Java Programming Fifth Edition: Characters, Strings, and StringBuilder - PDF

Java Programming Fifth Edition Characters, Strings, and the StringBuilder  Objectives Identify problems that can occur when you manipulate string data Manipulate characters Declare a String object Compare String values Use other String methods...

Java Programming Fifth Edition Characters, Strings, and the StringBuilder  Objectives Identify problems that can occur when you manipulate string data Manipulate characters Declare a String object Compare String values Use other String methods Java Programming, Fifth Edition 2  Objectives (continued) Convert Strings to numbers Learn about the StringBuilder and StringBuffer classes Java Programming, Fifth Edition 3  Identifying Problems That Can Occur When You Manipulate String Data Manipulating characters and groups of characters – Provides some challenges for beginning Java programmer String is a class – Each created String class object – String variable name not simple data type – Reference Variable that holds memory address Java Programming, Fifth Edition 4  Identifying Problems That Can Occur When You Manipulate String Data (continued) Compare two Strings using the == operator – Not comparing values – Comparing computer memory locations Compare contents of memory locations more frequently than memory locations themselves Java Programming, Fifth Edition 5  Identifying Problems That Can Occur When You Manipulate String Data (continued) Java Programming, Fifth Edition 6  Identifying Problems That Can Occur When You Manipulate String Data (continued) Classes to use when working with character data – Character Instances hold single character value Defines methods that can manipulate or inspect single-character data – String Class for working with fixed-string data – Unchanging data composed of multiple characters Java Programming, Fifth Edition 7  Identifying Problems That Can Occur When You Manipulate String Data (continued) Classes to use when working with character data (continued) – StringBuilder and StringBuffer Class for storing and manipulating changeable data composed of multiple characters Java Programming, Fifth Edition 8  Manipulating Characters Character class – Contains standard methods for testing values of characters – Methods that begin with “is” Such as isUpperCase() Return Boolean value Can be used in comparison statements – Methods that begin with “to” Such as toUpperCase() Return character that has been converted to stated format Java Programming, Fifth Edition 9  Manipulating Characters (continued) Java Programming, Fifth Edition 10 Java Programming, Fifth Edition 11  Declaring a String Object Literal string – Sequence of characters enclosed within double quotation marks – Unnamed object, or anonymous object of String class String variable – Named object of String class Class String – Defined in java.lang.String – Automatically imported into every program Java Programming, Fifth Edition 12  Declaring a String Object (continued) Declare String variable – String itself is distinct from variable used to refer to it Create String object String aGreeting = new String("Hello"); String aGreeting = "Hello"; – Can create String object without: Using keyword new Explicitly calling class constructor Java Programming, Fifth Edition 13  Comparing String Values String is a class – Each created String is a class object String variable name – Reference variable – Refers to location in memory Rather than to particular value Assign new value to String – Address held by String altered Java Programming, Fifth Edition 14  Comparing String Values (continued) Java Programming, Fifth Edition 15  Comparing String Values (continued) Immutable – Objects that cannot be changed – Such as String Making simple comparisons between Strings – Often produces misleading results – Compare Strings with == operator Compare memory addresses Not values Java Programming, Fifth Edition 16  Comparing String Values (continued) equals() method – Evaluates contents of two String objects to determine if they are equivalent – Returns true if objects have identical contents – public boolean equals(String s) equalsIgnoreCase() method – Ignores case when determining if two Strings equivalent – Useful when users type responses to prompts in programs Java Programming, Fifth Edition 17  Comparing String Values (continued) Java Programming, Fifth Edition 18  Comparing String Values (continued) compareTo() method – Compares two Strings and returns: Zero – Only if two Strings refer to same value Negative number – If calling object “less than” argument Positive number – If calling object “more than” argument if (aWord.compareTo(anotherWord) < 0) Java Programming, Fifth Edition 19  Using Other String Methods toUpperCase() and toLowerCase() – Convert any String to uppercase or lowercase equivalent length() method – Returns length of String Java Programming, Fifth Edition 20  Using Other String Methods (continued) indexOf() method – Determines whether specific character occurs within String – Returns position of character – First position of String begins with zero – Return value is –1 If character does not exist in String Java Programming, Fifth Edition 21  Using Other String Methods (continued) charAt() method – Requires integer argument – Indicates position of character that method returns endsWith() method and startsWith() method – Take String argument – Return true or false if String object does or does not end or start with specified argument Java Programming, Fifth Edition 22  Using Other String Methods (continued) replace() method – Replace all occurrences of some character within String toString() method – Not part of String class – Converts any object to String – Converts primitive data types to Strings String theString; int someInt = 4; theString = Integer.toString(someInt); Java Programming, Fifth Edition 23  Using Other String Methods (continued) Concatenation – Join simple variable to String String aString = "My age is " + myAge; – Plus sign (+) Java Programming, Fifth Edition 24  Using Other String Methods (continued) substring() method – Extract part of String – Takes two integer arguments Start position End position – Length of extracted substring Difference between second integer and first integer Java Programming, Fifth Edition 25 Java Programming, Fifth Edition 26  Converting Strings to Numbers Integer class – Part of java.lang – Automatically imported into programs – Convert String to integer – parseInt() method Takes String argument Returns its integer value Wrapper – Class or object “wrapped around” simpler element Java Programming, Fifth Edition 27  Converting Strings to Numbers (continued) Integer class valueOf() method – Convert String to Integer class object Integer class intValue() method – Extract simple integer from wrapper class Double class – Wrapper class – Imported into programs automatically – parseDouble() method Takes String argument Returns its double value Java Programming, Fifth Edition 28  Converting Strings to Numbers (continued) Java Programming, Fifth Edition 29  Learning About the StringBuilder and StringBuffer Classes Value of String fixed – After String created is immutable StringBuilder, StringBuffer classes – Alternative to String class – When String will be modified – Can use anywhere you would use String – Part of java.lang package – Automatically imported into every program Java Programming, Fifth Edition 30  Learning About the StringBuilder and StringBuffer Classes (continued) StringBuilder – More efficient StringBuffer – Thread safe – Use in multithreaded programs Java Programming, Fifth Edition 31  Learning About the StringBuilder and StringBuffer Classes (continued) Create StringBuilder object StringBuilder eventString = new StringBuilder ("Hello there"); – Must use: Keyword new Constructor name Initializing value between constructor’s parentheses Java Programming, Fifth Edition 32  Learning About the StringBuilder and StringBuffer Classes (continued) Buffer – Memory block – Might or might not contain String – String might not occupy entire buffer Length of String can be different from length of buffer – Capacity Actual length of buffer Java Programming, Fifth Edition 33  Learning About the StringBuilder and StringBuffer Classes (continued) setLength() method – Change length of String in StringBuilder object length property – Attribute of StringBuilder class – Identifies number of characters in String contained in StringBuilder capacity() method – Find capacity of StringBuilder object Java Programming, Fifth Edition 34 Java Programming, Fifth Edition 35  Learning About the StringBuilder and StringBuffer Classes (continued) Using StringBuilder objects – Provides improved computer performance over String objects – Can insert or append new contents into StringBuilder StringBuilder constructors public StringBuilder () public StringBuilder (int capacity) public StringBuilder (String s) Java Programming, Fifth Edition 36  Learning About the StringBuilder and StringBuffer Classes (continued) append() method – Adds characters to end of StringBuilder object insert() method – Adds characters at specific location within StringBuilder object setCharAt() method – Changes character at specified position within StringBuilder object Java Programming, Fifth Edition 37  Learning About the StringBuilder and StringBuffer Classes (continued) charAt() method – Accepts argument that is offset of character position from beginning of String – Returns character at that position Java Programming, Fifth Edition 38 Java Programming, Fifth Edition 39  You Do It Using String class methods Converting a String to an integer Using StringBuilder methods Java Programming, Fifth Edition 40  Don’t Do It Don’t attempt to compare Strings using the standard comparison operators Don’t forget to use the new operator and the constructor when declaring initialized StringBuilder objects Java Programming, Fifth Edition 41  Summary String variables – References Character class – Instances can hold single character value Each String class object – Immutable – equals() method toString() method – Converts any object to String Java Programming, Fifth Edition 42  Summary (continued) Integer.parseInt() method – Takes String argument – Returns integer value Double.parseDouble() method – Takes String argument – Returns double value StringBuilder class – Improves performance when string’s contents must change Java Programming, Fifth Edition 43