lecture 2 ( strings )

7/31/2019 Lecture 2 ( Strings )

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Strings and Text I/O


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Motivations

Often you encounter the problems that involve string

processing and file input and output. Suppose you need towrite a program to replace all occurrences of a word with a

new word in a file. How do you solve this problem? This

lecture introduces strings and text files, which will enable

you to solve this problem.


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Objectives To use the String class to process fixed strings.

To use the Character class to process a single character. To use the StringBuilder/StringBuffer class to process flexible

strings.

To learn how to pass arguments to the main method from thecommand line.

To discover file properties and to delete and rename files usingthe File class.

To write data to a file using the PrintWriter class.

To read data from a file using the Scanner class.


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The String Class Constructing a String:

String message = "Welcome to Java; String message = new String("Welcome to Java);

String s = new String();

Obtaining String length and Retrieving Individual Characters ina string

String Concatenation (concat)

Substrings (substring(index), substring(start, end))

Comparisons (equals, compareTo)

String Conversions Finding a Character or a Substring in a String

Conversions between Strings and Arrays

Converting Characters and Numeric Values to Strings


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Constructing Strings

String newString = new String(stringLiteral);

String message = new String("Welcome to Java");

Since strings are used frequently, Java provides a

shorthand initializer for creating a string:

String message = "Welcome to Java";


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Strings Are Immutable

A String object is immutable; its contents cannot be changed.

Does the following code change the contents of the string?

String s = "Java";

s = "HTML";


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Trace Code

String s = "Java";

s = "HTML";

: String

String object for "Java"

s

After executing String s = "Java"; After executing s = "HTML";

: String


: String

String object for "HTML"

Contents cannot be changed

This string object is

now unreferenceds

animation


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Trace Code

String s = "Java";

s = "HTML";

: String


s

After executing String s = "Java"; After executing s = "HTML";

: String


: String

String object for "HTML"

Contents cannot be changed

This string object is

now unreferenceds

animation


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Interned Strings

Since strings are immutable and are frequentlyused, to improve efficiency and save memory, the

JVM uses a unique instance for string literals with

the same character sequence. Such an instance is

called interned. For example, the following

statements:


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Examples

display

s1 == s2 is falses1 == s3 is true

A new object is created if you use thenew operator.

If you use the string initializer, no newobject is created if the interned object isalready created.

String s1 = "Welcome to Java";

String s2 = new String("Welcome to Java");


System.out.println("s1 == s2 is " + (s1 == s2));

System.out.println("s1 == s3 is " + (s1 == s3));

: String

Interned string object for

"Welcome to Java"

: String

A string object for

"Welcome to Java"

s1

s2

s3


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Trace CodeString s1 = "Welcome to Java";



: StringInterned string object for

"Welcome to Java"

s1

animation


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: String

Interned string object for"Welcome to Java"

: String

A string object for

"Welcome to Java"

s1

s2


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: String

Interned string object for"Welcome to Java"

: String

A string object for

"Welcome to Java"

s1

s2

s3


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String Comparisons

java.lang.String

+equals(s1: String): boolean

+equalsIgnoreCase(s1: String):

boolean

+compareTo(s1: String): int

+compareToIgnoreCase(s1: String):

int

+startsWith(prefix: String): boolean

+endsWith(suffix: String): boolean

Returns true if this string is equal to string s1.

Returns true if this string is equal to string s1 case-

insensitive.

Returns an integer greater than 0, equal to 0, or less than 0to indicate whether this string is greater than, equal to, or

less than s1.

Same as compareTo except that the comparison is case-

insensitive.

Returns true if this string starts with the specified prefix.

Returns true if this string ends with the specified suffix.


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String Comparisons

equals

String s1 = new String("Welcome);

String s2 = "welcome";

if (s1.equals(s2)){

// s1 and s2 have the same contents

}

if (s1 == s2) {

// s1 and s2 have the same reference

}


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String Comparisons, cont.

compareTo(Object object)

String s1 = new String("Welcome);

String s2 = "welcome";

if (s1.compareTo(s2) > 0) {

// s1 is greater than s2

}

else if (s1.compareTo(s2) == 0) {

// s1 and s2 have the same contents

}

else

// s1 is less than s2


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String Length, Characters, and

Combining Strings

java.lang.String

+length(): int

+charAt(index: int): char

+concat(s1: String): String

Returns the number of characters in this string.

Returns the character at the specified index from this string.

Returns a new string that concatenate this string with string s1.


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Finding String Length

Finding string length using the length()method:

message = "Welcome";

message.length() (returns 7)


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Retrieving Individual Characters

in a StringDo not use message[0]

Use message.charAt(index) Index starts from 0

W e l c o m e t o J a v a

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

message

Indices

message.charAt(0) message.charAt(14)message.length() is 15


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String Concatenation

String s3 = s1.concat(s2);

String s3 = s1 + s2;

s1 + s2 + s3 + s4 + s5 same as

(((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5);


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Extracting Substrings

java.lang.String

+subString(beginIndex: int):

String

+subString(beginIndex: int,endIndex: int): String

Returns this strings substring that begins with the character at the

specified beginIndex and extends to the end of the string

.

Returns this strings substring that begins at the specifiedbeginIndex and extends to the character at index endIndex1.

Note that the character at endIndex is not part of the substring.


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Extracting Substrings

You can extract a single character from a string using thecharAt method. You can also extract a substring from astring using the substring method in the String class.

String s1 = "Welcome to Java";String s2 = s1.substring(0, 11) + "HTML";

W e l c o m e t o J a v a

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

message

Indices

message.substring(0, 11) message.substring(11)


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Converting, Replacing, and Splitting

Strings

java.lang.String

+toLowerCase(): String

+toUpperCase(): String

+trim(): String+replace(oldChar: char,

newChar: char): String

+replaceFirst(oldString: String,

newString: String): String

+replaceAll(oldString: String,

newString: String): String

+split(delimiter: String):

String[]

Returns a new string with all characters converted to lowercase.

Returns a new string with all characters converted to uppercase.

Returns a new string with blank characters trimmed on both sides.Returns a new string that replaces all matching character in this

string with the new character.

Returns a new string that replaces the first matching substring in

this string with the new substring.

Returns a new string that replace all matching substrings in this

string with the new substring.

Returns an array of strings consisting of the substrings split by the

delimiter.


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Examples

"Welcome".toLowerCase() returns a new string, welcome."Welcome".toUpperCase() returns a new string,WELCOME." Welcome ".trim() returns a new string, Welcome."Welcome".replace('e', 'A') returns a new string, WAlcomA."Welcome".replaceFirst("e", "AB") returns a new string,

WABlcome."Welcome".replace("e", "AB") returns a new string,WABlcomAB."Welcome".replace("el", "AB") returns a new string,

WABlcome.


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Splitting a String

String[] tokens = "Java#HTML#Perl".split("#", 0);for (int i = 0; i < tokens.length; i++)

System.out.print(tokens[i] + " ");

Java HTML Perldisplays


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Matching, Replacing and Splitting by PatternsYou can match, replace, or split a string by specifying a pattern.

This is an extremely useful and powerful feature, commonlyknown as regular expression. Regular expression is complex tobeginning students. For this reason, two simple patterns areused in this section.

"Java".matches("Java");

"Java".equals("Java");

"Java is fun".matches("Java.*");

"Java is cool".matches("Java.*");


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Matching, Replacing and Splitting by Patterns

The replaceAll, replaceFirst, and split methods can be used witha regular expression. For example, the following statement

returns a new string that replaces $, +, or # in "a+b$#c" by the

string NNN.String s = "a+b$#c".replaceAll("[$+#]", "NNN");

System.out.println(s);

Here the regular expression [$+#] specifies a pattern that

matches $, +, or #. So, the output is aNNNbNNNNNNc.


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Matching, Replacing and Splitting by Patterns

The following statement splits the string into an array of stringsdelimited by some punctuation marks.

String[] tokens = "Java,C?C#,C++".split("[.,:;?],4);

for (int i = 0; i < tokens.length; i++)

System.out.println(tokens[i]);

Output:Java

C

C#

C++


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Finding a Character or a Substring

in a String

java.lang.String

+indexOf(ch: char): int

+indexOf(ch: char, fromIndex:int): int

+indexOf(s: String): int

+lastIndexOf(ch: int): int

+lastIndexOf(s: String): int

Returns the index of the first occurrence of ch in the string.

Returns -1 if not matched.

Returns the index of the first occurrence of ch after fromIndex inthe string. Returns -1 if not matched.

Returns the index of the first occurrence of string s in this string.


Returns the index of the last occurrence of ch in the string.


Returns the index of the last occurrence of string s. Returns -1 if

not matched.


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Finding a Character or a

Substring in a String

"Welcome to Java".indexOf('W')returns 0.

"Welcome to Java".indexOf('x')returns -1.

"Welcome to Java".indexOf('o', 5)returns 9.

"Welcome to Java".indexOf("come")returns 3.

"Welcome to Java".indexOf("Java", 5)returns 11.

"Welcome to Java".indexOf("java", 5)returns -1.

"Welcome to Java".lastIndexOf('a')

returns 14.


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Convert Character and Numbers

to Strings

The String class provides several static valueOf

methods for converting a character, an array of

characters, and numeric values to strings. Thesemethods have the same name valueOf with

different argument types char, char[], double, long,

int, and float. For example, to convert a double

value to a string, use String.valueOf(5.44). The

return value is string consists of characters 5, .,

4, and 4.


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Problem: Finding PalindromesObjective: Checking whether a string

is a palindrome: a string that reads thesame forward and backward.


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import java.util.Scanner;

public class CheckPalindrome {

/** Main method */

public static void main(String[] args) {

// Create a Scanner

Scanner input = new Scanner(System.in);

// Prompt the user to enter a string

System.out.print("Enter a string: ");

String s = input.nextLine();

if (isPalindrome(s))

System.out.println(s + " is a palindrome");

else

System.out.println(s + " is not a palindrome");

} 33

/** Check if a string is a palindrome */


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/ Check if a string is a palindrome /

public static boolean isPalindrome(String s) {

// The index of the first character in the string

int low = 0;

// The index of the last character in the string

int high = s.length() - 1;

while (low < high) {

if (s.charAt(low) != s.charAt(high))

return false; // Not a palindrome

low++;

high--;

}

return true; // The string is a palindrome

}

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The Character Classjava.lang.Character

+Character(value: char)

+charValue(): char

+compareTo(anotherCharacter: Character): int+equals(anotherCharacter: Character): boolean

+isDigit(ch: char): boolean

+isLetter(ch: char): boolean

+isLetterOrDigit(ch: char): boolean

+isLowerCase(ch: char): boolean+isUpperCase(ch: char): boolean

+toLowerCase(ch: char): char

+toUpperCase(ch: char): char

Constructs a character object with char value

Returns the char value from this object

Compares this character with anotherReturns true if this character equals to another

Returns true if the specified character is a digit

Returns true if the specified character is a letter

Returns true if the character is a letter or a digit

Returns true if the character is a lowercase letterReturns true if the character is an uppercase letter

Returns the lowercase of the specified character

Returns the uppercase of the specified character


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ExamplesCharacter charObject = new Character('b');

charObject.compareTo(new Character('a')) returns 1

charObject.compareTo(new Character('b')) returns 0charObject.compareTo(new Character('c')) returns -1

charObject.compareTo(new Character('d') returns2

charObject.equals(new Character('b')) returns truecharObject.equals(new Character('d')) returns false

P bl C ti E h L tt i


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Problem: Counting Each Letter ina String

This example gives a program that counts the

number of occurrence of each letter in a string.

Assume the letters are not case-sensitive.


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public class CountEachLetter {

/** Main method */

public static void main(String[] args) { // Create a Scanner





// Invoke the countLetters method to count each letter

int[] counts = countLetters(s.toLowerCase());

// Display results for (int i = 0; i < counts.length; i++) {

if (counts[i] != 0)

System.out.println((char)('a' + i) + " appears " +

counts[i] + ((counts[i] == 1) ? " time" : " times"));

}

} 38


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/** Count each letter in the string */

public static int[] countLetters(String s) {

int[] counts = new int[26];

for (int i = 0; i < s.length(); i++) {

if (Character.isLetter(s.charAt(i)))

counts[s.charAt(i) - 'a']++;

}

return counts;

}

}39


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StringBuilder and StringBuffer

The StringBuilder/StringBuffer class isan alternative to the String class. In general, a

StringBuilder/StringBuffer can be used wherever a

string is used. StringBuilder/StringBuffer is moreflexible than String. You can add, insert, or

append new contents into a string buffer, whereas

the value of a String object is fixed once the string

is created.


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StringBuilder Constructors

java.lang.StringBuilder

+StringBuilder()

+StringBuilder(capacity: int)

+StringBuilder(s: String)

Constructs an empty string builder with capacity 16.

Constructs a string builder with the specified capacity.

Constructs a string builder with the specified string.


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Modifying Strings in the Builderjava.lang.StringBuilder

+append(data: char[]): StringBuilder

+append(v: aPrimitiveType): StringBuilder

+append(s: String): StringBuilder

+delete(startIndex: int, endIndex: int):

StringBuilder

+deleteCharAt(index: int): StringBuilder

+insert(offset: int, data: char[]):

StringBuilder

+insert(offset: int, b: aPrimitiveType):

StringBuilder

+insert(offset: int, s: String): StringBuilder

+replace(startIndex: int, endIndex: int, s:

String): StringBuilder

+reverse(): StringBuilder

+setCharAt(index: int, ch: char): void

Appends a char array into this string builder.

Appends a primitive type value as a string to this

builder.

Appends a string to this string builder.

Deletes characters from startIndex to endIndex.

Deletes a character at the specified index.

Inserts data into this builder at the position offset.

Inserts a value converted to a string into this builder.

Inserts a string into this builder at the position offset.

Replaces the characters in this builder from startIndex

to endIndex with the specified string.

Reverses the characters in the builder.

Sets a new character at the specified index in this

builder.


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Examples

stringBuilder.append("Java");

stringBuilder.insert(11, "HTML and ");stringBuilder.delete(8, 11) changes the builder to WelcomeJava.stringBuilder.deleteCharAt(8) changes the builder to

Welcome o Java.stringBuilder.reverse() changes the builder to avaJ otemocleW.stringBuilder.replace(11, 15, "HTML")

changes the builder to Welcome to HTML.stringBuilder.setCharAt(0, 'w') sets the builder to welcometo Java.


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Problem: Checking Palindromes

Ignoring Non-alphanumeric Characters



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p j

public class PalindromeIgnoreNonAlphanumeric {

/** Main method */


// Create a Scanner





// Display result

System.out.println("Ignoring non-alphanumeric characters,

\nis "

+ s + " a palindrome? " + isPalindrome(s));

} 45


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/** Return true if a string is a palindrome */

public static boolean isPalindrome(String s) {

// Create a new string by eliminating non-alphanumeric chars

String s1 = filter(s);

// Create a new string that is the reversal of s1

String s2 = reverse(s1);

// Compare if the reversal is the same as the original string

return s2.equals(s1);

}

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/** Create a new string by eliminating non-alphanumeric chars

*/

public static String filter(String s) {

// Create a string builder

StringBuilder stringBuilder = new StringBuilder();

// Examine each char in the string to skip alphanumeric char

for (int i = 0; i < s.length(); i++) {

if (Character.isLetterOrDigit(s.charAt(i))) {

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

}

}

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/** Create a new string by reversing a specified string */

public static String reverse(String s) {

StringBuilder stringBuilder = new StringBuilder(s);

stringBuilder.reverse(); // Invoke reverse inStringBuilder

return stringBuilder.toString();

}

}

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Main Method Is Just a Regular Method

public class A {


String[] strings = {"New York",

"Boston", "Atlanta"};B.main(strings);

}

}

class B {


for (int i = 0; i < args.length; i++)

System.out.println(args[i]);

}

}

You can call a regular method by passing actualparameters. Can you pass arguments to main? Of

course, yes. For example, the main method in class

B is invoked by a method in A, as shown below:


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Problem: Calculator

Objective: Write a program that will perform

binary operations on integers. The program

receives three parameters: an operator and twointegers.

java Calculator 2 + 3

java Calculator 2 - 3

java Calculator 2 / 3

java Calculator 2 * 3

Th Fil Cl


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The File ClassThe File class is intended to provide an abstraction that

deals with most of the machine-dependent complexities

of files and path names in a machine-independent

fashion. The filename is a string. The File class is a

wrapper class for the file name and its directory path.


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java.io.File

+File(pathname: String)

+File(parent: String, child: String)

+File(parent: File, child: String)

+exists(): boolean

+canRead(): boolean+canWrite(): boolean

+isDirectory(): boolean

+isFile(): boolean

+getAbsolutePath(): String

+getName(): String

+getPath(): String

+delete(): boolean

+renameTo(dest: File): boolean

Creates a File object for the specified pathname. The pathname may be adirectory or a file.

Creates a File object for the child under the directory parent. child may be a

filename or a subdirectory.

Creates a File object for the child under the directory parent. parent is a File

object. In the preceding constructor, the parent is a string.

Returns true if the file or the directory represented by the File object exists.

Returns true if the file represented by the File object exists and can be read.Returns true if the file represented by the File object exists and can be written.

Returns true if the File object represents a directory.

Returns true if the File object represents a file.

Returns the complete absolute file or directory name represented by the File

object.

Returns the last name of the complete directory and file name represented by

the File object. For example, new File("c:\\book\\test.dat").getName() returnstest.dat.

Returns the complete directory and file name represented by the File object.

For example, new File("c:\\book\\test.dat").getPath() returns c:\book\test.dat.

Deletes this file. The method returns true if the deletion succeeds.

Renames this file. The method returns true if the operation succeeds.

Obtaining file properties and manipulating file

P bl E l Fil P ti


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Problem: Explore File Properties

TestFileClass Run

Objective: Write a program that demonstrates how to

create files in a platform-independent way and use themethods in the File class to obtain their properties. Figure

16.1 shows a sample run of the program on Windows, and

Figure 16.2 a sample run on Unix.

T t I/O


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Text I/OA File object encapsulates the properties of a file ora path, but does not contain the methods forreading/writing data from/to a file. In order toperform I/O, you need to create objects usingappropriate Java I/O classes. The objects contain

the methods for reading/writing data from/to a file.This section introduces how to read/write stringsand numeric values from/to a text file using theScanner and PrintWriter classes.

W iti D t U i P i tW it


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Writing Data Using PrintWriter

WriteData Run

java.io.PrintWriter

+PrintWriter(filename: String)

+print(s: String): void

+print(c: char): void

+print(cArray: char[]): void

+print(i: int): void

+print(l: long): void+print(f: float): void

+print(d: double): void

+print(b: boolean): void

Also contains the overloaded

println methods.

Also contains the overloaded

printf methods.

Creates a PrintWriter for the specified file.

Writes a string.

Writes a character.

Writes an array of character.

Writes an int value.

Writes a long value.

Writes a float value.

Writes a double value.

Writes a boolean value.

A println method acts like a print method; additionally it

prints a line separator. The line separator string is defined

by the system. It is \r\n on Windows and \n on Unix.The printf method was introduced in 3.6, Formatting

Console Output and Strings.

R di D U i S


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Reading Data Using Scannerjava.util.Scanner

+Scanner(source: File)

+Scanner(source: String)

+close()

+hasNext(): boolean

+next(): String

+nextByte(): byte+nextShort(): short

+nextInt(): int

+nextLong(): long

+nextFloat(): float

+nextDouble(): double

+useDelimiter(pattern: String):

Scanner

Creates a Scanner that produces values scanned from the specified file.

Creates a Scanner that produces values scanned from the specified string.

Closes this scanner.

Returns true if this scanner has another token in its input.

Returns next token as a string.

Returns next token as a byte.Returns next token as a short.

Returns next token as an int.

Returns next token as a long.

Returns next token as a float.

Returns next token as a double.

Sets this scanners delimiting pattern.

ReadData Run

P bl R l i T t


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Problem: Replacing Text

Write a class named ReplaceText that replaces a string in a text

file with a new string. The filename and strings are passed as

command-line arguments as follows:

java ReplaceText sourceFile targetFile oldString newString

For example, invokingjava ReplaceText FormatString.java t.txt StringBuilder StringBuffer

replaces all the occurrences of StringBuilder by StringBuffer in

FormatString.java and saves the new file in t.txt.

ReplaceText Run

(GUI) Fil Di l


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(GUI) File Dialogs

ReadFileUsingJFileChooser Run

lecture 2 ( strings )

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