String in java, string constructors and operations

Java Strings
• In Java, a string is a sequence of characters. For example, "hello" is a string
containing a sequence of characters 'h', 'e', 'l', 'l', and 'o'.
• We use double quotes to represent a string in Java. For example
/ create a string
String type = "Java programming“
Here, we have created a string variable named type. The variable is initialized with
the string Java Programming.

Example: Create a String in Java
class Main {
public static void main(String[] args) {
// create strings
String first = "Java";
String second = "Python";
String third = "JavaScript";
// print strings
System.out.println(first); // print Java
System.out.println(second); // print Python
System.out.println(third); // print JavaScript
}
}

String length
• The length() method returns the number of characters in a string.
Example
class Main {
String str1 = "Java is fun";
// returns the length of str1
int length = str1.length();
System.out.println(str1.length());
}
}
// Output: 11

String length
length() Syntax
• The syntax of the string's length() method is:
string.length()
length() Arguments
The length() method doesn't take any arguments.
length() Return Value
The length() method returns the length of a given string.

Example: Java String length()
class Main {
String str1 = "Java";
String str2 = "";
System.out.println(str1.length()); // 4
System.out.println("Java".length()); // 4
// length of empty string
System.out.println(str2.length()); // 0
// new line is considered a single character
System.out.println("Javan".length()); // 5
System.out.println("Learn Java".length()); // 10
}

String constructors
• In Java, strings are represented by the String class, which is part of the
java.lang package (automatically imported in every Java program).
• The String class provides multiple ways (constructors) to create and
initialize string objects.

String constructors in java
• String()
• String(String str)
• String(char chars[ ])
• String(char chars[ ], int startIndex, int count)
• String(byte byteArr[ ])
• String(byte byteArr[ ], int startIndex, int count)

String()
• To create an empty string, we will call the default constructor. The
general syntax to create an empty string in Java program is as follows:
String s = new String();
• It will create a string object in the heap area with no value.

String(String str)
• This constructor will create a new string object in the heap area and
stores the given value in it. The general syntax to construct a string
object with the specified string str is as follows:
String st = new String(String str);
For example:
String s2 = new String("Hello Java");
Here, the object str contains Hello Java.

String(char chars[ ])
• This string constructor creates a string object and stores the array of characters in
it. The general syntax to create a string object with a specified array of characters
is as follows:
String str = new String(char char[])
For example:
char chars[] = { 'a', 'b', 'c', 'd' };
String s3 = new String(chars);
The object reference variable s3 contains the address of the value stored in the heap
area.

//we will create a string object and store an array of
characters in it.
package stringPrograms;
public class Science {
public static void main(String[ ] args)
{
char chars[] = {'s', 'c', 'i', 'e', 'n', 'c', 'e'};
String s = new String(chars);
System.out.println(s);
}
}
Output:
science

4. String(char chars[ ], int startIndex, int count)
• This constructor creates and initializes a string object with a subrange
of a character array.
• The argument startIndex specifies the index at which the subrange
begins and count specifies the number of characters to be copied. The
general syntax to create a string object with the specified subrange of
character array is as follows:

4. String(char chars[ ], int startIndex, int count)
String str = new String(char chars[ ], int startIndex, int count);
For example:
char chars[] = { 'w', 'i', 'n', 'd', 'o', 'w', 's' };
String str = new String(chars, 2, 3);
The object str contains the address of the value ”ndo” stored in the heap area
because the starting index is 2 and the total number of characters to be copied is 3.

public class Windows {
public static void main(String[] args)
{
char chars[] = { 'w', 'i', 'n', 'd', 'o', 'w', 's' };
String s = new String(chars, 0,4);
}
}
Output:
wind

Java program where we will create a string object that contains the same characters sequence as
another string object.
public class MakeString {
{
char chars[] = { 'F', 'A', 'N' };
String s2 = new String(s1);
System.out.println(s1);
System.out.println(s2);
}
}
Output:
FAN
FAN

5. String(byte byteArr[ ])
• This type of string constructor constructs a new string object by decoding the given array of
bytes (i.e., by decoding ASCII values into the characters) according to the system’s default
character set.
public class ByteArray {
{
byte b[] = { 97, 98, 99, 100 }; // Range of bytes: -128 to 127. These byte values will be
converted into corresponding characters.
String s = new String(b);
}
}
Output:
abcd

Special string operations in JAVA
• String literals
• Concatenation of strings
• String Conversion using toString( )
• Character extraction
• String comparison

string literals
string literal in your program, Java automatically constructs a String object.
Thus string literal can be used to initialize a String object.
For example, the following code fragment creates two equivalent strings:
char chars[] = { 'a', 'b', 'c' };
String s2 = "abc"; // use string literal
● String object is created for every string literal, you can use a string literal
any place you can use a String object.
For example, you can call methods directly on a quoted string as if it were an object
reference, as the following statement shows.
It calls the length( ) method on the string “abc”. As expected, it prints “3”.
System.out.println("abc".length());

Concatenation of Strings
Java does not allow operators to be applied to String objects.
The one exception to this rule is the + operator, which concatenates two
strings, producing a String object as the result.
String age = "9";
String s = "He is " + age + " years old.";
This displays the string “He is 9 years old.”
String Concatenation with Other Data Types
You can concatenate strings with other types of data.
Be careful when you mix other types of operations with string concatenation
expressions,
however.

String in java, string constructors and operations

String Conversion using toString( )
• If you want to represent any object as a string, toString() method comes into
existence.
• The toString() method returns the String representation of the object.
• If you print any object, Java compiler internally invokes the toString() method on
the object. So overriding the toString() method, returns the desired output, it can
be the state of an object etc. depending on your implementation.
• By overriding the toString() method of the Object class, we can return values of
the object, so we don't need to write much code.

Understanding problem without toString() method
class Student{
int rollno;
String name;
String city;
Student(int rollno, String name, String city){
this.rollno=rollno;
this.name=name;
this.city=city;
}
public static void main(String args[]){
Student s1=new Student(101,"Raj","lucknow");
Student s2=new Student(102,"Vijay","ghaziabad");
System.out.println(s1);//compiler writes here s1.toString()
}
}
Output:
Student@1fee6fc
in the above example, printing s1 and s2 prints the hashcode values of
the objects but I want to print the values of these objects. Since Java
compiler internally calls toString() method, overriding this method will
return the specified values.

Example of Java toString() method
class Student{
int rollno;
String name;
String city;
Student(int rollno, String name, String city){
this.rollno=rollno;
this.name=name;
this.city=city;
}
public String toString(){//overriding the toString() method
return rollno+" "+name+" "+city;
}
Student s1=new Student(101,"Raj","lucknow");
Student s2=new Student(102,"Vijay","ghaziabad");
}
}
Output:
101 Raj lucknow
102 Vijay ghaziabad

Character extraction
• The String class in Java provides several ways to extract characters from a String object
1. charAt(int index):
This method is used to return the char value at the specified index.
The value of the index must be nonnegative and specify a location within the string.
For example:
char ch;
ch = "abc".charAt(1);
Result:
assigns the value “b” to ch.

Character extraction
2. getChars( )
• To extract more than one character from a String object, we can use this method.
General form:
void getChars(int sourceStart, int sourceEnd, char target[ ], int targetStart)
• Here, sourceStart specifies the starting index of the substring, and sourceEnd specifies an
index that is one past the end of the desired substring. The variable target specifies the
resultant array.

class getCharsDemo
{
public static void main(String args[])
{
String s = "This is a demo of the getChars method.";
int start = 10;
int end = 14;
char buf[] = new char[end - start];
s.getChars(start, end, buf, 0);
System.out.println(buf);
}
}
Output:
demo

3. getBytes( ):
This is an alternative method to getChars( ). It stores the characters in a byte array.
General form:
byte[ ] getBytes( )
4. toCharArray( )
To convert all the characters of a String object into a character array, this method is used.
It returns a character array for the given string.
General form:
char[ ] toCharArray( )

String Comparison
• The String class in Java includes several methods to compare strings
or substrings
• Some of the important string operations in Java to compare strings are
given below:
1. equals( ) and equalsIgnoreCase( )
2. compareTo()
3. compareToIgnoreCase():

1. equals( ) and equalsIgnoreCase( )
equals( ) and equalsIgnoreCase( )
To compare two strings for equality, use equals( ). It has this general form:
boolean equals(Object str)
Here, str is the String object being compared with the invoking String object.
It returns true if the strings contain the same characters in the same order, and false otherwise.
The comparison is case-sensitive.
equalsIgnoreCase( ).
When it compares two strings, it considers A-Z to be the same as a-z.
It has this general form: boolean equalsIgnoreCase(String str)
Here, str is the String object being compared with the invoking String object. It, too, returns
true if the strings contain the same characters in the same order, and false otherwise.

class equalsDemo
{
{
String s1 = "Hello";
String s3 = "Good-bye";
String s4 = "HELLO";
System.out.println(s1 + " equals " + s2 + " -> " +
s1.equals(s2));
s1.equals(s3));
s1.equals(s4));
System.out.println(s1 + " equalsIgnoreCase " + s4 + " -> " +
s1.equalsIgnoreCase(s4));
}
}
Output :
Hello equals Hello -> true
Hello equals Good-bye -> false
Hello equals HELLO -> false
Hello equalsIgnoreCase HELLO -> true

The equals( ) method and the “==”operator perform different functions.
The equals( ) method compares the characters of a String object, whereas the ==
operator compares the references of two string objects to see whether they refer
to the same instance.
A simple program to demonstrate the above difference is given below:
The variable s1 is pointing to the String "Hello". The object pointed by s2 is
constructed with the help of s1. Thus, the values inside the two String objects are
the same, but they are distinct objects.

class Demo
{
{
String s2 = new String(s1);
System.out.println(s1 + " equals " + s2 + " -> " + s1.equals(s2));
System.out.println(s1 + " == " + s2 + " -> " + (s1 == s2));
}
}
Output:
Hello equals Hello -> true
Hello == Hello -> false

2. compareTo()
The compareTo() method of the Java String class compares the string lexicographically. It returns a
positive, negative, or zero value. To perform a sorting operation on strings, the compareTo() method
comes in handy.
A string is less than another if it comes before in the dictionary order.
A string is greater than another if it comes after in the dictionary order.
General form:
int compareTo(String str)
Here, str is the string being compared with the invoking string.

class SortStringFunc
{
static String arr[] = {"Now", "is", "the", "time", "for", "all", "good", "men","to", "come", "to",
"the", "aid", "of", "their", "country" };
{
for(int j = 0; j < arr.length; j++)
{
for(int i = j + 1; i < arr.length; i++)
{
if(arr[i].compareTo(arr[j]) < 0)
{
String t = arr[j];
arr[j] = arr[i];
arr[i] = t;
}
}
System.out.println(arr[j]);
}
}
}
Output:
Now
aid
all
come
country
for
good
is
men
of
the
the
their
time
to
to

3. compareToIgnoreCase():
This method is the same as compareTo( ), but it ignores the lowercase and
uppercase differences of the strings while comparing.

Searching Strings
The Java String class provides two methods that allow us to search a
character or substring in another string.
• indexOf( ): It searches the first occurrence of a character or substring.
• lastIndexOf( ): It searches the last occurrence of a character or
substring.

class indexOfDemo
{
{
String s = "Now is the time for all good men " + "to come to the aid of their
country.";
System.out.println("indexOf(t) = " + s.indexOf('t'));
System.out.println("lastIndexOf(t) = " + s.lastIndexOf('t'));
System.out.println("indexOf(the) = " + s.indexOf("the"));
System.out.println("lastIndexOf(the) = " + s.lastIndexOf("the"));
System.out.println("indexOf(t, 10) = " + s.indexOf('t', 10));
System.out.println("lastIndexOf(t, 60) = " + s.lastIndexOf('t', 60));
System.out.println("indexOf(the, 10) = " + s.indexOf("the", 10));
System.out.println("lastIndexOf(the, 60) = " + s.lastIndexOf("the", 60));
}
}
Output:
Now is the time for all good men to come to
their country.
indexOf(t) = 7
lastIndexOf(t) = 65
indexOf(the) = 7
lastIndexOf(the) = 55
indexOf(t, 10) = 11
lastIndexOf(t, 60) = 55
indexOf(the, 10) = 44
lastIndexOf(the, 60) = 55

Modifying a String
1. substring( )
2. concat( )
3. replace( )
4. trim( )

1. substring( )
• As the name suggests, ‘sub + string’, is a subset of a string. By subset, we
mean the contiguous part of a character array or string.
• The substring() method is used to fetch a substring from a string in Java.
General form:
• String substring(int startIndex)
• String substring(int startIndex, int endIndex)
• Here, startIndex specifies the beginning index, and endIndex specifies
the stopping point.
• The string returned contains all the characters from the beginning index,
up to, but not including, the ending index i.e [startindex,endindex-1]

/ Substring replacement.
class StringReplace
{
{
String org = "This is a test. This is, too.";
String search = "is";
String sub = "was";
String result = "";
int i;
do {
// replace all matching substrings
System.out.println(org);
i = org.indexOf(search);
if(i != -1)
{
result = org.substring(0, i);
result = result + sub;
result = result + org.substring(i + search.length());
org = result;
}
} while(i != -1);
}
}
Output :
This is a test. This is, too.
Thwas is a test. This is, too.
Thwas was a test. This is, too.
Thwas was a test. Thwas is, too.
Thwas was a test. Thwas was, too.

2. concat( )
• To concatenate two strings in Java, we can use the concat( ) method
apart from the “+” operator.
• General form:
• String concat(String str)
• This method creates a new object containing the invoking string with
the str appended to the end. This performs the same function as the +
operator. Refer to the comparison table below:

3. replace( )
• This method is used to replace a character with some other character in a string. It has two forms.
• The first replaces all occurrences of one character in the invoking string with another character.
• General form:
• String replace(char original, char replacement)
• Here, the original specifies the character to be replaced by the character specified by replacement.
The resulting string is returned.
• For example:
String s = "Hello".replace('l', 'w’);
Result: s= “Hewwo”

3. replace( )
• The second form replaces one character sequence with
another.
• General form:
• String replace(CharSequence original, CharSequence
replacement)

4. trim( )
• The trim( ) method returns a copy of the invoking string from which any leading and trailing
whitespace has been removed.
General form:
• String trim( )
For example:
• String s = " Hello World ".trim();
• Result: s= “Hello World”

Data Conversion Using valueOf( )
• For converting different data types into strings, the valueOf() method is used. It is a static method
defined in the Java String class.
General form:
• static String valueOf(double num)
• static String valueOf(long num)
• static String valueOf(Object ob)
• static String valueOf(char chars[ ])
For example:
int num=20;
String s1=String.valueOf(num);
s1 = s1+10; //concatenating string s1 with 10
Result s1=2010

Changing the Case of Characters in a String
• The method toLowerCase( ) converts all the characters in a
string from uppercase to lowercase.
• The toUpperCase( ) method converts all the characters in a
string from lowercase to uppercase.

// Demonstrate toUpperCase() and toLowerCase().
class ChangeCase
{
{
String s = "This is a test.";
System.out.println("Original: " + s);
String upper = s.toUpperCase();
String lower = s.toLowerCase();
System.out.println("Uppercase: " + upper);
System.out.println("Lowercase: " + lower);
}
}
Output:
Original: This is a test.
Uppercase: THIS IS A TEST.
Lowercase: this is a test.

Joining strings
• In Java, you can join strings using the String.join() method. This method
allows you to concatenate multiple strings with a specified delimiter.
String fruits = String.join(" ", "Orange", "Apple", "Mango");
System.out.println(fruits);
The join() method takes two parameters.
delimiter - the delimiter to be joined with the elements
elements - elements to be joined

Syntax()
String.join(CharSequence delimiter, CharSequence... elements)
Here, ... signifies there can be one or more CharSequence.
String.join(CharSequence delimiter, Iterable elements)

class Main {
String str1 = "I";
String str2 = "love";
String str3 = "Java";
// join strings with space between them
String joinedStr = String.join(" ", str1, str2, str3);
System.out.println(joinedStr);
}
}
// Output: I love Java

class Main {
String result;
result = String.join("-", "Java", "is", "fun");
System.out.println(result); // Java-is-fun
}
}

StringBuffer in Java
• StringBuffer is a class in the java.lang package that represents a mutable sequence
of characters.
• Unlike the String class, StringBuffer allows you to modify the content of the
string object after it has been created.

StringBuffer buffer = new StringBuffer("Hello, ");
buffer.append("World!"); // appends to the existing value
buffer.insert(7, "Java "); // inserts at a specified index
buffer.reverse(); // reverses the characters
System.out.println(buffer); // prints: "!dlroW avaJ ,olleH"

Key Features of StringBuffer
• Mutable: StringBuffer provides the flexibility to change the content, making it ideal
for scenarios where you have to modify strings frequently.
• Synchronized: Being thread-safe, it ensures that only one thread can access the
buffer's methods at a time, making it suitable for multi-threaded environments.
• Performance Efficient: For repeated string manipulation, using StringBuffer can be
more efficient than the String class.
• Method Availability: StringBuffer offers several methods to manipulate strings.
These include append(), insert(), delete(), reverse(), and replace().

When to Use StringBuffer?
• When you need to perform several modifications to strings,
using StringBuffer is an efficient choice.
• Due to the mutability of StringBuffer, it doesn't create a new object for every
modification, leading to less memory consumption and improved
performance.
• StringBuffer is especially beneficial in a multithreaded environment due to its
synchronized methods, ensuring thread safety

1. append()
• The append() method adds the specified value to the end of the current
string.
StringBuffer buffer = new StringBuffer("javaguides");
buffer.append(" - For Beginners");
System.out.println(buffer);
// Output: "javaguides - For Beginners"

2. insert()
• The insert() method adds the specified value at the given index.
import java.io.*;
class A {
{
StringBuffer sb = new StringBuffer("Hello ");
sb.insert(1, "Java");
// Now original string is changed
System.out.println(sb);
}
} Output
HJavaello

3. delete()
• The delete() method of the StringBuffer class deletes the string from the specified
beginIndex to endIndex-1.
import java.io.*;
class A {
{
StringBuffer sb = new StringBuffer("Hello");
sb.delete(1, 3);
}}
Output
Hlo

4.replace()
• The replace() method replaces the given string from the specified beginIndex and endIndex-1.
import java.io.*;
class A {
{
StringBuffer sb = new StringBuffer("Hello");
sb.replace(1, 3, "Java");
}
}
Output
HJavalo

5.reverse()
• The reverse() method of the StringBuilder class reverses the current String.
class StringBufferExample5{
StringBuffer sb=new StringBuffer("Hello");
sb.reverse();
System.out.println(sb);//prints olleH
}
}

6.capacity()
• the capacity() method of the StringBuffer class returns the current capacity of the
buffer.
• The default capacity of the buffer is 16.
• If the number of character increases from its current capacity, it increases the
capacity by (oldcapacity*2)+2.
• For example if your current capacity is 16, it will be (16*2)+2=34.

StringBuffer sb=new StringBuffer();
System.out.println(sb.capacity());//default 16
sb.append("Hello");
System.out.println(sb.capacity());//now 16
sb.append("java is my favourite language");
System.out.println(sb.capacity());//now (16*2)+2=34 i.e (oldcapacity*2)+2
}
}
Output:
16
16
34

7.ensureCapacity()
• The ensureCapacity() method of the StringBuffer class ensures that the
given capacity is the minimum to the current capacity.
• If it is greater than the current capacity, it increases the capacity by
(oldcapacity*2)+2. For example if your current capacity is 16, it will
be (16*2)+2=34.

StringBuffer sb=new StringBuffer();
System.out.println(sb.capacity());//default 16
sb.append("Hello");
sb.append("java is my favourite language");
System.out.println(sb.capacity());//now (16*2)+2=34 i.e (oldcapacity*2)+2
sb.ensureCapacity(10);//now no change
sb.ensureCapacity(50);//now (34*2)+2
}
}
Output:
16
16
34
34
70

Important Constructors of StringBuffer Class
Constructor with CharSequence
StringBuilder sb = new StringBuilder(CharSequence seq);
This constructor creates a StringBuilder instance that contains the same characters as the specified
CharSequence. Like the constructor with a string, the initial capacity will be the length of the CharSequence
plus 16.

public class StringBuilderExample {
// Default constructor
StringBuilder sb1 = new StringBuilder();
sb1.append("Hello, World!");
System.out.println(sb1);
// Constructor with initial capacity
StringBuilder sb2 = new StringBuilder(50);
sb2.append("Initial capacity set to 50.");
// Constructor with initial string
StringBuilder sb3 = new StringBuilder("Initial String");
sb3.append(" appended text.");
// Constructor with CharSequence
CharSequence seq = "CharSequence example";
StringBuilder sb4 = new StringBuilder(seq);
sb4.append(" additional text.");
}
}
output
Hello, World!
Initial capacity set to 50.
Initial String appended text.
CharSequence example additional text.

String builder
• A StringBuilder is a mutable sequence of characters used in
programming languages such as Java, C#, and others.
• It is designed to efficiently handle strings that undergo multiple
modifications, such as concatenations, insertions, deletions, or
appending operations.
• Unlike immutable string objects, StringBuilder allows for these
operations without creating new string objects, making it more
performance-efficient, especially in scenarios involving extensive
string manipulation.

• append: Appends the specified data to the end of the StringBuilder.
sb.append("Hello");
sb.append(123);
sb.append(true);
insert: Inserts the specified data at the specified position.
sb.insert(0, "Start: ");
delete: Removes the characters in a substring of this sequence.
sb.delete(5, 10);
deleteCharAt: Removes the character at the specified position.
sb.deleteCharAt(0);
commonly used methods of StringBuilder:

replace: Replaces the characters in a substring of this sequence with characters in the specified
string.
sb.replace(0, 5, "Hi");
reverse: Reverses the sequence of characters.
sb.reverse();
toString: Converts the StringBuilder to a String.
String result = sb.toString();
setLength: Sets the length of the character sequence.
sb.setLength(10);
commonly used methods of StringBuilder:

public class StringBuilderExample {
// Default constructor
StringBuilder sb = new StringBuilder();
// Appending different types of data
sb.append("Hello, ");
sb.append("World!");
sb.append(" Number: ").append(42);
sb.append(" Boolean: ").append(true);
System.out.println(sb.toString());
// Inserting text
sb.insert(7, "Java ");
// Deleting text
sb.delete(7, 12);
// Replacing text
sb.replace(7, 12, "Universe");
// Reversing the sequence
sb.reverse();
// Converting to String
String result = sb.toString();
System.out.println("Final result: " + result);
}
}
OUTPUT
Hello, World! Number: 42 Boolean: true
Hello, Java World! Number: 42 Boolean: true
Hello, World! Number: 42 Boolean: true
Hello, Universe! Number: 42 Boolean: true
eurt :naelooB 24 :rebmuN !esrevinU ,olleH
Final result: eurt :naelooB 24 :rebmuN !esrevinU ,olleH

String in java, string constructors and operations

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