3.3. if Statements

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3.2. boolean Data Type and Operations

Often in a program you need to compare two values, such as whether i is greater than j . Java provides six comparison operators (also known as relational operators ), shown in Table 3.1, which can be used to compare two values. The result of the comparison is a Boolean value: true or false . For example, the following statement displays true :

 System.out.println(   1   <   2   ); 

Table 3.1. Comparison Operators

Operator	Name	Example	Answer
<	less than	1 < 2	true
<=	less than or equal to	1 <= 2	true
>	greater than	1 > 2	false
>=	greater than or equal to	1 >= 2	false
==	equal to	1 == 2	false
!=	not equal to	1 != 2	true

Note

You can also compare characters . Comparing characters is the same as comparing the Unicodes of the characters. For example, ' a ' is larger than ' A ' because the Unicode of ' a ' is larger than the Unicode of ' A .'

Caution

The equality comparison operator is two equal signs ( == ), not a single equal sign ( = ). The latter symbol is for assignment.

A variable that holds a Boolean value is known as a Boolean variable . The boolean data type is used to declare Boolean variables . The domain of the boolean type consists of two literal values: true and false . For example, the following statement assigns true to the variable lightsOn :

   boolean   lightsOn =   true   ; 

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Boolean operators , also known as logical operators , operate on Boolean values to create a new Boolean value. Table 3.2 contains a list of Boolean operators. Table 3.3 defines the not ( ! ) operator. The not ( ! ) operator negates true to false and false to true . Table 3.4 defines the and ( && ) operator. The and ( && ) of two Boolean operands is true if and only if both operands are true . Table 3.5 defines the or ( ) operator. The or ( ) of two Boolean operands is true if at least one of the operands is true . Table 3.6 defines the exclusive or ( ^ ) operator. The exclusive or ( ^ ) of two Boolean operands is true if and only if the two operands have different Boolean values.

Table 3.2. Boolean Operators

Operator	Name	Description
!	not	logical negation
&&	and	logical conjunction
	or	logical disjunction
^	exclusive or	logical exclusion

Table 3.3. Truth Table for Operator !

p	!p	Example
true	false	!(1 > 2) is true , because (1 > 2) is false .
false	true	!(1 > 0) is false , because (1 > 0) is true .

Table 3.4. Truth Table for Operator &&

p1	p2	p1 && p2	Example
false	false	false	(2 > 3) && (5 > 5) is false , because either (2 > 3) or (5 > 5) is false .
false	true	false
true	false	false	(3 > 2) && (5 > 5) is false , because (5 > 5) is false .
true	true	true	(3 > 2) && (5 >= 5) is true , because (3 > 2) and (5 >= 5) are both true .

Table 3.5. Truth Table for Operator

p1	p2	p1 p2	Example
false	false	false	(2 > 3)(5 > 5) is false , because (2 > 3) and (5 > 5) are both false .
false	true	true
true	false	true	(3 > 2)(5 > 5) is true , because (3 > 2) is true .
true	true	true

Table 3.6. Truth Table for Operator ^

p1	p2	p1^p2	Example
false	false	false	(2 > 3) ^ (5 > 5) is true , because (2 > 3) is false and (5 > 1) is true .
false	true	true
true	false	true	(3 > 2) ^ (5 > 1) is false , because both (3 > 2) and (5 > 1) are true .
true	true	false

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Listing 3.1 gives a program that checks whether a number is divisible by 2 and 3 , whether a number is divisible by 2 or 3 , and whether a number is divisible by 2 or 3 but not both:

Listing 3.1. TestBoolean.java

 1   import   javax.swin ptionPane; 2 3   public class   TestBoolean { 4   public static void   main(String[] args) { 5   int   number =   18   ; 6 7 JOptionPane.showMessageDialog(   null   , 8   "Is "   + number + 9   "  \n  divisible by 2 and 3? "   + 10 (number %   2   ==       &&   number %   3   ==     ) 11 +   "  \n  divisible by 2 or 3? "   + 12 (number %   2   ==         number %   3   ==     ) + 13   "  \n  divisible by 2 or 3, but not both? "   14 + (number %   2   ==       ^   number %   3   ==     )); 15 } 16 } 

A long string is formed by concatenating the substrings in lines 8 “14. The three \n characters display the string in four lines. (number % 2 == 0 && number % 3 == 0) (line 10) checks whether the number is divisible by 2 and 3. (number % 2 == 0 number % 3 == 0) (line 12) checks whether the number is divisible by 2 or 3. (number % 2 == 0 ^ number % 3 == 0) (line 14) checks whether the number is divisible by 2 or 3 , but not both.

3.2.1. Unconditional vs. Conditional Boolean Operators

If one of the operands of an && operator is false , the expression is false ; if one of the operands of an operand is true , the expression is true . Java uses these properties to improve the performance of these operators.

When evaluating p1 && p2 , Java first evaluates p1 and then evaluates p2 if p1 is true ; if p1 is false , it does not evaluate p2 . When evaluating p1 p2 , Java first evaluates p1 and then evaluates p2 if p1 is false ; if p1 is true , it does not evaluate p2 . Therefore, && is referred to as the conditional or short-circuit AND operator, and is referred to as the conditional or short-circuit OR operator.

Java also provides the & and operators. The & operator works exactly the same as the && operator, and the operator works exactly the same as the operator with one exception: the & and operators always evaluate both operands. Therefore, & is referred to as the unconditional AND operator, and is referred to as the unconditional OR operator. In some rare situations when needed, you can use the & and operators to guarantee that the right-hand operand is evaluated regardless of whether the left-hand operand is true or false . For example, the expression (width < 2) & (height “ “ < 2) guarantees that (height “ “ < 2) is evaluated. Thus the variable height will be decremented regardless of whether width is less than 2 or not.

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Tip

Avoid using the & and operators. The benefits of the & and operators are marginal. Using them will make the program difficult to read and could cause errors. For example, the expression (x != 0) & (100 / x) results in a runtime error if x is 0. However, (x != 0) && (100 / x) is fine. If x is , (x != 0) is false. Since && is a short-circuit operator, Java does not evaluate (100 / x) and returns the result as false for the entire expression (x != 0) && (100 / x).

Note

The & and operators can also apply to bitwise operations . See Appendix G, "Bit Manipulations," for details.

Note

As shown in the preceding chapter, a char value can be cast into an int value, and vice versa. A Boolean value, however, cannot be cast into a value of other types, nor can a value of other types be cast into a Boolean value.

Note

true and false are literals, just like a number such as 10 , so they are not keywords, but you cannot use them as identifiers, just as you cannot use 10 as an identifier.

3.2.2. Example: Determining Leap Year

This section presents a program that lets the user enter a year in a dialog box and checks whether it is a leap year.

A year is a leap year if it is divisible by 4 but not by 100 or if it is divisible by 400 . So you can use the following Boolean expression to check whether a year is a leap year:

 (year %   4   ==     && year %   100   !=     )  (year %   400   ==     ) 

Listing 3.2 gives the program. Two sample runs of the program are shown in Figure 3.1.

Figure 3.1. The program determines whether a year is a leap year.

Listing 3.2. LeapYear.java

(This item is displayed on pages 71 - 72 in the print version)

 1   import   javax.swing.JOptionPane; 2 3   public class   LeapYear { 4   public static void   main(String args[]) { 5  // Prompt the user to enter a year  

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 6   String yearString = JOptionPane.showInputDialog  (   "Enter a year"   );  7 8  // Convert the string into an int value  9   int   year =   Integer.parseInt(yearString);   10 11  // Check if the year is a leap year  12   boolean   isLeapYear = 13 (year %   4   ==     && year %   100   !=     )  (year %   400   ==     ); 14 15  // Display the result in a message dialog box  16  JOptionPane.showMessageDialog(   null   ,  17  year +   " is a leap year? "   + isLeapYear);  18 } 19 } 

3.2.3. Example: A Simple Math Learning Tool

This example creates a program to let a first grader practice addition. The program randomly generates two single-digit integers number1 and number2 and displays a question such as "What is 7 + 9?" to the student, as shown in Figure 3.2(a). After the student types the answer in the input dialog box, the program displays a message dialog box to indicate whether the answer is true or false, as shown in Figure 3.2(b).

Figure 3.2. The program generates an addition question and grades the student's answer.

There are many good ways to generate random numbers . For now, generate the first integer using System.currentTimeMillis() % 10 and the second using System.currentTimeMillis() * 7 % 10 . Listing 3.3 gives the program. Lines 5 “6 generate two numbers, number1 and number2 . Line 11 displays a dialog box and obtains an answer from the user. The answer is graded in line 15 using a Boolean expression number1 + number2 == answer .

Listing 3.3. AdditionTutor.java

 1   import   javax.swing.*; 2 3   public class   AdditionTutor { 4   public static void   main(String[] args) { 5   int   number1 = (   int   )(System.currentTimeMillis() %   10   ); 6   int   number2 = (   int   )(System.currentTimeMillis() *   7 % 10   ); 7 8 String answerString = JOptionPane.showInputDialog 9 (   "What is "   + number1 +   " + "   + number2 +   "?"   ); 10 11   int   answer = Integer.parseInt(answerString); 12 13 JOptionPane.showMessageDialog(   null   , 14 number1 +   " + "   + number2 +   " = "   + answer +   " is "   + 15 (   number1 + number2 == answer   )); 16 } 17 }