Class 06: Conditionals

Held Tuesday, February 2

Summary

• Boolean values and expressions
• The if statement
• The switch statement
• Lab: J4: Boolean Expressions and Conditionals
  • Experiments: J4: Boolean Expressions and Conditionals

Contents

Notes

• Don't forget to continue reading ahead in the labs.
• Don't forget that lab J3 is due at the start of class tomorrow.
• I've started to separate out the experiment portions of labs. Look at the lab index for more information.
• A few of you have asked how you might access code files if I don't include an explicit link to them. Note that there is a link to Code at the top of every online lab.
• Only do J4.2 and J4.4.

Boolean Values

• In working with Java, we've encountered three kinds of values (more or less)
  • Primitive numeric values (int and its ilk)
  • Built-in object values (String)
  • User-defined object values (Point)
• There are additional kinds of primitive values that we will be concerned with. The simpest of these is the boolean value.
• Booleans are truth values: they are either true or false. They have no other possible values.
• You can compute boolean values with boolean expressions.
• You can compare two primitive numeric values with
  • x < y (less than)
  • x <= y (less than or equal to)
  • x == y (equal to; note there are two equals signs)
  • x != y (not equal to)
  • x >= y (greater than or equal to)
  • x > y (greater than)
• You can typically compare any two objects with the equals method. (It does need to be defined in most cases.)
• It is also possible to define new boolean methods. For example, consider ``is this point within 1 unit of the origin?''

    /**
     * Determine if the current point is within one unit of the origin.
     */
    public boolean nearOrigin() {
      return (this.distanceFromOrigin() < 1.0);
    } // nearOrigin()
  

• It is also possible to combine booleans with boolean operators,
  • b1 && b2 (and)
  • b1 || b2 (or)
  • !b (not)
• For example, we might call a point ``bothersome'' if it is within half a unit from either axis. These operations will help define such a function.

    /**
     * Determine if the current point is bothersome.  That is, it is
     * within half a unit from either axis.
     */
    public boolean bothersome() {
      return (Math.abs(this.xcoordinate) < 0.5) ||
             (Math.abs(this.ycoordinate) < 0.5);
    } // bothersome()
  

• As we'll soon see, there are many uses of boolean values, particularly relative to control structures for programs.
  • Control structures influence the flow of control within the program; they select which operations are performed.

Simple Conditionals

• The if or conditional statement is one of the simplest control structures.
• Java provides the two basic forms of the if statement.
• The simplest one:

  if (test) {
    statements;
  } // if
  

  This executes the statement only when the test holds.
  • The test must return a boolean (truth) value.
  • The test must be surrounded by parentheses.
• This is a common structure in recipes. For example, ``For fluffier pancakes, decrease the milk by 10%.''
• It is also possible to suggest an alternate if the test does not hold.

  if (test) {
    statements;
  } // if
  else {
    statements;
  } // not test
  

  This 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. (Some other languages do.)

More Conditionals

• Often, you need to do more than select between two values, one true and one false.
• While it is possible to do this with a sequence of if statements, such a solution is neither readable nor efficient.
• As an alternative, Java provides the switch statement (like the case statement in Pascal).
• It takes the form

  switch (integer-valued-expression) {
    case value1:
      stuff-to-do;
      break;
    case value2:
      stuff-to-do;
      break;
    ...
    default:
      stuff-to-do;
      break;
  }
  

• If you leave out a break, execution will continue into the next case.
• You can switch on byte, short, int, long, or char valued expressions.
• break has other uses, which you'll see later.

Lab

Do lab J4: conditionals.

Reflection

What are some other ways we might use conditionals with the Point class?

What are some other ways in which you might expect to use conditionals?

History

• Created Monday, January 11, 1999.
• Added short summary on Friday, January 22, 1999.
• Filled in the details on Tuesday, February 2, 1999.