9 Copyright © 2005, Oracle. All rights reserved. Using Strings, String Buffer, Wrapper, and Text-Formatting Classes

9Copyright © 2005, Oracle. All rights reserved.

Using Strings, String Buffer, Wrapper, and Text-Formatting Classes

9-2 Copyright © 2005, Oracle. All rights reserved.

Objectives

After completing this lesson, you should be able to do the following:

• Create strings in Java

• Use the conversion methods that are provided by the predefined wrapper classes

• Use the StringBuffer class for manipulating character data

• Introduce the DateFormat, DecimalFormat, and MessageFormat classes

• Examine standard output and serialization


What Is a String?

• String is a class.

• A String object holds a sequence of characters.

• String objects are read-only (immutable); their values cannot be changed after creation.

• The String class represents all strings in Java.


Creating a String

• Assign a double-quoted constant to a String variable:

• Concatenate other strings:

• Use a constructor:

String category = "Action";

String empName = firstName + " " + lastName;

String empName = new String(“Bob Smith");


Concatenating Strings

• Use the + operator to concatenate strings.

• You can concatenate primitives and strings.

• The String class has a concat() instance method that can be used to concatenate strings.

System.out.println("Name = " + empName);

int age = getAge();

System.out.println("Age = " + age);


Performing Operations on Strings

• Find the length of a string:

• Find the character at a specific index:

• Return a substring of a string:

int length(); String str = "Comedy";

int len = str.length();

char charAt(int index); String str = "Comedy";

char c = str.charAt(1);

String substring

(int beginIndex,

int endIndex);

String str = "Comedy";

String sub =

str.substring(2,4);


Performing More Operations on Strings

• Convert to uppercase or lowercase:

• Trim whitespace:

• Find the index of a substring:

String toUpperCase();

String toLowerCase();

String caps =

str.toUpperCase();

String trim(); String nospaces =

str.trim();

int indexOf (String str);

int lastIndexOf

(String str);

int index =

str.indexOf("me");


Comparing String Objects

• Use equals()if you want case to count:

• Use equalsIgnoreCase()if you want to ignore case:

• Do not use ==.

String passwd = connection.getPassword();

if (passwd.equals("fgHPUw"))… // Case is important

String cat = getCategory();

if (cat.equalsIgnoreCase("Drama"))…

// We just want the word to match


Producing Strings from Other Objects

• Use the Object.toString() method.

• Your class can override toString().

• System.out.println() automatically calls an object’s toString() method if a reference is passed to it.

public Class Movie {…

public String toString () {

return name + " (" + Year + ")";

}…

Movie mov = new Movie(…);

System.out.println(mov);


Producing Strings from Primitives

• Use String.valueOf():

• There is a version of System.out.println()for each primitive type:

String seven = String.valueOf(7);

String onePoint0 = String.valueOf(1.0f);

int count;

…

System.out.println(count);


Producing Primitives from Strings

• Use the primitive wrapper classes.

• There is one wrapper class for each primitive type:– Integer wraps the int type.– Float wraps the float type.– Character wraps the char type.– Boolean wraps the boolean type.– And so on…

• Wrapper classes provide methods to convert a String to a primitive, and primitives to a String.


Wrapper Class Conversion Methods

Example: Use the methods to process data from fields as they are declared.

String qtyVal = "17";

String priceVal = "425.00";

int qty = Integer.parseInt(qtyVal);

float price = Float.parseFloat(priceVal);

float itemTotal = qty * price;


Changing the Contents of a String

• Use the StringBuffer class for modifiable strings of characters:

• Use StringBuffer if you need to keep adding characters to a string.

Note: StringBuffer has a reverse() method.

public String reverseIt(String s) {

StringBuffer sb = new StringBuffer();

for (int i = s.length() - 1; i >= 0; i--)

sb.append(s.charAt(i));

return sb.toString();

}


Formatting Classes

The java.text package contains:

• An abstract class called Format with the format () method shown in the following example:

• Classes that format locale-sensitive information such as dates, numbers, and messages– DateFormat, NumberFormat, and MessageFormat

public abstract class Format … { public final String format(Object obj){ //Formats an object and produces a string. } …}


Using the SimpleDateFormat Class

The SimpleDateFormat:

• Is a concrete class for formatting and parsing dates in a locale-sensitive manner

• Allows you to start by choosing any user-defined patterns for date–time formatting

• Uses time-pattern string to display the date:– y year 1996– M month in year July or 07– m minute in hour 30


Using the MessageFormat Class

The MessageFormat:

• Is a concrete class for constructing language neutral messages, displayed for end users

• Takes a set of objects, formats them, and then inserts the formatted strings into the pattern at the appropriate places

• Differs from other Format classes, in that you create a MessageFormat object

• Is typically set dynamically at run time


Using DecimalFormat

The DecimalFormat:

• Is a concrete subclass of NumberFormat for formatting decimal numbers

• Allows for a variety of parameters and for localization to Western, Arabic, or Indic numbers

• Uses standard number notation in format

public DecimalFormat(String pattern);


Guided Practice

1. What is the output of each code fragment?a.

b.

String s = new String("Friday");

if(s == "Friday")

System.out.println("Equal A");

if(s.equals("Friday"))

System.out.println("Equal B");

int num = 1234567;

System.out.println(String.valueOf(num).charAt(3));


Guided Practice

2. What is the output of each code fragment?a.

b.

String s1 = "Monday";

String s2 = "Tuesday";

System.out.println(s1.concat(s2).substring(4,8));

// s3 begins with 2 spaces and ends with 2 spaces

String s3 = " Monday ";

System.out.println(s3.indexOf("day"));

System.out.println(s3.trim().indexOf("day"));


Using Regular Expressions

• Matches character sequences against patterns specified by regular expressions

• Includes a Matcher class which is the engine that performs match operations

• Employs a Pattern class to provide a compiled representation of a regular expression

Pattern p = Pattern.compile("a*b");

Matcher m = p.matcher("aaaaab");

boolean b = m.matches();


About System.out.println

Understanding System.out.println()• System is a class in the java.lang package.

• out is a public final static (class) variable.– Declared as a PrintStream object reference

• println() is an overloaded method of the PrintStream class.– PrintStream is a FilterOutputStream that

subclasses OutputStream.

• System.err is also provided as a PrintStream object reference to write to standard error.


About OutputStream and PrintStream

• OutputStream provides basic byte I/O operations:– write(int b) to write one byte– write(byte[] b) to write an array of bytes– write(byte[] b,int off,int len) to write a

subset of an array of bytes– flush() and close() to flush and close the stream

• PrintStream is a subclass of (Filter)Output Stream, which– Converts Unicode to environment byte encoding– Terminates lines in a platform-independent way– Flushes the output stream


What Is Object Serialization?

Serialization is a lightweight persistence mechanism for saving and restoring streams of bytes containing primitives and objects.

• A class indicates that its instances can be serialized by:– Implementing java.io.Serializable or

java.io.Externalizable interface– Ensuring that all its fields are serializable, including

other objects referenced– Using the transient modifier to prevent fields

from being saved and restored


Serialization Streams, Interfaces,and Modifiers

• Example of implementing java.io.Serializable– Mark fields with the transient modifier to prevent

them from being saved; that is, to protect the information.

• Write object with java.io.ObjectOutputStream.

• Read object with java.io.ObjectInputStream.

import java.io.Serializable;public class Member implements Serializable { private int id; private String name; private transient String password; …}


Summary

In this lesson, you should have learned how to:

• Create strings in Java

• Use the conversion methods that are provided by the predefined wrapper classes

• Use the StringBuffer class for manipulating character data

• Manipulate objects by using the DateFormat, DecimalFormat, and MessageFormat classes


Practice 9: Overview

This practice covers:

• Creating a new Order class

• Populating and formatting orderDate• Formatting existing orderDate values with the

GregorianCalendar class

• Formatting orderTotal

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9 Copyright © 2005, Oracle. All rights reserved. Using Strings, String Buffer, Wrapper, and Text-Formatting Classes