- 2.1 Introduction
- 2.2 Your First Program in Java: Printing a Line of Text
- 2.3 Modifying Your First Java Program
- 2.4 Displaying Text with printf
- 2.5 Another Application: Adding Integers
- 2.6 Memory Concepts
- 2.7 Arithmetic
- 2.8 Decision Making: Equality and Relational Operators
- 2.9 Wrap-Up
- Summary
- Self-Review Exercises
- Answers to Self-Review Exercises
- Exercises
- Making a Difference
This chapter is from the book
This chapter is from the book
This chapter is from the book
2.8 Decision Making: Equality and Relational Operators
A condition is an expression that can be true or false . This section introduces Java's if selection statement, which allows a program to make a decision based on a condition's value. For example, the condition "grade is greater than or equal to 60" determines whether a student passed a test. If the condition in an if statement is true, the body of the if statement executes. If the condition is false, the body does not execute. We'll see an example shortly.
Conditions in if statements can be formed by using the equality operators ( == and != ) and relational operators ( > , < , >= and <= ) summarized in Fig. 2.14. Both equality operators have the same level of precedence, which is lower than that of the relational operators. The equality operators associate from left to right. The relational operators all have the same level of precedence and also associate from left to right.
Fig 2.14. Equality and relational operators.
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Standard algebraic equality or relational operator |
Java equality or relational operator |
Sample Java condition |
Meaning of Java condition |
|
Equality operators |
|||
|
= |
== |
x == y |
x is equal to y |
|
|
!= |
x != y |
x is not equal to y |
|
Relational operators |
|||
|
> |
> |
x > y |
x is greater than y |
|
< |
< |
x < y |
x is less than y |
|
|
>= |
x >= y |
x is greater than or equal to y |
|
|
<= |
x <= y |
x is less than or equal to y |
Figure 2.15 uses six if statements to compare two integers input by the user. If the condition in any of these if statements is true, the statement associated with that if statement executes; otherwise, the statement is skipped. We use a Scanner to input the integers from the user and store them in variables number1 and number2. The program compares the numbers and displays the results of the comparisons that are true.
Fig 2.15. Compare integers using if statements, relational operators and equality operators.
1// Fig. 2.15: Comparison.java2// Compare integers using if statements, relational operators3// and equality operators.4import java.util.Scanner; // program uses class Scanner5 6public class Comparison7 { 8// main method begins execution of Java application9public static void main( String[] args )10 { 11// create Scanner to obtain input from command line12 Scanner input =new Scanner( System.in );13 14int number1; // first number to compare15int number2; // second number to compare16 17 System.out.print("Enter first integer: " ); // prompt18 number1 = input.nextInt();// read first number from user19 20 System.out.print("Enter second integer: " ); // prompt21 number2 = input.nextInt();// read second number from user22 23if ( number1 == number2 )24System.out.printf( "%d == %d\n", number1, number2 );25 26if ( number1 != number2 )27System.out.printf( "%d != %d\n", number1, number2 );28 29if ( number1 < number2 )30System.out.printf( "%d < %d\n", number1, number2 );31 32if ( number1 > number2 )33System.out.printf( "%d > %d\n", number1, number2 );34 35if ( number1 <= number2 )36System.out.printf( "%d <= %d\n", number1, number2 );37 38if ( number1 >= number2 )39System.out.printf( "%d >= %d\n", number1, number2 );40 }// end method main41 }// end class Comparison
The declaration of class Comparison begins at line 6
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The class's main method (lines 9–40) begins the execution of the program. Line 12
Scanner input =
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declares Scanner variable input and assigns it a Scanner that inputs data from the standard input (i.e., the keyboard).
Lines 14–15
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declare the int variables used to store the values input from the user.
Lines 17–18
System.out.print(
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prompt the user to enter the first integer and input the value, respectively. The input value is stored in variable number1.
Lines 20–21
System.out.print(
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prompt the user to enter the second integer and input the value, respectively. The input value is stored in variable number2.
Lines 23–24
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compare the values of number1 and number2 to determine whether they're equal. An if statement always begins with keyword if, followed by a condition in parentheses. An if statement expects one statement in its body, but may contain multiple statements if they're enclosed in a set of braces ({}). The indentation of the body statement shown here is not required, but it improves the program's readability by emphasizing that the statement in line 24 is part of the if statement that begins at line 23. Line 24 executes only if the numbers stored in variables number1 and number2 are equal (i.e., the condition is true). The if statements in lines 26–27, 29–30, 32–33, 35–36 and 38–39 compare number1 and number2 using the operators !=, <, >, <= and >=, respectively. If the condition in one or more of the if statements is true, the corresponding body statement executes.
Common Programming Error 2.6
Confusing the equality operator, ==, with the assignment operator, =, can cause a logic error or a syntax error. The equality operator should be read as "is equal to" and the assignment operator as "gets" or "gets the value of." To avoid confusion, some people read the equality operator as "double equals" or "equals equals."
Good Programming Practice 2.11
Placing only one statement per line in a program enhances program readability.
There's no semicolon (;) at the end of the first line of each if statement. Such a semicolon would result in a logic error at execution time. For example,
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would actually be interpreted by Java as
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where the semicolon on the line by itself—called the empty statement—is the statement to execute if the condition in the if statement is true. When the empty statement executes, no task is performed. The program then continues with the output statement, which always executes, regardless of whether the condition is true or false, because the output statement is not part of the if statement.
Common Programming Error 2.7
Placing a semicolon immediately after the right parenthesis of the condition in an if statement is normally a logic error.
Note the use of white space in Fig. 2.15. Recall that the compiler normally ignores white space. So, statements may be split over several lines and may be spaced according to your preferences without affecting a program's meaning. It's incorrect to split identifiers and strings. Ideally, statements should be kept small, but this is not always possible.
Error-Prevention Tip 2.9
A lengthy statement can be spread over several lines. If a single statement must be split across lines, choose breaking points that make sense, such as after a comma in a comma-separated list, or after an operator in a lengthy expression. If a statement is split across two or more lines, indent all subsequent lines until the end of the statement.
Figure 2.16 shows the operators discussed so far in decreasing order of precedence. All but the assignment operator, =, associate from left to right. The assignment operator, =, associates from right to left, so an expression like x = y = 0 is evaluated as if it had been written as x = (y = 0), which first assigns the value 0 to variable y, then assigns the result of that assignment, 0, to x.
Fig 2.16. Precedence and associativity of operators discussed.
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Operators |
Associativity |
Type |
|||
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* |
/ |
% |
left to right |
multiplicative |
|
|
+ |
– |
left to right |
additive |
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|
< |
<= |
> |
>= |
left to right |
relational |
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== |
!= |
left to right |
equality |
||
|
= |
right to left |
assignment |
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Good Programming Practice 2.12
When writing expressions containing many operators, refer to the operator precedence chart (Appendix A). Confirm that the operations in the expression are performed in the order you expect. If, in a complex expression, you're uncertain about the order of evaluation, use parentheses to force the order, exactly as you'd do in algebraic expressions.