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2.5 Another Application: Adding Integers

Our next application reads (or inputs) two integers (whole numbers, such as –22, 7, 0 and 1024) typed by a user at the keyboard, computes their sum and displays it. This program must keep track of the numbers supplied by the user for the calculation later in the program. Programs remember numbers and other data in the computer's memory and access that data through program elements called variables. The program of Fig. 2.7 demonstrates these concepts. In the sample output, we use bold text to identify the user's input (i.e., 45 and 72 ).

Fig 2.7. Addition program that displays the sum of two numbers.

 1   // Fig. 2.7: Addition.java
 2   // Addition program that displays the sum of two numbers.
 3   import java.util.Scanner; // program uses class Scanner
 4
 5   public class Addition
 6   {
 7      // main method begins execution of Java application
 8      public static void main( String[] args )
 9      {
10         // create a Scanner to obtain input from the command window
11         Scanner input = new Scanner( System.in );                  
12
13         int number1; // first number to add   
14         int number2; // second number to add  
15         int sum; // sum of number1 and number2
16
17         System.out.print( "Enter first integer: " ); // prompt
18         number1 = input.nextInt(); // read first number from user
19
20         System.out.print( "Enter second integer: " ); // prompt
21         number2 = input.nextInt(); // read second number from user
22
23         sum = number1 + number2; // add numbers, then store total in sum
24
25         System.out.printf( "Sum is %d\n", sum ); // display sum
26      } // end method main
27   } // end class Addition

Import Declarations

Lines 1–2

// Fig. 2.7: Addition.java
// Addition program that displays the sum of two numbers.

state the figure number, file name and purpose of the program.

A great strength of Java is its rich set of predefined classes that you can reuse rather than "reinventing the wheel." These classes are grouped into packages—named groups of related classes—and are collectively referred to as the Java class library, or the Java Application Programming Interface (Java API). Line 3

import java.util.Scanner; // program uses class Scanner

is an import declaration that helps the compiler locate a class that's used in this program. It indicates that this example uses Java's predefined Scanner class (discussed shortly) from package java.util .

Declaring Class Addition

Line 5

public class Addition

begins the declaration of class Addition. The file name for this public class must be Addition.java. Remember that the body of each class declaration starts with an opening left brace (line 6) and ends with a closing right brace (line 27).

The application begins execution with the main method (lines 8–26). The left brace (line 9) marks the beginning of method main's body, and the corresponding right brace (line 26) marks its end. Method main is indented one level in the body of class Addition, and the code in the body of main is indented another level for readability.

Declaring and Creating a Scanner to Obtain User Input from the Keyboard

A variable is a location in the computer's memory where a value can be stored for use later in a program. All Java variables must be declared with a name and a type before they can be used. A variable's name enables the program to access the value of the variable in memory. A variable's name can be any valid identifier. A variable's type specifies what kind of information is stored at that location in memory. Like other statements, declaration statements end with a semicolon (;).

Line 11

Scanner input = new Scanner( System.in );

is a variable declaration statement that specifies the name (input) and type (Scanner) of a variable that's used in this program. A Scanner enables a program to read data (e.g., numbers and strings) for use in a program. The data can come from many sources, such as the user at the keyboard or a file on disk. Before using a Scanner, you must create it and specify the source of the data.

The = in line 11 indicates that Scanner variable input should be initialized (i.e., prepared for use in the program) in its declaration with the result of the expression to the right of the equals sign—new Scanner(System.in). This expression uses the new keyword to create a Scanner object that reads characters typed by the user at the keyboard. The standard input object, System.in , enables applications to read bytes of information typed by the user. The Scanner translates these bytes into types (like ints) that can be used in a program.

Declaring Variables to Store Integers

The variable declaration statements in lines 13–15

int number1; // first number to add
int number2; // second number to add
int sum; // sum of number1 and number2

declare that variables number1, number2 and sum hold data of type int —they can hold integer values (whole numbers such as 72, –1127 and 0). These variables are not yet initialized. The range of values for an int is –2,147,483,648 to +2,147,483,647. [Note: Actual int values may not contain commas.]

Other types of data include float and double , for holding real numbers, and char , for holding character data. Real numbers contain decimal points, such as 3.4, 0.0 and –11.19. Variables of type char represent individual characters, such as an uppercase letter (e.g., A), a digit (e.g., 7), a special character (e.g., * or %) or an escape sequence (e.g., the newline character, \n). The types int, float, double and char are called primitive types. Primitive-type names are keywords and must appear in all lowercase letters. Appendix D summarizes the characteristics of the eight primitive types (boolean, byte, char, short, int, long, float and double).

Several variables of the same type may be declared in a single declaration with the variable names separated by commas (i.e., a comma-separated list of variable names). For example, lines 13–15 can also be written as:

int number1, // first number to add
    number2, // second number to add
    sum, // sum of number1 and number2

Prompting the User for Input

Line 17

System.out.print( "Enter first integer: " ); // prompt

uses System.out.print to display the message "Enter first integer: ". This message is called a prompt because it directs the user to take a specific action. We use method print here rather than println so that the user's input appears on the same line as the prompt. Recall from Section 2.2 that identifiers starting with capital letters typically represent class names. So, System is a class. Class System is part of package java.lang . Notice that class System is not imported with an import declaration at the beginning of the program.

Obtaining an int as Input from the User

Line 18

number1 = input.nextInt(); // read first number from user

uses Scanner object input's nextInt method to obtain an integer from the user at the keyboard. At this point the program waits for the user to type the number and press the Enter key to submit the number to the program.

Our program assumes that the user enters a valid integer value. If not, a runtime logic error will occur and the program will terminate. Chapter 11, Exception Handling: A Deeper Look, discusses how to make your programs more robust by enabling them to handle such errors. This is also known as making your program fault tolerant.

In line 18, we place the result of the call to method nextInt (an int value) in variable number1 by using the assignment operator, = . The statement is read as "number1 gets the value of input.nextInt()." Operator = is called a binary operator, because it has two operandsnumber1 and the result of the method call input.nextInt(). This statement is called an assignment statement, because it assigns a value to a variable. Everything to the right of the assignment operator, =, is always evaluated before the assignment is performed.

Prompting for and Inputting a Second int

Line 20

System.out.print( "Enter second integer: " ); // prompt

prompts the user to input the second integer. Line 21

number2 = input.nextInt(); // read second number from user

reads the second integer and assigns it to variable number2.

Using Variables in a Calculation

Line 23

sum = number1 + number2; // add numbers then store total in sum

is an assignment statement that calculates the sum of the variables number1 and number2 then assigns the result to variable sum by using the assignment operator, =. The statement is read as "sum gets the value of number1 + number2." In general, calculations are performed in assignment statements. When the program encounters the addition operation, it performs the calculation using the values stored in the variables number1 and number2. In the preceding statement, the addition operator is a binary operator—its two operands are the variables number1 and number2. Portions of statements that contain calculations are called expressions. In fact, an expression is any portion of a statement that has a value associated with it. For example, the value of the expression number1 + number2 is the sum of the numbers. Similarly, the value of the expression input.nextInt() is the integer typed by the user.

Displaying the Result of the Calculation

After the calculation has been performed, line 25

System.out.printf( "Sum is %d\n", sum ); // display sum

uses method System.out.printf to display the sum. The format specifier %d is a placeholder for an int value (in this case the value of sum)—the letter d stands for "decimal integer." The remaining characters in the format string are all fixed text. So, method printf displays "Sum is ", followed by the value of sum (in the position of the %d format specifier) and a newline.

Calculations can also be performed inside printf statements. We could have combined the statements at lines 23 and 25 into the statement

System.out.printf( "Sum is %d\n", ( number1 + number2 ) );

The parentheses around the expression number1 + number2 are not required—they're included to emphasize that the value of the entire expression is output in the position of the %d format specifier.

Java API Documentation

For each new Java API class we use, we indicate the package in which it's located. This information helps you locate descriptions of each package and class in the Java API documentation. A web-based version of this documentation can be found at

download.oracle.com/javase/6/docs/api/

You can download it from

www.oracle.com/technetwork/java/javase/downloads/index.html

Appendix E shows how to use this documentation.

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