STRINGS AND STRINGS MANIPULATION

CONTENTS1. What Is String?

2. Creating and Using Strings

• Declaring, Creating, Reading and Printing

3. Manipulating Strings

• Comparing, Concatenating, Searching, Extracting Substrings, Splitting

4. Other String Operations

• Replacing Substrings, Deleting Substrings, Changing Character Casing, Trimming

CONTENTS5. Building and Modifying Strings

• Using StringBuilder Class

6. Formatting Strings

WHAT IS STRING?

FREQUENTLY ASKED QUESTIONS FAQ• What is string ?

• Why string is final?

• What are the ways to declare and initialize the String Object?

• What is the difference b/w Reference values and literal strings?

• What is the difference b/w + opretor and concat() method?

• What is the effect when comparing strings with == and equals() ?

• What is Difference b/w String class and String Buffer?

• What is String pool in Java?

• What does intern() method do in Java?

• Why String is thread-safe in Java?

• What is the difference b/w System.out.println(1+2+” text”) and System.out.println(” text”+1+2) ?

WHAT IS STRING?• String is:

• A sequence of characters

• Each character is a Unicode character

• Represented by the String (java.lang.String) data type in Java

• Example:

String s = "Hello, Java";

H e l l o , J a v as

JAVA.LANG.STRING• We use java.lang.String to work with strings in Java

• String objects contain an immutable (read-only) sequence of characters

• Use Unicode in order to support multiple languages and alphabets

• Stores strings in the dynamic memory (managed heap)

• java.lang.String is class

• It is reference type

JAVA.LANG.STRING • String objects are like arrays of characters (char[])

• Have fixed length (String.length())

• Elements can be accessed by index

• Using charAt() method

• The index is in the range 0...length()-1

String s = "Hello!";int len = s.length(); // len = 6char ch = s.charAt(1); // ch = 'e‘`

0 1 2 3 4 5

H e l l o !

index =index =index =index =

s.charAt(index) =s.charAt(index) =s.charAt(index) =s.charAt(index) =

STRINGS – FIRST EXAMPLE

String s = "Stand up, stand up, Balkan superman.";

System.out.printf("s = \"%s\"%n", s);System.out.printf("s.length() = %d%n", s.length());

for (int i = 0; i < s.length(); i++) { System.out.printf("s[%d] = %c%n", i, s.charAt(i));}

STRINGS – FIRST EXAMPLELive Demo

Creating and Using StringsCreating and Using Strings

Declaring, Creating, Reading and Printing

DECLARING STRINGS• We use Java String class for declaring string variables:

String str;

CREATING STRINGS• Before initializing a string variable is equal to nunullll

• Strings can be initialized by:

• Assigning a string literal to the string variable

• Assigning the value of another string variable

• Assigning the result of operation of type string

CREATING STRINGS (2)• Not initialized variables has value of nullnull

• Assigning a string literal

• Assigning another string variable

• Assigning the result of string operation

String s; // s is equal to null

String s = "I am string literal!";

String s2 = s;

String s = "I'm " + 42 + " years old.";

READING AND PRINTING STRINGS• Reading strings from the console

• Use the method input.nextLine()

String s = input.nextLine();

• Printng Strings to the console

• Use methods print() and println()

• Printng Strings to the console

• Use methods print() and println()

System.out.print("Please enter your name: "); String name = input.nextLine();System.out.printf("Hello, %s!%n", name);

Reading and Reading and Printing StringsPrinting Strings

Live Demo

MANIPULATING STRINGSComparing, Concatenating, Searching, Extracting Substrings, Splitting

COMPARING STRINGS• There are a number of ways to compare two strings:

• Dictionary-based string comparison

• Case-insensitive

• Case-sensitive

int result = str1.compareToIgnoreCase(str2);// result == 0 if str1 equals str2// result < 0 if str1 if before str2// result > 0 if str1 if after str2

str1.compareTo(str2);

COMPARING STRINGS (2)

• Equality checking by equalsIgnoreCase()

• Performs case-insensitive compare

• Returns boolean value

• The case-sensitive equals() method

if (str1.equalsIgnoreCase(str2)){ …}

if (str1.equals(str2)){ …}

COMPARING STRINGS (3)

• Operators == and != does not check for equality!

• These operators returns boolean value, but check if the addresses of the object are equal

• Use equals() and equalsIgnoreCase() instead

String str1 = new String("Hello");String str2 = str1;System.out.println((str1==str2)); // true

String str1 = "Hello";String str2 = "Hello";System.out.println((str1==str2)); // true!!!

String str1 = new String("Hello");String str2 = new String("Hello");System.out.println((str1==str2)); // This is false!

COMPARING STRINGS – EXAMPLE • Finding the first in a lexicographical order string from a given list of strings

String[] towns = {“Jamshoro", “hyderabad", “Qasimabad",

“Latifabad", “Kotri", “Heerabad"};String firstTown = towns[0];for (int i=1; i<towns.length; i++) { String currentTown = towns[i]; if (currentTown.compareTo(firstTown) < 0) { firstTown = currentTown; }}System.out.println("First town: " + firstTown);

COMPARING STRINGSLive Demo

CONCATENATING STRINGS• There are two ways to combine strings:

• Using the concat() method

• Using the + or the += operator

• Any object can be appended to string

String str = str1.concat(str2);

String str = str1 + str2 + str3;String str += str1;

String name = "Peter";int age = 22;String s = name + " " + age; // "Peter 22"

CONCATENATING STRINGS – EXAMPLE

String firstName = "Svetlin";String lastName = "Nakov";

String fullName = firstName + " " + lastName;System.out.println(fullName);

int age = 26;

String nameAndAge = "Name: " + fullName + "\nAge: " + age;

System.out.println(nameAndAge);// Name: Svetlin Nakov// Age: 26

CONCATENATING STRINGSLive Demo

SEARCHING STRINGS• Finding a character or substring within given string

• First occurrence

• First occurrence starting at given position

• Last occurrence

• Last occurrence before given position

indexOf(String str)

indexOf(String str, int fromIndex)

lastIndexOf(String)

lastIndexOf(String, int fromIndex)

SEARCHING STRINGS – EXAMPLEString str = "Java Programming Course";

int index = str.indexOf("Java"); // index = 0index = str.indexOf("Course"); // index = 17index = str.indexOf("COURSE"); // index = -1// indexOf is case sensetive. -1 means not foundindex = str.indexOf("ram"); // index = 9index = str.indexOf("r"); // index = 6index = str.indexOf("r", 7); // index = 9index = str.indexOf("r", 10); // index = 20

00 11 22 33 44 55 66 77 88 99 1010 1111 1212 ……

JJ aa vv aa PP rr oo gg rr aa mm mm ……

i =i =

s.charAt(i) =) =s.charAt(i) =) =

SEARCHING STRINGSLive Demo

EXTRACTING SUBSTRINGS• Extracting substrings

• str.substring(int beginIndex, int endIndex)

• lastIndex is not included

• str.substring(int beginIndex)

String filename = "C:\\Pics\\Rila2005.jpg";String name = filename.substring(8, 16);// name is Rila2005

String filename = "C:\\Pics\\Summer2005.jpg";String nameAndExtension = filename.substring(8);// nameAndExtension is Rila2005.jpg

00 11 22 33 44 55 66 77 88 99 1010 1111 1212 1313 1414 1515 1616 1717 1818 1919

CC :: \\ \\ PP ii cc ss \\\\ RR ii ll aa 22 00 00 55 .. jj pp gg

EXTRACTING SUBSTRINGSLive Demo

SPLITTING STRINGS• To split a string by given separator(s) use the following method:

• String regex – String with special format

• We can list the character which we want to use for separator in square brackets […]

String[] split(String regex)

String[] parts = "Ivan; Petar,Gosho".split("[;,]");// this wil separate the stirng into three parts// "Ivan", " Petar" and "Gosho"

SPLITTING STRINGS - EXAMPLE

String listOfBeers = "Amstel, Zagorka, Tuborg, Becks.";

String[] beers = listOfBeers.split("[ ,.]");System.out.println("Available beers are:");for (String beer : beers) {

if (!"".equalsIgnoreCase(beer)) {System.out.println(beer);

}}

SPLITTING STRINGSLive Demo

OTHER STRING OPERATIONSReplacing Substrings, Changing Character Casing, Trimming

REPLACING SUBSTRINGS• replace(String, String) – replaces all occurrences of given string with another

• The result is new string (strings are immutable)

String cocktail = "Vodka + Martini + Cherry";String replaced = cocktail.replace("+", "and");// Vodka and Martini and Cherry

CHANGING CHARACTER CASING• Using method toLowerCase()

• Using method toUpperCase()

String alpha = "aBcDeFg";String lowerAlpha = alpha.toLowerCase(); // abcdefgSystem.out.println(lowerAlpha);

String alpha = "aBcDeFg";String upperAlpha = alpha.toUpperCase(); // ABCDEFGSystem.out.println(upperAlpha);

TRIMMING WHITE SPACE• Using method trim()

String s = " example of white space ";String clean = s.trim();System.out.println(clean);

OTHER STRING OPERATIONSLive Demo

BUILDING AND MODIFYING STRINGS

Using StringBuilder Class

CONSTRUCTING STRINGS• Strings are immutable

• concat(), replace(), trim(), ... return new string, do not modify the old one

• Do not use "+" for strings in a loop!

• It runs very inefficiently!

public static string dupChar(char ch, int count){ String result = ""; for (int i=0; i<count; i++) result += ch; return result;} Bad practice.

Avoid this!

CHANGING THE CONTENTS OF A STRING – STRINGBUILDER

• Use the java.lang.StringBuilder class for modifiable strings of characters:

• Use StringBuilder if you need to keep adding characters to a string

public static String reverseIt(String s) { StringBuilder sb = new StringBuilder(); for (int i = s.length()-1; i >= 0; i--) sb.append(s.charAt(i)); return sb.ToString();}

THE STRINGBUILDER CLASS

• StringBuilder keeps a buffer memory, allocated in advance

• Most operations use the buffer memory and do not allocate new objects

H e l l o , J a v a !StringBuilder:

length() = 11capacity() = 15

StringBuilder:

length() = 11capacity() = 15 used buffer

(length())used buffer(length())

unusedbufferunusedbuffer

CapacityCapacity

THE STRINGBUILDER CLASS (2)

• StringBuilder(int capacity) constructor allocates in advance buffer memory of a given size

• By default 16 characters are allocated

• capacity() holds the currently allocated space (in characters)

• charAt(int index) gives access to the char value at given position

• length() hold the length of the string in the buffer

THE STRINGBUILDER CLASS (3)• append(…) appends string or other object after the last character in the buffer

• delete(int start, int end) removes the characters in given range

• insert(int offset, String str) inserts given string (or object) at given position

• replace(int start, int end, String str) replaces all occurrences of a substring with given string

• toString() converts the StringBuilder to String object

STRINGBUILDER – EXAMPLE• Extracting all capital letters from a string

public static String extractCapitals(String s) { StringBuilder result = new StringBuilder(); for (int i = 0; i < s.length(); i++) { char ch = s.charAt(i); if (Character.isUpperCase(ch)) { result.append(ch); } } return result.toString();}

HOW THE + OPERATOR DOES STRING CONCATENATIONS?

• Consider following string concatenation:

• It is equivalent to this code:

• Actually several new objects are created and leaved to the garbage collector

• What happens when using + in a loop?

String result = str1 + str2;

StringBuffer sb = new StringBuffer();sb.append(str1);sb.append(str2);String result = sb.toString();

USING STRINGBUILDERLive Demo

FORMATTING STRINGSUsing toString() and String.format()

METHOD TOSTRING()• All classes have this public virtual method

• Returns a human-readable, culture-sensitive string representing the object

• Most Java Platform types have own implementation of toString()

METHOD STRING.FORMAT()• Applies templates for formatting strings

• Placeholders are used for dynamic text

• Like System.out.printf(…)

String template = "If I were %s, I would %s.";String sentence1 = String.format( template, "developer", "know Java");System.out.println(sentence1);// If I were developer, I would know Java.

String sentence2 = String.format( template, "elephant", "weigh 4500 kg");System.out.println(sentence2);// If I were elephant, I would weigh 4500 kg.

FORMATTING DATES• When we print Dates we use prefix t or T

• d, e – day (with/without leading zero)

• m – month

• y, Y – year (2 or 4 digits)

• H, M, S – hour, minute, second

Date now = (new GregorianCalendar()).getTime();System.out.printf("Now is " +

"%1$td.%1$tm.%1$tY %1$tH:%1$tM:%1$tS", now);// Now is 23.05.2006 21:09:32

FORMATTING STRINGSLive Demo

EXERCISES Write a program that reads a string, reverses it and prints it on the console. Example:

"sample" "elpmas".

Write a program to check if in a given expression the brackets are put correctly. Example of correct expression: ((a+b)/5-d). Example of incorrect expression: )(a+b)).

Write a program that finds how many times a substring is contained in a given text (perform case insensitive search).

Example: The target substring is "in". The text is as follows:

The result is: 9.

We are living in a yellow submarine. We don't have anything else. Inside the submarine is very tight. So we are drinking all the day. We will move out of it in 5 days.

You are given a text. Write a program that changes the text in all regions identified by the tags <upcase> and </upcase> to uppercase. The tags cannot be nested. Example:

The expected result:

We are living in a <upcase>yellow submarine</upcase>. We don't have <upcase>anything</upcase> else.

We are living in a YELLOW SUBMARINE. We don't have ANYTHING else.

Write a program that parses an URL address given in the format:

and extracts from it the [protocol], [server] and [resource] elements. For example from the URL http://www.devbg.org/forum/index.php following information should be extracted: [protocol] =

"http", [server] = "www.devbg.org", [resource] = "/forum/index.php"

[protocol]://[server]/[resource]

Write a program that extracts from a given text all the sentences that contain given word. Example: The word is "in". The text is:

The expected result is:

Consider that the sentences are separated by "." and the words – by non-letter symbols.

We are living in a yellow submarine. We don't have anything else. Inside the submarine is very tight. So we are drinking all the day. We will move out of it in 5 days.

We are living in a yellow submarine.We will move out of it in 5 days.

We are given a string containing a list of forbidden words and a text containing some of these words. Write a program that replaces the forbidden words with asterisks. Example:

Words: "Java, JVM, Microsoft"

The expected result:

Microsoft announced its next generation Java compiler today. It uses advanced parser and special optimizer for the Microsoft JVM.

********* announced its next generation **** compiler today. It uses advanced parser and special optimizer for the ********* ***.

Write a program that reads a string from the console and lists all the different letters in the string along with information how many times each letter is found.

Write a program that reads a string from the console and lists all the different words in the string with information how many times each word is found.

Write a program that reads a string from the console and replaces all series of consecutive identical letters with a single one. Example: "aaaaabbbbbcdddeeeedssaa" -> "abcdedsa".

Write a program that reads a list of words, separated by spaces (' ') , and prints the list in an alphabetical order.

Write a program that lets the user input a string of maximum 20 characters. If the length of the string is less, the rest of the characters should be filled with '*'. Print the string into the console.