• Definition:
  • A method that returns true or false
• Common Uses:
  • Indicate success or failure of a process
  • Conduct a test

• The Idea:
  • The method is called to change the state of the system (e.g., generate output, modify a collection)
  • The method might or might not succeed
• Using a Boolean Method:
  • Instead of making the method void, have it return true on success and false on failure

public static boolean printSquareRoot(double x)
{
  boolean    ok;
  double     squareRoot;

  if (x >= 0.) 
  {
    squareRoot = Math.sqrt(x);
    JMUConsole.printf("The root of %f is %f.\n", x, squareRoot);
    ok = true;
  } 
  else 
  {
    ok = false;
  }


  return ok;
}
  

• The Idea:
  • Replace an expression that conducts a test with a method call that conducts the test
• Advantages/Benefits:
  • Increase readability
  • Reduce code duplication

• The Original Code:

  • if (((dieOne + dieTwo) == 7) || ((dieOne + dieTwo) == 11))
    {
      // Make Payout
    }
    

• Using a Boolean Method:

  if (wins(dieOne, dieTwo))
  {
    // Make Payout
  }
  

• An Inelegant Implementation:

  • public static boolean wins(int dieOne, int dieTwo)
    {
      boolean result;
      int     total;
    
      total = dieOne + dieTwo;
    
      if ((total == 7) || (total == 11)) result = true;
      else                               result = false;
    
      return result;
    }
    

• An Elegant Implementation:

  public static boolean wins(int dieOne, int dieTwo)
  {
    int     total;
  
    total = dieOne + dieTwo;
  
    return ((total == 7) || (total == 11));
  }
  

• Readability:
  • isOdd(x) is much easier to read and understand than ((x % 2) == 1)
• Reduced Code Duplication:
  • It is difficult to modify duplicate code consistently (which we can see with a simple example)

• The Original Code:

  • if (height >= 62) 
    {
      if (sex == 'M')
      {
        // Case 1
      } 
      else
      {
        // Case 2
      }
    }
    else
    {
      if (sex == 'M')
      {
        // Case 3
      }
      else
      {
        // Case 4
      }
    }
      

• The Necessary Modification:
  • Account for both 'M' and 'm'

• The New Code:

  • if (height >= 62) 
    {
      if ((sex == 'M') || (sex == 'm')) 
      {
        // Case 1
      }
      else
      {
        // Case 2
      }
    }
    else
    {
      if ((sex == 'M') || (sex == 'm')) 
      {
        // Case 3
      }
      else
      {
        // Case 4
      }
    }
      

• The Important Observation:
  • Because of code duplication, the change had to be made in multiple places

• Using a Test Method:

  • if (height >= 62) 
    {
      if (isMale(sex)) // Case 1
      else             // Case 2
    }
    else
    {
      if (isMale(sex)) // Case 3
      else             // Case 4
    }
      

• The Original and Modified Test Methods:

  • public static boolean isMale(char sex)
    {
      return (sex == 'M');
    }
      

  • public static boolean isMale(char sex)
    {
      return ((sex == 'M') || (sex == 'm'));
    }
      

• A Common Situation:
  • There are related test methods (e.g., isOdd() and isEven())
• A Design that is Difficult to Modify:
  • Making the test methods independent (e.g., use (x%2) == 1 in isOdd() and (x%2) == 0 in isEven())
• A Better Design:
  • Have one test method invoke another (e.g., have isEven() return !isOdd())

• An Implementation that is Error-Prone:

  • public static boolean isLowIncome(double income)
    {
      return (income < 20000.0);
    }
    
    public static boolean isMiddleIncome(double income)
    {
      return ((income >= 20000.0) && (income < 75000.0));
    }
    
    public static boolean isHighIncome(double income)
    {
      return (income >= 75000.0);
    }
      

• A Better Implementation:

  • public static boolean isLowIncome(double income)
    {
      return (income < 20000.0);
    }
    
    public static boolean isMiddleIncome(double income)
    {
      return ((!isLowIncome()) && (income < 75000.0));
    }
    
    public static boolean isHighIncome(double income)
    {
      return (!isLowIncome(income) && !isMiddleIncome(income));
    }
      

• Recall:
  • In Java a Boolean method returns a boolean (i.e., true or false)
• An Observation:
  • Sometimes it is more convenient to return 0/1 (because numeric values can be used in other calculations)

One Implementation

public static boolean isPassing(int grade)
{
  return grade>=60;
}

public static int creditsEarned(int grade, int creditsPossible)
{
  if (isPassing(grade)) return creditsPossible;
  else                  return 0;
}

An Alternative Implementation

public static int isPassing(int grade)
{
  return grade/60;
}

public static int creditsEarned(double grade, int creditsPossible)
{
  return isPassing(grade)*creditsPossible;
}