Java Strings Guide

STRINGS AND STRINGS MANIPULATION

CONTENTS
1. 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

CONTENTS
5. Building and Modifying Strings
• Using StringBuilder Class
6. Formatting Strings

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";

s H e l l o , J a v a

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 = 6
char ch = s.charAt(1); // ch = 'e„`

index = 0 1 2 3 4 5
s.charAt(index) = H e l l o !

STRINGS – FIRST EXAMPLE

String s = “Hidaya Institute of Science & Tchnology.";

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 EXAMPLE
Live Demo

Creating 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 null
• 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 null

String s; // s is equal to null
• Assigning a string literal
String s = "I am string literal!";
• Assigning another string variable

String s2 = s;
• Assigning the result of string operation

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()

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

MANIPULATING STRINGS
Comparing, Concatenating, Searching, Extracting Substrings, Splitting

COMPARING STRINGS
• There are a number of ways to compare two strings:
• Dictionary-based string comparison
• Case-insensitive

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

• Case-sensitive

str1.compareTo(str2);

COMPARING STRINGS (2)
• Equality checking by equalsIgnoreCase()

• Performs case-insensitive compare
• Returns boolean value

if (str1.equalsIgnoreCase(str2)){
…
}

• The case-sensitive equals() method

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!!!

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 STRINGS
Live Demo

CONCATENATING STRINGS
• There are two ways to combine strings:
• Using the concat() method

String str = str1.concat(str2);
• Using the + or the += operator
String str = str1 + str2 + str3;
String str += str1;
• Any object can be appended to string
String name = "Peter";
int age = 22;
String s = name + " " + age; //  "Peter 22"

CONCATENATING STRINGS – EXAMPLE
String firstName = "Shahjahan";
String lastName = "Samoon";

String fullName = firstName + " " + lastName;

int age = 23;

String nameAndAge = "Name: " + fullName +
"nAge: " + age;
System.out.println(nameAndAge);
// Name: Shahjahan Samoon
// Age: 23

CONCATENATING STRINGS
Live Demo

SEARCHING STRINGS
• Finding a character or substring within given string
• First occurrence

indexOf(String str)
• First occurrence starting at given position

indexOf(String str, int fromIndex)
• Last occurrence

lastIndexOf(String)
• Last occurrence before given position

lastIndexOf(String, int fromIndex)

SEARCHING STRINGS – EXAMPLE
String str = "Java Programming Course";

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

i = 0 1 2 3 4 5 6 7 8 9 10 11 12 …

s.charAt(i) = J a v a P r o g r a m m …

SEARCHING STRINGS
Live Demo

EXTRACTING SUBSTRINGS
• Extracting substrings
• str.substring(int beginIndex, int endIndex)
• lastIndex is not included
String filename = "C:PicsRila2005.jpg";
String name = filename.substring(8, 16);
// name is Rila2005
• str.substring(int beginIndex)
String filename = "C:PicsSummer2005.jpg";
String nameAndExtension = filename.substring(8);
// nameAndExtension is Rila2005.jpg

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

C : P i c s R i l a 2 0 0 5 . j p g

EXTRACTING SUBSTRINGS
Live Demo

SPLITTING STRINGS
• To split a string by given separator(s) use the following method:
String[] split(String regex)
• String regex – String with special format
• We can list the character which we want to use for separator in square brackets […]

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 STRINGS
Live Demo

OTHER STRING OPERATIONS
Replacing 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()

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

• Using method toUpperCase()

String alpha = "aBcDeFg";
String upperAlpha = alpha.toUpperCase(); // ABCDEFG
System.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 OPERATIONS
Live 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
Capacity

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

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

• StringBuilder keeps a buffer memory, allocated in advance
• Most operations use the buffer memory and do not allocate new objects

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:

String result = str1 + str2;
• It is equivalent to this code:

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

• Actually several new objects are created and leaved to the garbage collector
• What happens when using + in a loop?

USING STRINGBUILDER
Live Demo

FORMATTING STRINGS
Using 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 STRINGS
Live 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:

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.

The result is: 9.

 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:

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

The expected result:

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:

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

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 "

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

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.

The expected result is:

We are living in a yellow submarine.

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

 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:

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

Words: "Java, JVM, Microsoft"
The expected result:

********* 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.

Java Strings Guide

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