Decision Making: Equality and Relational Operators

Decision Making Equality and Relational Operators

A condition is an expression that can be either true or false. This section introduces a simple version of Java's if statement that allows a program to make a decision based on the value of a condition. 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 will 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.

The application of Fig. 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 assignment statement associated with that if statement executes. The program uses a Scanner to input the two integers from the user and store them in variables number1 and number2. Then the program compares the numbers and displays the results of the comparisons that are true.

Figure 2.15. Equality and relational operators.

(This item is displayed on pages 57 - 58 in the print version)

The declaration of class Comparison begins at line 6

public class Comparison

The class's main method (lines 941) begins the execution of the program.

Line 12

Scanner input = new Scanner( System.in );

declares Scanner variable input and assigns it a Scanner that inputs data from the standard input (i.e., the keyboard).

Lines 1415

int number1; // first number to compare int number2; // second number to compare

declare the int variables used to store the values input from the user.

Lines 1718

System.out.print( "Enter first integer: " ); // prompt number1 = input.nextInt(); // read first number from user

prompt the user to enter the first integer and input the value, respectively. The input value is stored in variable number1.

Lines 2021

System.out.print( "Enter second integer: " ); // prompt number2 = input.nextInt(); // read second number from user

prompt the user to enter the second integer and input the value, respectively. The input value is stored in variable number2.

Lines 2324

if ( number1 == number2 ) System.out.printf( "%d == %d ", number1, number2 );

declare an if statement that compares the values of the variables number1 and number2 to determine whether they are equal. An if statement always begins with keyword if, followed by a condition in parentheses. An if statement expects one statement in its body. 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 on 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 at lines 2627, 2930, 3233, 3536 and 3839 compare number1 and number2 with the operators !=, <, >, <= and >=, respectively. If the condition in any of the if statements is true, the corresponding body statement executes.

Common Programming Error 2.9

Common Programming Error 2.10

Common Programming Error 2.11

Common Programming Error 2.12

Good Programming Practice 2.15

Good Programming Practice 2.16

Note that there is 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,

if ( number1 == number2 ); // logic error System.out.printf( "%d == %d ", number1, number2 );

would actually be interpreted by Java as

if ( number1 == number2 ) ; // empty statement System.out.printf( "%d == %d ", number1, number2 );

where the semicolon on the line by itselfcalled the empty statementis the statement to execute if the condition in the if statement is true. When the empty statement executes, no task is performed in the program. 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 2.13

Note the use of white space in Fig. 2.15. Recall that white-space characters, such as tabs, newlines and spaces, are normally ignored by the compiler. So statements may be split over several lines and may be spaced according to the programmer's preferences without affecting the meaning of a program. It is incorrect to split identifiers and strings. Ideally, statements should be kept small, but this is not always possible.

Good Programming Practice 2.17

Figure 2.16 shows the precedence of the operators introduced in this chapter. The operators are shown from top to bottom in decreasing order of precedence. All these operators, with the exception of the assignment operator, =, associate from left to right. Addition is left associative, so an expression like x + y + z is evaluated as if it had been written as ( x + y ) + z. 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, as we will soon see, first assigns the value 0 to variable y and then assigns the result of that assignment, 0, to x.

Good Programming Practice 2.18