5.5 Methods that Return boolean Values

A particularly effective way of improving the clarity of programs is through the use of boolean methods - queries that return either true or false. Often the boolean expressions that control selection or repetition statements are quite complex. Trying to read through such an expression can cause a reader to get caught up in details and lose track of the flow of the program. If a boolean method with some descriptive identifier is used in place of such an expression, the resulting code should be much easier to follow. An identifier of a boolean method usually begins with "is", indicating that the value returned by the method is the answer to some true/false question.

Example 1

The fragment
  if (13 <= age && age <= 19)
could be replaced by
  if (isTeen(age))
where isTeen is a method whose definition could be
public static boolean isTeen (int n)
{
  if (n >= 13 && n <= 19)
    return true;
  else
    return false;
}

In Example 1, the value returned by the method is always identical to the value of the expression n >= 13 && n <= 19. We can take advantage of this to make the definition of the method more concise, as follows. 

public static boolean isTeen (int n)
{
  return >= 13 && n <= 19;
}

Exercise 5.5

  1. Write boolean methods that could be used to make each fragment
more readable.
(a) while (n % 2 1)
	.
	.
(b) if ('0' <= c && c <= '9')
	.
	.
(c) do
	.
	.
    while (nextChar =='.' || nextChar ==',' ||
	   nextChar == '?' || nextChar == ':' ||
	   nextChar == ';' || nextChar == '!');
  2. Write a boolean method isDi visible with two int parameters. The method should return true if and only if the first parameter value is exactly divisible by the second.

  3. Write a boolean method isLetter that returns true if and only if its single char parameter is a letter of the alphabet (either upper case or lower case).

  4. Write a boolean method isPrime that returns true if and only if its int parameter is a prime number.

  5. Write two boolean methods, both called isPass, that could be used with the following fragment. 
    System.out.println("Letter Grade? (y/n)");
char response = In.getChar();
System.out.println("Value of grade?");
if (response == 'y')
{
  char mark = In.getChar();
  if (isPass(mark))
    System.out.println("Pass");
}
else // assume numerical grade
{
  int mark = In.getlnt();
  if (isPass(mark))
    System.out.println("Pass");
}
    The methods should return true if and only if the mark is a pass (at least 'D' for a letter grade and at least 50 for a numerical grade).