Outline of Class 10: Conditionals and Reflection

Held: Thursday, February 19, 1998

Administrivia

• Journal assignment 8 is now ready.
• I've worked out a set of resolutions for problem set 3.
• I've also worked out a set of resolutions for much of problem set 4.
• If possible, I will return all the assignments I can find in my office.
• A review sheet for the first exam is now available.
• We'll continue to spend time on computing assignment 5.
• We'll also begin computing assignment 6.
• Next Tuesday, we'll return to problem solving. In preparation for that class, please read chapter 3 in Thinking Mathematically.
• I'm thinking about an alternative final project for the class. What do you think about working together to provide an online guide to techniques for problem solving?
  • Those who are more inclined toward programming can develop programs that help illustrate issues.
  • Those who are more inclined toward problem solving can develop a number of sample resolutions.
  • Those who are more inclined toward reflection can work on descriptive text and an overall structure.
  • Some would also find new problems (to create a reasonable "problem bank") and research other techniques of problem solving.
  • Of course, we'll all be expected to work at least a little bit on each part.
• To turn in a programming assignment, mail it to me with a command like
  % elm rebelsky < File.java

Java, Revisited

• At this point, it may be helpful to reflect on Java's perspective on what a program is, and what issues relate to programs.
• In Java, a program is a collection of cooperating objects.
• The cooperation is governed, in part, by a main method that creates an initial set of objects and tells them to do some basic things.
• In addition to the world of the program, Java also deals with a "metaworld" (ideal world) in which generic (ideal) objects reside. You populate your programs with copies of these generic objects.
  • The generic objects are called classes.
• You can build your own classes. For now, the main aspect of a class will be the methods (capabilities) it provides.
  • Many classes provide a main method. This method says "if I controlled the world, this is what I'd have it do."
  • Most methods provide more general utility. For example, the class rebelsky.io.SimpleInput provides a readDouble method that allows you to read values that the user types.
• Methods take zero or more inputs (parameters) and return zero or one values.
• Methods contain zero or more variables (containers that hold values) and zero or more statements (instructions for doing things).
• Variables, parameters, and methods are all typed. That is, we describe the type of object that the represent (and, therefore, the capabilities).
  • Types are similar to classes.
• Right now, we know a few simple types.
  • double - numbers
  • String - sequences of characters
  • rebelsky.io.SimpleInput - things that can read values
  • rebelsky.io.SimpleOutput - things that can print values
  • void - things that do nothing (used primarily as a return type from methods that don't return anything).

Methods, Revisited

• Why does Java include methods? Why do we use them when writing algorithms/programs?
• There are a number of reasons.
• Methods clarify algorithms. Rather than writing some complicated formula (such as X*9/5 + 32), you can represent them by a more readable name (such as centigradeToFahrenheit).
• Methods allow you to divide the work in your programs and to think at a higher level. Once you have a method for converting Centigrade to Fahrenheit, you can use it in a variety of situations
  • Converting a single value
  • Building a table
  • Determining the Centigrade value that corresponds to a particular Fahrenheit value (basically, you can try to run the method "backwards" by guessing).
• Methods allow you to extract common code. As you write larger and larger programs, you'll find that you're doing that more and more.

The if Statement

• The if or conditional statement is one of the simplest control structures.
• Java provides the two basic forms of the if statement.
• if (test) statement executes the statement only when the test holds.
  • Note that the test must return a boolean (truth) value.
  • Note that the test must be surrounded by parentheses.
• if (test) statement else statement selects between the two statements. It executes the first if the test evaluates to true and the second if it evalutes to false.
• Java does not provide a special facility for nesting if statements.

Exercise

See absolute values, letter grades, and grade averaging, in computing exercise 5 for an example and some exercises.