaluated?

• for and while test test at the start
• do tests test at the end

Comparing loops (3)

Definite and indefinite iteration

• Definite iteration: we know the number of iterations in advance
• while, do, and for allow both definite and indefinite iteration
• for is especially suitable for definite iteration
• while and do are especially suitable for indefinite iteration

Loop Bugs and Tricks

• Empty statement (;) as loop body
• for loops without init, test, or update
• See WeirdLoops.java

Key Points

• Highest importance: if, while, for
• Medium: iterators, ArrayLists
• Lowest: do while, switch, conditional operator

Exercise

Write a method that prints the powers of 2 from 2⁰ = 1 to 2¹⁰ = 1024.

• Use while
• Use do while
• Use for
• Add parameter n; print the powers of 2 to 2ⁿ
• Do we need an if statement to test n ≥ 0?
• Don't print, but return an ArrayList of the powers

Lab 5 Introduction

• Instructions
• Algorithms

(6.5) Drawing with loops ...

Loops and conditionals can result in some interesting graphical programs.

Example

Bullseye, BullseyePanel (Listings 6.5–6.6)

• for loop draws concentric circles with decreasing radius
• Conditional selects the color
• Result: a series of rings alternating black and white

Another Example

Boxes, BoxesPanel (Listings 6.7–6.8)

• Uses loop to draw collection of random boxes
• Color and fill properties are conditionally determined from box's height and width
• paintComponent method generates a new collection of boxes each time it runs
  • Hide and reshow all or part of window to see

(6.6) Dialog boxes

Pop-up windows that engage the user briefly

• Choose a file
• Confirm a command
• Just read a message
• Etc.

Dialog boxes: Methods (1)

The class javax.swing.JOptionPane has useful methods for creating dialog boxes:

• showInputDialog
• showConfirmDialog
• showMessageDialog

Dialog boxes: Methods (2)

Get user input, using msg as prompt:

static String showInputDialog(Object msg)

Dialog boxes: Methods (3)

Show a confirmation dialog, with Yes, No, Cancel buttons:

static int showConfirmDialog (Component parent, 
                              Object msg)

Dialog box appears over the parent argument; if null, then at center of screen.

Returns int values such as JOptionPane.YES_OPTION

Dialog boxes: Methods (4)

Show a message with an OK button:

static void showMessageDialog (Component parent,
                               Object msg)

Example

EvenOdd (Listing 6.9)

Demonstrates the three types of dialog box

Chapter Summary

• Additional control statements:
  • Selection: switch
  • Iteration: do while, for
• Control statements can achieve interesting graphical effects