Transcript CSC110_Programming_5_StringObject(1)

String and Scanner Objects
CSC 110 – INTRO TO
COMPUTING - PROGRAMMING
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The String Class
• Java has no primitive data type that holds a series of
characters.
• The String class from the Java standard library is
used for this purpose.
• In order to be useful, the a variable must be created to
reference a String object.
String number;
• Notice the S in String is upper case.
• By convention, class names should always begin with
an upper case character.
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Primitive vs. Reference Variables
• Primitive variables actually contain the value
that they have been assigned.
number = 25;
• The value 25 will be stored in the memory
location associated with the variable number.
• Objects are not stored in variables, however.
Objects are referenced by variables.
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Primitive vs. Reference Variables
• When a variable references an object, it contains the
memory address of the object’s location.
• Then it is said that the variable references the object.
String cityName = "Charleston";
The object that contains the
character string “Charleston”
cityName
Address to the object
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Charleston
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String Objects
• A variable can be assigned a String literal.
String value = "Hello";
• Strings are the only objects that can be created in
this way.
• A variable can be created using the new keyword.
String value = new String("Hello");
• This is the method that all other objects must use when
they are created.
See example: StringDemo.java
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StringDemo.java
// A simple program demonstrating String objects.
public class StringDemo
{
public static void main(String[] args)
{
String greeting = "Good morning, ";
String name = "Herman";
System.out.println(greeting + name);
}
}
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The String Methods
• Since String is a class, objects that are
instances of it have methods.
• One of those methods is the length method.
stringSize = value.length();
• This statement runs the length method on the
object pointed to by the value variable.
See example: StringLength.java
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StringLength.java
// This program demonstrates the String class's length method.
public class StringLength
{
public static void main(String[] args)
{
String name = "Herman";
int stringSize;
stringSize = name.length();
System.out.println(name + " has " + stringSize + " characters.");
}
}
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String Methods
• The String class contains many methods that
help with the manipulation of String objects.
• String objects are immutable, meaning that
they cannot be changed.
• Many of the methods of a String object can
create new versions of the object.
See example: StringMethods.java
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StringMethods.java
// This program demonstrates a few of the String methods.
public class StringMethods
{
public static void main(String[] args)
{
String message = "Java is Great Fun!";
String upper = message.toUpperCase();
String lower = message.toLowerCase();
char letter = message.charAt(2);
int stringSize = message.length();
System.out.println(message);
System.out.println(upper);
System.out.println(lower);
System.out.println(letter);
System.out.println(stringSize);
}
}
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Scope
• Scope refers to the part of a program that has
access to a variable’s contents.
• Variables declared inside a method (like the
main method) are called local variables.
• Local variables’ scope begins at the declaration
of the variable and ends at the end of the
method in which it was declared.
See example: Scope.java (This program contains
an intentional error.)
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Scope.java
// This program can't find its variable.
public class Scope
{
public static void main(String[] args)
{
System.out.println(value); // ERROR!
int value = 100;
}
}
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Commenting Code
• Java provides three methods for commenting
code.
Comment
Style
Description
//
Single line comment. Anything after the // on the line will be
ignored by the compiler.
/* … */
Block comment. Everything beginning with /* and ending with
the first */ will be ignored by the compiler. This comment type
cannot be nested.
/** … */
Javadoc comment. This is a special version of the previous block
comment that allows comments to be documented by the javadoc
utility program. Everything beginning with the /** and ending
with the first */ will be ignored by the compiler. This comment
type cannot be nested.
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Commenting Code
• Javadoc comments can be built into HTML
documentation.
• See example: Comment3.java
• To create the documentation:
– Run the javadoc program with the source file as an
argument
– Ex: javadoc Comment3.java
• The javadoc program will create index.html
and several other documentation files in the same
directory as the input file.
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Comment3.java
/**
This class creates a program that calculates company payroll.
*/
public class Comment3
{
/**
The main method is the program's starting point.
*/
public static void main(String[] args)
{
double payRate;
// Holds the hourly pay rate
double hours;
// Hours holds the hours worked
int employeeNumber; // Holds the employee number
// The Remainder of This Program is Omitted.
}
}
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Commenting Code
• Example index.html:
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Programming Style
• Although Java has a strict syntax, whitespace
characters are ignored by the compiler.
• The Java whitespace characters are:
–
–
–
–
–
space
tab
newline
carriage return
form feed
See example: Compact.java
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Compact.java
public class Compact {public static void main(String[] args){int
shares=220; double averagePrice=14.67; System.out.println(
"There were "+shares+" shares sold at $"+averagePrice+
" per share.");}}
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Indentation
• Programs should use proper indentation.
• Each block of code should be indented a few spaces
from its surrounding block.
• Two to four spaces are sufficient.
• Tab characters should be avoided.
– Tabs can vary in size between applications and devices.
– Most programming text editors allow the user to replace the
tab with spaces.
See example: Readable.java
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Readable.java
/**
This example is much more readable than Compact.java.
*/
public class Readable
{
public static void main(String[] args)
{
int shares = 220;
double averagePrice = 14.67;
System.out.println("There were " + shares +
" shares sold at $" +
averagePrice + " per share.");
}
}
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The Scanner Class
• To read input from the keyboard we can use the
Scanner class.
• The Scanner class is defined in java.util, so we
will use the following statement at the top of our
programs:
import java.util.Scanner;
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The Scanner Class
• Scanner objects work with System.in
• To create a Scanner object:
Scanner keyboard = new Scanner (System.in);
• Scanner class methods are listed in Table 218 in the text.
• See example: Payroll.java
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Payroll.java
import java.util.Scanner; // Needed for the Scanner class
/**
This program demonstrates the Scanner class.
*/
public class Payroll
{
public static void main(String[] args)
{
String name;
// To hold a name
int hours;
// Hours worked
double payRate; // Hourly pay rate
double grossPay; // Gross pay
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Payroll.java (cont)
// Create a Scanner object to read input.
Scanner keyboard = new Scanner(System.in);
// Get the user's name.
System.out.print("What is your name? ");
name = keyboard.nextLine();
// Get the number of hours worked this week.
System.out.print("How many hours did you work this week? ");
hours = keyboard.nextInt();
// Get the user's hourly pay rate.
System.out.print("What is your hourly pay rate? ");
payRate = keyboard.nextDouble();
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Payroll.java (cont)
// Calculate the gross pay.
grossPay = hours * payRate;
// Display the resulting information.
System.out.println("Hello, " + name);
System.out.println("Your gross pay is $" + grossPay);
}
}
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