©2004 Brooks/Cole Chapter 7 Strings and Characters

©2004 Brooks/Cole

Chapter 7

Strings and Characters

Figures ©2004 Brooks/ColeCS 119: Intro to Java Fall 2005

A String is a Sequence of Characters

• Literal strings are enclosed in double quotes – "hello"

• Strings are objects that belong to the String class

• Each character in the string has an index– first character has index 0


Strings are Objects

• Variables which refer to Strings are reference variables– Variable memory stores the address of the

String object– Data (character sequence) is stored in a

different part of memory


Creating Strings

• Declaring a String Variable String message;

• A literal string is created automatically.– Assignment stores the address in a variable.message = "hello";

• A String object can be created with new• This is known as instantiationmessage = new String("hello");• The String class has several constructors

(overloaded)


String Creation

String string1 = new String("Hello");

String string2 = new String("Hello there");


String Concatenation

• We can join two strings into a larger string using the + operatorString s1 = "Hot ";

String s2 = "Dog";

String s3 = s1 + s2;


Concatenation creates a new String

String message = "Start";

message = message + "le";


Strings are immutable

• Once created, a String object cannot be modified– String is created with a certain amount of

memory

• All the String methods that appear to modify a String actually return a new String

• There is a StringBuffer class that is mutable


String I/O

• We have used print and println to output Strings to the console

• We use the next and nextLine methods of the Scanner class to read String data from the keyboard

• JOptionPane.showInputDialog also reads String data


String Methods

• length() returns the number of characters in the string

• concat() can be used (like +) to concatenate strings

• equals() can be used to check if two strings contain the same sequence of characters

• compareTo() indicates whether one string comes before another


Comparing Strings for equality

• The == operator checks whether the same value is stored in two different variables.

• For reference variables, what is compared is two addresses

• To compare whether two strings have the same sequence of characters stored in them, use the equals method.


Comparing with ==

• s1==s2 is true• s1==s3 is false• s1==s4 is true• s2==s3 is false• s2==s4 is true• s2==s4 is false

String s1, s2, s3, s4;

s1 = "java";

s2 = "java";

s3 = new String("java");

s4 = s1;


Different Reference Variables Can Reference Equal Strings

• Comparisonss1==s2 is false

s1.equals(s2) is true

String s1 = new String( "Help");

String s2 = new String( "Help");


Comparing Strings with compareTo

• compareTo does a character by character comparison– uses unicode values to

determine order– case-sensitive

• s1.CompareTo(s2) is– negative if s1 comes before

s2– 0 if s1 and s2 have the same

sequence of characters– positive if s1 comes after s2

• Examples"Hello" comes after "Goodbye"

"hello" comes after "Hello"

"SMITH" comes after "JONES"

"123" comes after "1227"

"Beehive" comes before "Behop"

"123" comes before "abc"


More String methods

• indexOf( char ch), indexOf( String str) return the position of a character or sequence of characters in the String– Returns -1 if absent

– Overloaded versions of indexOf and lastIndexOf

• toUpperCase(), toLowerCase() return strings with all letters of the same case


Substrings

• substring( int start, int next) returns a String containing the characters with indexes start through next-1 in the original String– "appaloosa".substring( 2, 5) ---> "pal"– "Helpless".substring( 0, 4) ---> "Help"– "Helpless".substring( 4) ---> "less"


Character Class

• Like the other primitive types, there is a wrapper class, Character, for the char type– Can be used to create objects with a single char

value for data – Has class methods useful for handling character

data


Character Methods

• Boolean methods that test whether a character belongs to a particular group of characters– isDigit, isLetter, isWhitespace, isUpperCase,

isLowerCase

• Methods to convert characters from one case to another– toUpperCase, toLowerCase


Converting Primitive Values to Strings

• String class has valueOf method which is overloaded to work for any primitive type– String valueOf( boolean b)– String valueOf( int I)– String valueOf( double d)– String valueOf( char c)

• Wrapper classes have a static toString method– String toString( boolean b) in the

Boolean class for example


Converting Strings to Primitive Values

• Wrapper class have methods to convert a String to a primitive value– Integer.parseInt( String intString)– Integer.valueOf( String intString)

• If the string is not appropriate for the primitive type, an error will occur


StringBuffer Class

• A StringBuffer is very similar to a String except that a StringBuffer object can be modified

• This means you can add, insert or replace characters within the StringBuffer rather than having to create a new object for each change.

• A StringBuffer is created with a capacity which is the number of characters it can hold. – The capacity can change if necessary


Creating a StringBuffer

• StringBuffer() creates a StringBuffer with a capacity of 16

• StringBuffer(int length) creates a StringBuffer with a capacity of length

• StringBuffer( String str) creates a StringBuffer with a capacity of str.length() + 16


Initial Storage of a StringBuffer Object

StringBuffer str = new StringBuffer( "This cannot be");


Modifying the StringBuffer

str.insert( 4, " I know");

str.replace( 12, 18, "to");


The StringBuffer After the Append

str.append( "correct");

Documents

©2004 Brooks/Cole Chapter 7 Strings and Characters