Version 5.1¹

Two class days

Handouts: WeirdLoops.java

First Day

(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

Sketch of syntax (simplified):

switch (expr) {
  case value1:
    statement;
    ...
    break;
  case value2:
    statement;
    ...
    break;
  default:
    statement;
    ...
    [break;]
}

• The expr and values must be either int or char in type—not boolean, not any floating point type, and not any other integer type—of course, also not any reference type.
• []'s mean that the last break; is optional, but why is it a good idea?

Example

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 
}

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");

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 (a) comparisons other than equality, and (b) 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
  

• It is an expression, not a *statement.
  • It has a value and normally does not perform an action.
  • It selects between two expressions, not statements.
  • There are always two expressions, not one.

• Can be used to abbreviate alternate assignments to the same variable, or as an argument in a method call (pp. 274–275)

(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 6.2 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.

Example:

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

Equivalent to:

{
  int i = 0;
  while (i < 10)
  {
    System.out.println(i);
    i++;
  }
}

Syntax:

for (init; test; update)
  statement

Semantics: equivalent to

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

Simple usage with ++ or -- in the update:

count++ // p. 279
num--   // p. 280

The init may contain a variable declaration.

• If it does, the scope of the variable is only within the (block of the) for loop.
• Incidentally, variables can be declared in any block, and their scope is the block.

The update can perform any kind of update; normally, it changes the variable initialized in the init.

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. The outer loop prints one line per iteration. The inner loop prints one * per iteration. Each line is one * longer than the previous.

Iterators and for loops

New syntax in Java 1.5 (5): enhanced for statement (for-each loop): given a collection with an iterator,

for (type elt : iterator)
  statement;

Equivalent to:

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

This also works for Iterables as well as Iterators. An Iterable object is one one that has an iterator, which can be got by calling the object's iterator() method. Lists and other collections are usually Iterables.²

for (type elt : collection)
  statement;

Equivalent to:

Iterator iterator = collection.iterator(); // gets the iterator while
(iterator.hasNext()) { elt = iterator.next(); statement; }

Example: foreach.java

Comparing loops

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

2. for and while evaluate test at the start; do evaluates test at the end.

3. while and do allow indefinite iteration (where the number of iterations cannot be determined in advance).

   Actually, so does for, but it is especially well suited for for definite iteration (where the number of iterations is knowable in advance), such as counting loops ("repeat 10 times").

Key Points

• Most important: if, while, and for statements
  • Be fluent in them, reading and writing!
• Medium importance (in this course): iterators, ArrayLists
  • These tend to become more important in the long run, though, because they provide useful abstractions that can be applied to many types of sequences
• Useful, but less important: do while, switch, and conditional operator
  • At least try to recognize and understand them. You may choose never write them yourself, but still come across them in code written by others.

Loop Bugs and Tricks

• ; (empty statement) as loop body
• for loops without init, test, update
  • Default test is true, i.e., loop forever
  • Default init and update are empty statements (do nothing)
• See WeirdLoops.java
  • Which of these are bugs?
  • Which are stylistic abominations?

Second Day

Exercises

Variations on Exercise 6.12 (p. 295):

Write and test a method that prints the powers of 2 from 2⁰ = 1 to 2¹⁰ = 1024. The method has no parameters and returns no value.

• Use a while statement.
• Use a do while statement.
• Use a for statement.
• Add a parameter n to the method; print the first n powers of 2.
• Discuss: do we need an if statement to check that n ≥ 0?
• Instead of printing the powers of two, have the method create and return an ArrayList storing the powers (1, 2, …, 2ⁿ).

Introduction to Lab 5

• Lab 5
• Discuss algorithms for selection of radius and location

(6.5) Drawing with loops and conditionals

Loops and conditionals can result in some interesting graphical programs.

Listings 6.5–6.6: Bullseye, BullseyePanel

for loop draws concentric circles with decreasing radius; the conditional selects the color. The result is a series of rings alternating black and white.

Listings 6.7–6.8: Boxes, BoxesPanel

Uses a loop to draw a collection of random boxes; their color and fill properties are conditionally determined from the box's height and width.

Interesting — the paintComponent method generates a new collection of boxes each time it runs, so if you hide and reshow the window, you get a new picture (and if you hide and reshow a part of it, it looks strange)!

(6.6) Dialog boxes

Dialog boxes are pop-up windows that engage the user briefly, e.g., to choose a file, to confirm a command, or simply to read a message.

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

• static String showInputDialog(Object msg) — uses msg to request user input into a text input field and returns the input as a String;

• static int showConfirmDialog (Component parent, Object msg) — provides Yes, No, and Cancel buttons; returns int values such as JOptionPane.YES_OPTION.

• static void showMessageDialog (Component parent, Object msg) — displays a message and an OK button to close the dialog box.

The parent argument represents the component over which the dialog box appears; if it is null, the dialog box is placed in the center of the screen.

Listing 6.9 EvenOdd

Demonstrates the three types of dialog box.