chapter 3: string

Chapter 3: String

String and String Method

Chapter Objectives

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String Class Commonly Used String Methods Parsing Numeric Strings

The class String

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Contains operations to manipulate strings. String:

Sequence of zero or more characters. Enclosed in double quotation marks. Is processed as a single unit . Null or empty strings have no characters. “ “ Every character has a relative position , the first

character is in position 0 .

Java Programming: From Problem Analysis to Program Design, Second Edition 4

The class String

Java system automatically makes the class String available (i.e no need to import this class )

Example :

Consider the following declaration :

String sentence ;

sentence = “programming with java”;

The class String

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Length of the string is the number of characters in it. When determining the length of a string, blanks

count. Example :

“ “ has length = 0 “abc” has length = 3, position of a = 0, b= 1, c= 2 “a boy” has length = 5

Strings and the Operator +

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Operator + can be used to concatenate two strings, or a string and a numeric value or character.

ExampleString str;int num1, num2;num1 = 12;num2 = 26;str = "The sum = " + num1 + num2;

After this statement executes, the string assigned to str is:"The sum = 1226";

Strings and the Operator +

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Consider the following statement:str = "The sum = " + (num1 + num2);

In this statement, because of the parentheses, you first evaluate num1 + num2.

Because num1 and num2 are both int variables, num1 + num2 = 12 + 26 = 38. After this statement executes, the string assigned to str is:

"The sum = 38";

Some Commonly Used String Methods

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Suppose that

String sentence = "Programming with Java";. Then each character in sentence and its

position is as follows:

charAt (index):

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method header how to use it

char charAt (int index)

char ch=sentence.charAt(3) ;System.out.println (sentence.charAt(4+2));

Returns the character at the position specified by

index

note :index should be >= 0 and <length

indexOf (ch):

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int indexOf (char ch) int i=sentence.indexOf('J');char ch ='a';System.out.println (sentence.indexOf(ch));System.out.println (sentence.indexOf('p'));

Returns the index of the first occurrence of the character specified by ch; If the character specified by ch does not appear in the string, it returns –1

indexOf (ch,pos):

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int indexOf (char ch,int pos)

char ch ='a';System.out.println (sentence.indexOf(ch,10));System.out.println (sentence.indexOf('m',9));

Returns the index of the first occurrence of the character specified by ch; The parameter pos specifies where to begin the search; If the character specified by ch does not appear in the string, it returns –1

indexOf (str):

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int indexOf (String str) System.out.println (sentence.indexOf("with"));System.out.println (sentence.indexOf("ing"));System.out.println (sentence.indexOf("the"));

Returns the index of the first occurrence of the string specified by str; If the string specified by str does not appear in the string, it returns –1

indexOf (str,pos):

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int indexOf (String str,int pos) int i=sentence.indexOf("a", 10) ;int i=sentence.indexOf("Pr", 10) ;

Returns the ??Returns the index of the first occurrence of the String specified by str; The parameter pos specifies where to begin the search; If the string specified by str does not appear in the string, it returns -1

concat(str)

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String concat(String str) Stringn Str=sentence.concat(" is fun.");

Returns the string that is this string concatenated with str

Programming with Java is fun. Stringn Str=sentence + "is fun.";

length()

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int length() int len=sentence.length();

Returns the length of the string

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replace (ToBeReplaced,

ReplacedWith )

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String replace(char chR,char chRW )

String nStr=sentence.replace('a', '*') ;

Returns the string in which every occurrence of charToBeReplaced is replaced with charReplacedWith

Progr*mming with J*v*

substring (beginIndex,endIndex)

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String substring (int beginIndex, int endIndex);

String subStr=sentence.substring(5,11) ;

Returns the string which is a substring of this

string beginning at beginIndex until endIndex

– 1

amming

note :1. if we did not specify the endIndex, substring of this string beginning at beginIndex until the end of the string.2. beginIndex<endIndex

toLowerCase()

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String toLowerCase() String lowStr=sentence.toLowerCase() ;

Returns the string that is the same as this string,

except that all uppercase letters of this string are

replaced with their equivalent lowercase letters

programming with java

toUpperCase()

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String toUpperCase() String uppStr=sentence.toUpperCase() ;

Returns the string that is the same as this string,

except that all lowercase letters of this string are

replaced with their equivalent uppercase letters

PROGRAMMING WITH JAVA

equals(str)

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boolean equals(String str)

boolean b=sentence.equals ("programming with java") ;

Returns true if this string is same as str

False

compareTo(str)

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int compareTo(String str)

int i=sentence.compareTo ("programming with java") ;

Compares two strings character by characterReturns a negative value if this string is less than str Returns 0 if this string is same as strReturns a positive value if this string is greater than str

Java Programming: From Problem Analysis to Program Design, Second Edition

Examples on string methods

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String s1 , s2 , s3 ;

s1 = “abcdefeg” ;

System.out.println( s1.length() ); // 8

System.out.println(s1.charAt(3)); //d

System.out.println(s1.indexOf(‘e’)); //4

System.out.println(s1.indexOf(“cd”)); //2

System.out.println(s1.toUpperCase()); //ABCDEFEG


Examples on string methods

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System.out.println(s1.substring(1 , 4)); //bcdSystem.out.println(s1 + “xyz”); // abcdefegxyzSystem.out.println( s1.replace(‘d’ ,’D’)); // abcDefegSystem.out.println(s1.charAt(4) ); // eSystem.out.println(s1.indexOf(‘b’)); // 1System.out.println(s1.indexOf(‘e’,5)); // 6


Parsing Numeric Strings

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Integer, Float, and Double are classes designed to convert a numeric string into a number.

These classes are called wrapper classes. parseInt is a method of the class Integer, which

converts a numeric integer string into a value of the type int.

parseFloat is a method of the class Float and is used to convert a numeric decimal string into an equivalent value of the type float.

parseDouble is a method of the class Double, which is used to convert a numeric decimal string into an equivalent value of the type double.



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A string consisting of only integers or decimal numbers is called a numeric string.

To convert a string consisting of an integer to a value of the type int, we use the following expression:

Integer.parseInt(strExpression)• Example:

int i=Integer.parseInt("6723"); --> 6723int j=Integer.parseInt("-823");--> -823



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To convert a string consisting of a decimal number to a value of the type float, we use the following expression:

Float.parseFloat(strExpression)• Example:

float f1=Float.parseFloat("34.56"); -> 34.56float f2=Float.parseFloat("-542.97"); -> -542.97

To convert a string consisting of a decimal number to a value of the type double, we use the following expression:

Double.parseDouble(strExpression)• Example:

double y=Double.parseDouble("345.78");--> 345.78double z=Double.parseDouble("-782.873");--> -

782.873

What is the value of each variable

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String sentence;

sentence="Now is the time for the birthday party.";

int index = sentence.indexOf("birthday");

String str1 = sentence.substring(index, index + 14); int len =sentence.length() ;

chapter 3: string

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