Java How To Program Chapter 2 PDF

Java™ How to Program, 10/e Late Objects Version © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Java application  A computer pr...

Java™ How to Program, 10/e Late Objects Version © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Java application  A computer program that executes when you use the java command to launch the Java Virtual Machine (JVM).  Sample program in Fig. 2.1 displays a line of text. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. Commenting Your Programs  Comments // Fig. 2.1: Welcome1.java  // indicates that the line is a comment.  Used to document programs and improve their readability.  Compiler ignores comments.  A comment that begins with // is an end-of-line comment—it terminates at the end of the line on which it appears.  Traditional comment, can be spread over several lines as in  This type of comment begins with.  All text between the delimiters is ignored by the compiler. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Javadoc comments  Delimited by.  All text between the Javadoc comment delimiters is ignored by the compiler.  Enable you to embed program documentation directly in your programs.  The javadoc utility program (online Appendix G) reads Javadoc comments and uses them to prepare program documentation in HTML format. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. Using Blank Lines  Blank lines, space characters and tabs  Make programs easier to read.  Together, they’re known as white space (or whitespace).  White space is ignored by the compiler. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. Declaring a class  Class declaration public class Welcome1  Every Java program consists of at least one class that you define.  class keyword introduces a class declaration and is immediately followed by the class name.  Keywords (Appendix C) are reserved for use by Java and are always spelled with all lowercase letters. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. Filename for a public Class  A public class must be placed in a file that has a filename of the form ClassName.java, so class Welcome1 is stored in the file Welcome1.java. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. Class Names and Identifiers  By convention, begin with a capital letter and capitalize the first letter of each word they include (e.g., SampleClassName).  A class name is an identifier—a series of characters consisting of letters, digits, underscores (_) and dollar signs ($) that does not begin with a digit and does not contain spaces.  Java is case sensitive—uppercase and lowercase letters are distinct—so a1 and A1 are different (but both valid) identifiers. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. Class Body  A left brace, {, begins the body of every class declaration.  A corresponding right brace, }, must end each class declaration. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. Declaring a Method public static void main( String[] args )  Starting point of every Java application.  Parentheses after the identifier main indicate that it’s a program building block called a method.  Java class declarations normally contain one or more methods.  main must be defined as shown; otherwise, the JVM will not execute the application.  Methods perform tasks and can return information when they complete their tasks.  Keyword void indicates that this method will not return any information. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Body of the method declaration  Enclosed in left and right braces.  Statement System.out.println("Welcome to Java Programming!");  Instructs the computer to perform an action  Display the characters contained between the double quotation marks.  Together, the quotation marks and the characters between them are a string—also known as a character string or a string literal.  White-space characters in strings are not ignored by the compiler.  Strings cannot span multiple lines of code. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  System.out object  Standard output object.  Allows a Java application to display information in the command window from which it executes.  System.out.println method  Displays (or prints) a line of text in the command window.  The string in the parentheses the argument to the method.  Positions the output cursor at the beginning of the next line in the command window.  Most statements end with a semicolon. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. Compiling Your First Java Application  Open a command window and change to the directory where the program is stored.  Many operating systems use the command cd to change directories.  To compile the program, type javac Welcome1.java  If the program contains no compilation errors, preceding command creates a.class file (known as the class file) containing the platform- independent Java bytecodes that represent the application.  When we use the java command to execute the application on a given platform, these bytecodes will be translated by the JVM into instructions that are understood by the underlying operating system. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. Executing the Welcome1 Application  To execute this program in a command window, change to the directory containing Welcome1.java—C:\examples\ch02\ fig02_01 on Microsoft Windows or ~/Documents/ examples/ch02/fig02_01 on Linux/OS X.  Next, type java Welcome1.  This launches the JVM, which loads the Welcome1.class file.  The command omits the.class file-name extension; otherwise, the JVM will not execute the program.  The JVM calls class Welcome1’s main method. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Class Welcome2, shown in Fig. 2.3, uses two statements to produce the same output as that shown in Fig. 2.1.  New and key features in each code listing are highlighted.  System.out’s method print displays a string.  Unlike println, print does not position the output cursor at the beginning of the next line in the command window.  The next character the program displays will appear immediately after the last character that print displays. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Newline characters indicate to System.out’s print and println methods when to position the output cursor at the beginning of the next line in the command window.  Newline characters are whitespace characters.  The backslash (\) is called an escape character.  Indicates a “special character”  Backslash is combined with the next character to form an escape sequence—\n represents the newline character.  Complete list of escape sequences http://docs.oracle.com/javase/specs/jls/se7/html/ jls-3.html#jls-3.10.6. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  System.out.printf method  f means “formatted”  displays formatted data  Multiple method arguments are placed in a comma-separated list.  Calling a method is also referred to as invoking a method.  Java allows large statements to be split over many lines.  Cannot split a statement in the middle of an identifier or string.  Method printf’s first argument is a format string  May consist of fixed text and format specifiers.  Fixed text is output as it would be by print or println.  Each format specifier is a placeholder for a value and specifies the type of data to output.  Format specifiers begin with a percent sign (%) and are followed by a character that represents the data type.  Format specifier %s is a placeholder for a string. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Integers  Whole numbers, like –22, 7, 0 and 1024)  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. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Helps the compiler locate a class that is used in this program.  Rich set of predefined classes that you can reuse rather than “reinventing the wheel.”  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).  You use import declarations to identify the predefined classes used in a Java program. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Variable declaration statement Scanner input = new Scanner( System.in );  Specifies the name (input) and type (Scanner) of a variable that is used in this program.  Variable  A location in the computer’s memory where a value can be stored for use later in a program.  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.  The name can be any valid identifier.  A variable’s type specifies what kind of information is stored at that location in memory. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Scanner  Enables a program to read data for use in a program.  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 equals sign (=) in a declaration indicates that the variable should be initialized (i.e., prepared for use in the program) with the result of the expression to the right of the equals sign.  The new keyword creates an object.  Standard input object, System.in, enables applications to read bytes of data typed by the user.  Scanner object translates these bytes into types that can be used in a program. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Variable declaration statements 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.  Range of values for an int is –2,147,483,648 to +2,147,483,647.  The int values you use in a program may not contain commas.  Several variables of the same type may be declared in one declaration with the variable names separated by commas. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Prompt  Output statement that directs the user to take a specific action.  Class System  Part of package java.lang.  Class System is not imported with an import declaration at the beginning of the program. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Scanner method nextInt number1 = input.nextInt(); // read first number from user  Obtains an integer from the user at the keyboard.  Program waits for the user to type the number and press the Enter key to submit the number to the program.  The result of the call to method nextInt is placed in variable number1 by using the assignment operator, =.  “number1 gets the value of input.nextInt().”  Operator = is called a binary operator—it has two operands.  Everything to the right of the assignment operator, =, is always evaluated before the assignment is performed. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Arithmetic sum = number1 + number2; // add numbers then store total in sum  Assignment statement that calculates the sum of the variables number1 and number2 then assigns the result to variable sum by using the assignment operator, =.  “sum gets the value of number1 + number2.”  Portions of statements that contain calculations are called expressions.  An expression is any portion of a statement that has a value associated with it. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Integer formatted output System.out.printf( "Sum is %d%n", sum );  Format specifier %d is a placeholder for an int value  The letter d stands for “decimal integer.” © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Variables  Every variable has a name, a type, a size (in bytes) and a value.  When a new value is placed into a variable, the new value replaces the previous value (if any)  The previous value is lost, so this process is said to be destructive. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Arithmetic operators are summarized in Fig. 2.11.  The asterisk (*) indicates multiplication  The percent sign (%) is the remainder operator  The arithmetic operators are binary operators because they each operate on two operands.  Integer division yields an integer quotient.  Any fractional part in integer division is simply truncated (i.e., discarded)—no rounding occurs.  The remainder operator, %, yields the remainder after division. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Arithmetic expressions in Java must be written in straight-line form to facilitate entering programs into the computer.  Expressions such as “a divided by b” must be written as a / b, so that all constants, variables and operators appear in a straight line.  Parentheses are used to group terms in expressions in the same manner as in algebraic expressions.  If an expression contains nested parentheses, the expression in the innermost set of parentheses is evaluated first. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Rules of operator precedence  Multiplication, division and remainder operations are applied first.  If an expression contains several such operations, they are applied from left to right.  Multiplication, division and remainder operators have the same level of precedence.  Addition and subtraction operations are applied next.  If an expression contains several such operations, the operators are applied from left to right.  Addition and subtraction operators have the same level of precedence.  When we say that operators are applied from left to right, we are referring to their associativity.  Some operators associate from right to left.  Complete precedence chart is included in Appendix A. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  As in algebra, it’s acceptable to place redundant parentheses (unnecessary parentheses) in an ex- pression to make the expression clearer. © Copyright 1992-2015 by Pearson Education, Inc. All Rights Reserved.  Condition  An expression that can be true or false.  if selection statement  Allows a program to make a decision based on a condition’s value.  Equality operators (== and !=)  Relational operators (>, = and